Merge branch 'master' into skin-serialisation

osu.Game.Rulesets.Mania/ManiaRuleset.cs

@@ -47,7 +47,7 @@ namespace osu.Game.Rulesets.Mania
 
         public override ScoreProcessor CreateScoreProcessor() => new ManiaScoreProcessor();
 
-        public override HealthProcessor CreateHealthProcessor(double drainStartTime) => new DrainingHealthProcessor(drainStartTime, 0.2);
+        public override HealthProcessor CreateHealthProcessor(double drainStartTime) => new DrainingHealthProcessor(drainStartTime, 0.5);
 
         public override IBeatmapConverter CreateBeatmapConverter(IBeatmap beatmap) => new ManiaBeatmapConverter(beatmap, this);
 

osu.Game.Rulesets.Mania/Objects/Drawables/DrawableManiaHitObject.cs

@@ -6,6 +6,7 @@ using JetBrains.Annotations;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
+using osu.Game.Audio;
 using osu.Game.Rulesets.Objects.Drawables;
 using osu.Game.Rulesets.UI.Scrolling;
 using osu.Game.Rulesets.Mania.UI;
@@ -24,6 +25,11 @@ namespace osu.Game.Rulesets.Mania.Objects.Drawables
         [Resolved(canBeNull: true)]
         private ManiaPlayfield playfield { get; set; }
 
+        /// <summary>
+        /// Gets the samples that are played by this object during gameplay.
+        /// </summary>
+        public ISampleInfo[] GetGameplaySamples() => Samples.Samples;
+
         protected override float SamplePlaybackPosition
         {
             get

osu.Game.Rulesets.Mania/UI/Column.cs

@@ -27,6 +27,12 @@ namespace osu.Game.Rulesets.Mania.UI
         public const float COLUMN_WIDTH = 80;
         public const float SPECIAL_COLUMN_WIDTH = 70;
 
+        /// <summary>
+        /// For hitsounds played by this <see cref="Column"/> (i.e. not as a result of hitting a hitobject),
+        /// a certain number of samples are allowed to be played concurrently so that it feels better when spam-pressing the key.
+        /// </summary>
+        private const int max_concurrent_hitsounds = OsuGameBase.SAMPLE_CONCURRENCY;
+
         /// <summary>
         /// The index of this column as part of the whole playfield.
         /// </summary>
@@ -38,6 +44,7 @@ namespace osu.Game.Rulesets.Mania.UI
         internal readonly Container TopLevelContainer;
         private readonly DrawablePool<PoolableHitExplosion> hitExplosionPool;
         private readonly OrderedHitPolicy hitPolicy;
+        private readonly Container<SkinnableSound> hitSounds;
 
         public Container UnderlayElements => HitObjectArea.UnderlayElements;
 
@@ -64,6 +71,12 @@ namespace osu.Game.Rulesets.Mania.UI
                     RelativeSizeAxes = Axes.Both
                 },
                 background,
+                hitSounds = new Container<SkinnableSound>
+                {
+                    Name = "Column samples pool",
+                    RelativeSizeAxes = Axes.Both,
+                    Children = Enumerable.Range(0, max_concurrent_hitsounds).Select(_ => new SkinnableSound()).ToArray()
+                },
                 TopLevelContainer = new Container { RelativeSizeAxes = Axes.Both }
             };
 
@@ -120,6 +133,8 @@ namespace osu.Game.Rulesets.Mania.UI
             HitObjectArea.Explosions.Add(hitExplosionPool.Get(e => e.Apply(result)));
         }
 
+        private int nextHitSoundIndex;
+
         public bool OnPressed(ManiaAction action)
         {
             if (action != Action.Value)
@@ -131,7 +146,15 @@ namespace osu.Game.Rulesets.Mania.UI
                 HitObjectContainer.Objects.FirstOrDefault(h => h.HitObject.StartTime > Time.Current) ??
                 HitObjectContainer.Objects.LastOrDefault();
 
-            nextObject?.PlaySamples();
+            if (nextObject is DrawableManiaHitObject maniaObject)
+            {
+                var hitSound = hitSounds[nextHitSoundIndex];
+
+                hitSound.Samples = maniaObject.GetGameplaySamples();
+                hitSound.Play();
+
+                nextHitSoundIndex = (nextHitSoundIndex + 1) % max_concurrent_hitsounds;
+            }
 
             return true;
         }

osu.Game.Tests/NonVisual/Multiplayer/StatefulMultiplayerClientTest.cs

@@ -53,9 +53,9 @@ namespace osu.Game.Tests.NonVisual.Multiplayer
                 Client.RoomSetupAction = room =>
                 {
                     room.State = MultiplayerRoomState.Playing;
-                    room.Users.Add(new MultiplayerRoomUser(55)
+                    room.Users.Add(new MultiplayerRoomUser(PLAYER_1_ID)
                     {
-                        User = new User { Id = 55 },
+                        User = new User { Id = PLAYER_1_ID },
                         State = MultiplayerUserState.Playing
                     });
                 };

osu.Game.Tests/OnlinePlay/TestSceneCatchUpSyncManager.cs

@@ -0,0 +1,223 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using NUnit.Framework;
+using osu.Framework.Bindables;
+using osu.Framework.Testing;
+using osu.Framework.Timing;
+using osu.Game.Screens.OnlinePlay.Multiplayer.Spectate;
+using osu.Game.Tests.Visual;
+
+namespace osu.Game.Tests.OnlinePlay
+{
+    [HeadlessTest]
+    public class TestSceneCatchUpSyncManager : OsuTestScene
+    {
+        private TestManualClock master;
+        private CatchUpSyncManager syncManager;
+
+        private TestSpectatorPlayerClock player1;
+        private TestSpectatorPlayerClock player2;
+
+        [SetUp]
+        public void Setup()
+        {
+            syncManager = new CatchUpSyncManager(master = new TestManualClock());
+            syncManager.AddPlayerClock(player1 = new TestSpectatorPlayerClock(1));
+            syncManager.AddPlayerClock(player2 = new TestSpectatorPlayerClock(2));
+
+            Schedule(() => Child = syncManager);
+        }
+
+        [Test]
+        public void TestMasterClockStartsWhenAllPlayerClocksHaveFrames()
+        {
+            setWaiting(() => player1, false);
+            assertMasterState(false);
+            assertPlayerClockState(() => player1, false);
+            assertPlayerClockState(() => player2, false);
+
+            setWaiting(() => player2, false);
+            assertMasterState(true);
+            assertPlayerClockState(() => player1, true);
+            assertPlayerClockState(() => player2, true);
+        }
+
+        [Test]
+        public void TestMasterClockDoesNotStartWhenNoneReadyForMaximumDelayTime()
+        {
+            AddWaitStep($"wait {CatchUpSyncManager.MAXIMUM_START_DELAY} milliseconds", (int)Math.Ceiling(CatchUpSyncManager.MAXIMUM_START_DELAY / TimePerAction));
+            assertMasterState(false);
+        }
+
+        [Test]
+        public void TestMasterClockStartsWhenAnyReadyForMaximumDelayTime()
+        {
+            setWaiting(() => player1, false);
+            AddWaitStep($"wait {CatchUpSyncManager.MAXIMUM_START_DELAY} milliseconds", (int)Math.Ceiling(CatchUpSyncManager.MAXIMUM_START_DELAY / TimePerAction));
+            assertMasterState(true);
+        }
+
+        [Test]
+        public void TestPlayerClockDoesNotCatchUpWhenSlightlyOutOfSync()
+        {
+            setAllWaiting(false);
+
+            setMasterTime(CatchUpSyncManager.SYNC_TARGET + 1);
+            assertCatchingUp(() => player1, false);
+        }
+
+        [Test]
+        public void TestPlayerClockStartsCatchingUpWhenTooFarBehind()
+        {
+            setAllWaiting(false);
+
+            setMasterTime(CatchUpSyncManager.MAX_SYNC_OFFSET + 1);
+            assertCatchingUp(() => player1, true);
+            assertCatchingUp(() => player2, true);
+        }
+
+        [Test]
+        public void TestPlayerClockKeepsCatchingUpWhenSlightlyOutOfSync()
+        {
+            setAllWaiting(false);
+
+            setMasterTime(CatchUpSyncManager.MAX_SYNC_OFFSET + 1);
+            setPlayerClockTime(() => player1, CatchUpSyncManager.SYNC_TARGET + 1);
+            assertCatchingUp(() => player1, true);
+        }
+
+        [Test]
+        public void TestPlayerClockStopsCatchingUpWhenInSync()
+        {
+            setAllWaiting(false);
+
+            setMasterTime(CatchUpSyncManager.MAX_SYNC_OFFSET + 2);
+            setPlayerClockTime(() => player1, CatchUpSyncManager.SYNC_TARGET);
+            assertCatchingUp(() => player1, false);
+            assertCatchingUp(() => player2, true);
+        }
+
+        [Test]
+        public void TestPlayerClockDoesNotStopWhenSlightlyAhead()
+        {
+            setAllWaiting(false);
+
+            setPlayerClockTime(() => player1, -CatchUpSyncManager.SYNC_TARGET);
+            assertCatchingUp(() => player1, false);
+            assertPlayerClockState(() => player1, true);
+        }
+
+        [Test]
+        public void TestPlayerClockStopsWhenTooFarAheadAndStartsWhenBackInSync()
+        {
+            setAllWaiting(false);
+
+            setPlayerClockTime(() => player1, -CatchUpSyncManager.SYNC_TARGET - 1);
+
+            // This is a silent catchup, where IsCatchingUp = false but IsRunning = false also.
+            assertCatchingUp(() => player1, false);
+            assertPlayerClockState(() => player1, false);
+
+            setMasterTime(1);
+            assertCatchingUp(() => player1, false);
+            assertPlayerClockState(() => player1, true);
+        }
+
+        [Test]
+        public void TestInSyncPlayerClockDoesNotStartIfWaitingOnFrames()
+        {
+            setAllWaiting(false);
+
+            assertPlayerClockState(() => player1, true);
+            setWaiting(() => player1, true);
+            assertPlayerClockState(() => player1, false);
+        }
+
+        private void setWaiting(Func<TestSpectatorPlayerClock> playerClock, bool waiting)
+            => AddStep($"set player clock {playerClock().Id} waiting = {waiting}", () => playerClock().WaitingOnFrames.Value = waiting);
+
+        private void setAllWaiting(bool waiting) => AddStep($"set all player clocks waiting = {waiting}", () =>
+        {
+            player1.WaitingOnFrames.Value = waiting;
+            player2.WaitingOnFrames.Value = waiting;
+        });
+
+        private void setMasterTime(double time)
+            => AddStep($"set master = {time}", () => master.Seek(time));
+
+        /// <summary>
+        /// clock.Time = master.Time - offsetFromMaster
+        /// </summary>
+        private void setPlayerClockTime(Func<TestSpectatorPlayerClock> playerClock, double offsetFromMaster)
+            => AddStep($"set player clock {playerClock().Id} = master - {offsetFromMaster}", () => playerClock().Seek(master.CurrentTime - offsetFromMaster));
+
+        private void assertMasterState(bool running)
+            => AddAssert($"master clock {(running ? "is" : "is not")} running", () => master.IsRunning == running);
+
+        private void assertCatchingUp(Func<TestSpectatorPlayerClock> playerClock, bool catchingUp) =>
+            AddAssert($"player clock {playerClock().Id} {(catchingUp ? "is" : "is not")} catching up", () => playerClock().IsCatchingUp == catchingUp);
+
+        private void assertPlayerClockState(Func<TestSpectatorPlayerClock> playerClock, bool running)
+            => AddAssert($"player clock {playerClock().Id} {(running ? "is" : "is not")} running", () => playerClock().IsRunning == running);
+
+        private class TestSpectatorPlayerClock : TestManualClock, ISpectatorPlayerClock
+        {
+            public Bindable<bool> WaitingOnFrames { get; } = new Bindable<bool>(true);
+
+            public bool IsCatchingUp { get; set; }
+
+            public IFrameBasedClock Source
+            {
+                set => throw new NotImplementedException();
+            }
+
+            public readonly int Id;
+
+            public TestSpectatorPlayerClock(int id)
+            {
+                Id = id;
+
+                WaitingOnFrames.BindValueChanged(waiting =>
+                {
+                    if (waiting.NewValue)
+                        Stop();
+                    else
+                        Start();
+                });
+            }
+
+            public void ProcessFrame()
+            {
+            }
+
+            public double ElapsedFrameTime => 0;
+
+            public double FramesPerSecond => 0;
+
+            public FrameTimeInfo TimeInfo => default;
+        }
+
+        private class TestManualClock : ManualClock, IAdjustableClock
+        {
+            public void Start() => IsRunning = true;
+
+            public void Stop() => IsRunning = false;
+
+            public bool Seek(double position)
+            {
+                CurrentTime = position;
+                return true;
+            }
+
+            public void Reset()
+            {
+            }
+
+            public void ResetSpeedAdjustments()
+            {
+            }
+        }
+    }
+}

osu.Game.Tests/Visual/Gameplay/TestSceneSpectator.cs

@@ -1,34 +1,32 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using NUnit.Framework;
 using osu.Framework.Allocation;
-using osu.Framework.Bindables;
 using osu.Framework.Graphics;
 using osu.Framework.Screens;
 using osu.Framework.Testing;
-using osu.Framework.Utils;
 using osu.Game.Beatmaps;
 using osu.Game.Database;
-using osu.Game.Online;
 using osu.Game.Online.Spectator;
-using osu.Game.Replays.Legacy;
 using osu.Game.Rulesets.Osu;
 using osu.Game.Rulesets.Osu.Replays;
 using osu.Game.Rulesets.UI;
-using osu.Game.Scoring;
 using osu.Game.Screens.Play;
 using osu.Game.Tests.Beatmaps.IO;
+using osu.Game.Tests.Visual.Multiplayer;
+using osu.Game.Tests.Visual.Spectator;
 using osu.Game.Users;
 
 namespace osu.Game.Tests.Visual.Gameplay
 {
     public class TestSceneSpectator : ScreenTestScene
     {
+        private readonly User streamingUser = new User { Id = MultiplayerTestScene.PLAYER_1_ID, Username = "Test user" };
+
         [Cached(typeof(SpectatorStreamingClient))]
         private TestSpectatorStreamingClient testSpectatorStreamingClient = new TestSpectatorStreamingClient();
 
@@ -214,9 +212,9 @@ namespace osu.Game.Tests.Visual.Gameplay
 
         private void waitForPlayer() => AddUntilStep("wait for player", () => Stack.CurrentScreen is Player);
 
-        private void start(int? beatmapId = null) => AddStep("start play", () => testSpectatorStreamingClient.StartPlay(beatmapId ?? importedBeatmapId));
+        private void start(int? beatmapId = null) => AddStep("start play", () => testSpectatorStreamingClient.StartPlay(streamingUser.Id, beatmapId ?? importedBeatmapId));
 
-        private void finish(int? beatmapId = null) => AddStep("end play", () => testSpectatorStreamingClient.EndPlay(beatmapId ?? importedBeatmapId));
+        private void finish(int? beatmapId = null) => AddStep("end play", () => testSpectatorStreamingClient.EndPlay(streamingUser.Id, beatmapId ?? importedBeatmapId));
 
         private void checkPaused(bool state) =>
             AddUntilStep($"game is {(state ? "paused" : "playing")}", () => player.ChildrenOfType<DrawableRuleset>().First().IsPaused.Value == state);
@@ -225,89 +223,17 @@ namespace osu.Game.Tests.Visual.Gameplay
         {
             AddStep("send frames", () =>
             {
-                testSpectatorStreamingClient.SendFrames(nextFrame, count);
+                testSpectatorStreamingClient.SendFrames(streamingUser.Id, nextFrame, count);
                 nextFrame += count;
             });
         }
 
         private void loadSpectatingScreen()
         {
-            AddStep("load screen", () => LoadScreen(spectatorScreen = new SoloSpectator(testSpectatorStreamingClient.StreamingUser)));
+            AddStep("load screen", () => LoadScreen(spectatorScreen = new SoloSpectator(streamingUser)));
             AddUntilStep("wait for screen load", () => spectatorScreen.LoadState == LoadState.Loaded);
         }
 
-        public class TestSpectatorStreamingClient : SpectatorStreamingClient
-        {
-            public readonly User StreamingUser = new User { Id = 55, Username = "Test user" };
-
-            public new BindableList<int> PlayingUsers => (BindableList<int>)base.PlayingUsers;
-
-            private int beatmapId;
-
-            public TestSpectatorStreamingClient()
-                : base(new DevelopmentEndpointConfiguration())
-            {
-            }
-
-            public void StartPlay(int beatmapId)
-            {
-                this.beatmapId = beatmapId;
-                sendState(beatmapId);
-            }
-
-            public void EndPlay(int beatmapId)
-            {
-                ((ISpectatorClient)this).UserFinishedPlaying(StreamingUser.Id, new SpectatorState
-                {
-                    BeatmapID = beatmapId,
-                    RulesetID = 0,
-                });
-
-                sentState = false;
-            }
-
-            private bool sentState;
-
-            public void SendFrames(int index, int count)
-            {
-                var frames = new List<LegacyReplayFrame>();
-
-                for (int i = index; i < index + count; i++)
-                {
-                    var buttonState = i == index + count - 1 ? ReplayButtonState.None : ReplayButtonState.Left1;
-
-                    frames.Add(new LegacyReplayFrame(i * 100, RNG.Next(0, 512), RNG.Next(0, 512), buttonState));
-                }
-
-                var bundle = new FrameDataBundle(new ScoreInfo(), frames);
-                ((ISpectatorClient)this).UserSentFrames(StreamingUser.Id, bundle);
-
-                if (!sentState)
-                    sendState(beatmapId);
-            }
-
-            public override void WatchUser(int userId)
-            {
-                if (!PlayingUsers.Contains(userId) && sentState)
-                {
-                    // usually the server would do this.
-                    sendState(beatmapId);
-                }
-
-                base.WatchUser(userId);
-            }
-
-            private void sendState(int beatmapId)
-            {
-                sentState = true;
-                ((ISpectatorClient)this).UserBeganPlaying(StreamingUser.Id, new SpectatorState
-                {
-                    BeatmapID = beatmapId,
-                    RulesetID = 0,
-                });
-            }
-        }
-
         internal class TestUserLookupCache : UserLookupCache
         {
             protected override Task<User> ComputeValueAsync(int lookup, CancellationToken token = default) => Task.FromResult(new User

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiSpectatorLeaderboard.cs

@@ -11,20 +11,17 @@ using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Testing;
 using osu.Framework.Timing;
-using osu.Framework.Utils;
 using osu.Game.Database;
-using osu.Game.Online;
 using osu.Game.Online.Spectator;
-using osu.Game.Replays.Legacy;
 using osu.Game.Rulesets.Osu.Scoring;
-using osu.Game.Scoring;
 using osu.Game.Screens.OnlinePlay.Multiplayer.Spectate;
 using osu.Game.Screens.Play.HUD;
+using osu.Game.Tests.Visual.Spectator;
 using osu.Game.Users;
 
 namespace osu.Game.Tests.Visual.Multiplayer
 {
-    public class TestSceneMultiplayerSpectatorLeaderboard : MultiplayerTestScene
+    public class TestSceneMultiSpectatorLeaderboard : MultiplayerTestScene
     {
         [Cached(typeof(SpectatorStreamingClient))]
         private TestSpectatorStreamingClient streamingClient = new TestSpectatorStreamingClient();
@@ -37,11 +34,11 @@ namespace osu.Game.Tests.Visual.Multiplayer
 
         private readonly Dictionary<int, ManualClock> clocks = new Dictionary<int, ManualClock>
         {
-            { 55, new ManualClock() },
-            { 56, new ManualClock() }
+            { PLAYER_1_ID, new ManualClock() },
+            { PLAYER_2_ID, new ManualClock() }
         };
 
-        public TestSceneMultiplayerSpectatorLeaderboard()
+        public TestSceneMultiSpectatorLeaderboard()
         {
             base.Content.AddRange(new Drawable[]
             {
@@ -54,7 +51,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
         [SetUpSteps]
         public new void SetUpSteps()
         {
-            MultiplayerSpectatorLeaderboard leaderboard = null;
+            MultiSpectatorLeaderboard leaderboard = null;
 
             AddStep("reset", () =>
             {
@@ -78,7 +75,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
                 var scoreProcessor = new OsuScoreProcessor();
                 scoreProcessor.ApplyBeatmap(playable);
 
-                LoadComponentAsync(leaderboard = new MultiplayerSpectatorLeaderboard(scoreProcessor, clocks.Keys.ToArray()) { Expanded = { Value = true } }, Add);
+                LoadComponentAsync(leaderboard = new MultiSpectatorLeaderboard(scoreProcessor, clocks.Keys.ToArray()) { Expanded = { Value = true } }, Add);
             });
 
             AddUntilStep("wait for load", () => leaderboard.IsLoaded);
@@ -95,46 +92,46 @@ namespace osu.Game.Tests.Visual.Multiplayer
         {
             AddStep("send frames", () =>
             {
-                // For user 55, send frames in sets of 1.
-                // For user 56, send frames in sets of 10.
+                // For player 1, send frames in sets of 1.
+                // For player 2, send frames in sets of 10.
                 for (int i = 0; i < 100; i++)
                 {
-                    streamingClient.SendFrames(55, i, 1);
+                    streamingClient.SendFrames(PLAYER_1_ID, i, 1);
 
                     if (i % 10 == 0)
-                        streamingClient.SendFrames(56, i, 10);
+                        streamingClient.SendFrames(PLAYER_2_ID, i, 10);
                 }
             });
 
-            assertCombo(55, 1);
-            assertCombo(56, 10);
+            assertCombo(PLAYER_1_ID, 1);
+            assertCombo(PLAYER_2_ID, 10);
 
-            // Advance to a point where only user 55's frame changes.
+            // Advance to a point where only user player 1's frame changes.
             setTime(500);
-            assertCombo(55, 5);
-            assertCombo(56, 10);
+            assertCombo(PLAYER_1_ID, 5);
+            assertCombo(PLAYER_2_ID, 10);
 
             // Advance to a point where both user's frame changes.
             setTime(1100);
-            assertCombo(55, 11);
-            assertCombo(56, 20);
+            assertCombo(PLAYER_1_ID, 11);
+            assertCombo(PLAYER_2_ID, 20);
 
-            // Advance user 56 only to a point where its frame changes.
-            setTime(56, 2100);
-            assertCombo(55, 11);
-            assertCombo(56, 30);
+            // Advance user player 2 only to a point where its frame changes.
+            setTime(PLAYER_2_ID, 2100);
+            assertCombo(PLAYER_1_ID, 11);
+            assertCombo(PLAYER_2_ID, 30);
 
             // Advance both users beyond their last frame
             setTime(101 * 100);
-            assertCombo(55, 100);
-            assertCombo(56, 100);
+            assertCombo(PLAYER_1_ID, 100);
+            assertCombo(PLAYER_2_ID, 100);
         }
 
         [Test]
         public void TestNoFrames()
         {
-            assertCombo(55, 0);
-            assertCombo(56, 0);
+            assertCombo(PLAYER_1_ID, 0);
+            assertCombo(PLAYER_2_ID, 0);
         }
 
         private void setTime(double time) => AddStep($"set time {time}", () =>
@@ -149,71 +146,6 @@ namespace osu.Game.Tests.Visual.Multiplayer
         private void assertCombo(int userId, int expectedCombo)
             => AddUntilStep($"player {userId} has {expectedCombo} combo", () => this.ChildrenOfType<GameplayLeaderboardScore>().Single(s => s.User?.Id == userId).Combo.Value == expectedCombo);
 
-        private class TestSpectatorStreamingClient : SpectatorStreamingClient
-        {
-            private readonly Dictionary<int, int> userBeatmapDictionary = new Dictionary<int, int>();
-            private readonly Dictionary<int, bool> userSentStateDictionary = new Dictionary<int, bool>();
-
-            public TestSpectatorStreamingClient()
-                : base(new DevelopmentEndpointConfiguration())
-            {
-            }
-
-            public void StartPlay(int userId, int beatmapId)
-            {
-                userBeatmapDictionary[userId] = beatmapId;
-                userSentStateDictionary[userId] = false;
-                sendState(userId, beatmapId);
-            }
-
-            public void EndPlay(int userId, int beatmapId)
-            {
-                ((ISpectatorClient)this).UserFinishedPlaying(userId, new SpectatorState
-                {
-                    BeatmapID = beatmapId,
-                    RulesetID = 0,
-                });
-                userSentStateDictionary[userId] = false;
-            }
-
-            public void SendFrames(int userId, int index, int count)
-            {
-                var frames = new List<LegacyReplayFrame>();
-
-                for (int i = index; i < index + count; i++)
-                {
-                    var buttonState = i == index + count - 1 ? ReplayButtonState.None : ReplayButtonState.Left1;
-                    frames.Add(new LegacyReplayFrame(i * 100, RNG.Next(0, 512), RNG.Next(0, 512), buttonState));
-                }
-
-                var bundle = new FrameDataBundle(new ScoreInfo { Combo = index + count }, frames);
-                ((ISpectatorClient)this).UserSentFrames(userId, bundle);
-                if (!userSentStateDictionary[userId])
-                    sendState(userId, userBeatmapDictionary[userId]);
-            }
-
-            public override void WatchUser(int userId)
-            {
-                if (userSentStateDictionary[userId])
-                {
-                    // usually the server would do this.
-                    sendState(userId, userBeatmapDictionary[userId]);
-                }
-
-                base.WatchUser(userId);
-            }
-
-            private void sendState(int userId, int beatmapId)
-            {
-                ((ISpectatorClient)this).UserBeganPlaying(userId, new SpectatorState
-                {
-                    BeatmapID = beatmapId,
-                    RulesetID = 0,
-                });
-                userSentStateDictionary[userId] = true;
-            }
-        }
-
         private class TestUserLookupCache : UserLookupCache
         {
             protected override Task<User> ComputeValueAsync(int lookup, CancellationToken token = default)

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiSpectatorScreen.cs

@@ -0,0 +1,313 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics;
+using osu.Framework.Testing;
+using osu.Game.Beatmaps;
+using osu.Game.Database;
+using osu.Game.Online.Spectator;
+using osu.Game.Screens.OnlinePlay.Multiplayer.Spectate;
+using osu.Game.Screens.Play;
+using osu.Game.Tests.Beatmaps.IO;
+using osu.Game.Tests.Visual.Spectator;
+using osu.Game.Users;
+
+namespace osu.Game.Tests.Visual.Multiplayer
+{
+    public class TestSceneMultiSpectatorScreen : MultiplayerTestScene
+    {
+        [Cached(typeof(SpectatorStreamingClient))]
+        private TestSpectatorStreamingClient streamingClient = new TestSpectatorStreamingClient();
+
+        [Cached(typeof(UserLookupCache))]
+        private UserLookupCache lookupCache = new TestUserLookupCache();
+
+        [Resolved]
+        private OsuGameBase game { get; set; }
+
+        [Resolved]
+        private BeatmapManager beatmapManager { get; set; }
+
+        private MultiSpectatorScreen spectatorScreen;
+
+        private readonly List<int> playingUserIds = new List<int>();
+        private readonly Dictionary<int, int> nextFrame = new Dictionary<int, int>();
+
+        private BeatmapSetInfo importedSet;
+        private BeatmapInfo importedBeatmap;
+        private int importedBeatmapId;
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            importedSet = ImportBeatmapTest.LoadOszIntoOsu(game, virtualTrack: true).Result;
+            importedBeatmap = importedSet.Beatmaps.First(b => b.RulesetID == 0);
+            importedBeatmapId = importedBeatmap.OnlineBeatmapID ?? -1;
+        }
+
+        public override void SetUpSteps()
+        {
+            base.SetUpSteps();
+
+            AddStep("reset sent frames", () => nextFrame.Clear());
+
+            AddStep("add streaming client", () =>
+            {
+                Remove(streamingClient);
+                Add(streamingClient);
+            });
+
+            AddStep("finish previous gameplay", () =>
+            {
+                foreach (var id in playingUserIds)
+                    streamingClient.EndPlay(id, importedBeatmapId);
+                playingUserIds.Clear();
+            });
+        }
+
+        [Test]
+        public void TestDelayedStart()
+        {
+            AddStep("start players silently", () =>
+            {
+                Client.CurrentMatchPlayingUserIds.Add(PLAYER_1_ID);
+                Client.CurrentMatchPlayingUserIds.Add(PLAYER_2_ID);
+                playingUserIds.Add(PLAYER_1_ID);
+                playingUserIds.Add(PLAYER_2_ID);
+                nextFrame[PLAYER_1_ID] = 0;
+                nextFrame[PLAYER_2_ID] = 0;
+            });
+
+            loadSpectateScreen(false);
+
+            AddWaitStep("wait a bit", 10);
+            AddStep("load player first_player_id", () => streamingClient.StartPlay(PLAYER_1_ID, importedBeatmapId));
+            AddUntilStep("one player added", () => spectatorScreen.ChildrenOfType<Player>().Count() == 1);
+
+            AddWaitStep("wait a bit", 10);
+            AddStep("load player second_player_id", () => streamingClient.StartPlay(PLAYER_2_ID, importedBeatmapId));
+            AddUntilStep("two players added", () => spectatorScreen.ChildrenOfType<Player>().Count() == 2);
+        }
+
+        [Test]
+        public void TestGeneral()
+        {
+            int[] userIds = Enumerable.Range(0, 4).Select(i => PLAYER_1_ID + i).ToArray();
+
+            start(userIds);
+            loadSpectateScreen();
+
+            sendFrames(userIds, 1000);
+            AddWaitStep("wait a bit", 20);
+        }
+
+        [Test]
+        public void TestPlayersMustStartSimultaneously()
+        {
+            start(new[] { PLAYER_1_ID, PLAYER_2_ID });
+            loadSpectateScreen();
+
+            // Send frames for one player only, both should remain paused.
+            sendFrames(PLAYER_1_ID, 20);
+            checkPausedInstant(PLAYER_1_ID, true);
+            checkPausedInstant(PLAYER_2_ID, true);
+
+            // Send frames for the other player, both should now start playing.
+            sendFrames(PLAYER_2_ID, 20);
+            checkPausedInstant(PLAYER_1_ID, false);
+            checkPausedInstant(PLAYER_2_ID, false);
+        }
+
+        [Test]
+        public void TestPlayersDoNotStartSimultaneouslyIfBufferingForMaximumStartDelay()
+        {
+            start(new[] { PLAYER_1_ID, PLAYER_2_ID });
+            loadSpectateScreen();
+
+            // Send frames for one player only, both should remain paused.
+            sendFrames(PLAYER_1_ID, 1000);
+            checkPausedInstant(PLAYER_1_ID, true);
+            checkPausedInstant(PLAYER_2_ID, true);
+
+            // Wait for the start delay seconds...
+            AddWaitStep("wait maximum start delay seconds", (int)(CatchUpSyncManager.MAXIMUM_START_DELAY / TimePerAction));
+
+            // Player 1 should start playing by itself, player 2 should remain paused.
+            checkPausedInstant(PLAYER_1_ID, false);
+            checkPausedInstant(PLAYER_2_ID, true);
+        }
+
+        [Test]
+        public void TestPlayersContinueWhileOthersBuffer()
+        {
+            start(new[] { PLAYER_1_ID, PLAYER_2_ID });
+            loadSpectateScreen();
+
+            // Send initial frames for both players. A few more for player 1.
+            sendFrames(PLAYER_1_ID, 20);
+            sendFrames(PLAYER_2_ID, 10);
+            checkPausedInstant(PLAYER_1_ID, false);
+            checkPausedInstant(PLAYER_2_ID, false);
+
+            // Eventually player 2 will pause, player 1 must remain running.
+            checkPaused(PLAYER_2_ID, true);
+            checkPausedInstant(PLAYER_1_ID, false);
+
+            // Eventually both players will run out of frames and should pause.
+            checkPaused(PLAYER_1_ID, true);
+            checkPausedInstant(PLAYER_2_ID, true);
+
+            // Send more frames for the first player only. Player 1 should start playing with player 2 remaining paused.
+            sendFrames(PLAYER_1_ID, 20);
+            checkPausedInstant(PLAYER_2_ID, true);
+            checkPausedInstant(PLAYER_1_ID, false);
+
+            // Send more frames for the second player. Both should be playing
+            sendFrames(PLAYER_2_ID, 20);
+            checkPausedInstant(PLAYER_2_ID, false);
+            checkPausedInstant(PLAYER_1_ID, false);
+        }
+
+        [Test]
+        public void TestPlayersCatchUpAfterFallingBehind()
+        {
+            start(new[] { PLAYER_1_ID, PLAYER_2_ID });
+            loadSpectateScreen();
+
+            // Send initial frames for both players. A few more for player 1.
+            sendFrames(PLAYER_1_ID, 1000);
+            sendFrames(PLAYER_2_ID, 10);
+            checkPausedInstant(PLAYER_1_ID, false);
+            checkPausedInstant(PLAYER_2_ID, false);
+
+            // Eventually player 2 will run out of frames and should pause.
+            checkPaused(PLAYER_2_ID, true);
+            AddWaitStep("wait a few more frames", 10);
+
+            // Send more frames for player 2. It should unpause.
+            sendFrames(PLAYER_2_ID, 1000);
+            checkPausedInstant(PLAYER_2_ID, false);
+
+            // Player 2 should catch up to player 1 after unpausing.
+            waitForCatchup(PLAYER_2_ID);
+            AddWaitStep("wait a bit", 10);
+        }
+
+        [Test]
+        public void TestMostInSyncUserIsAudioSource()
+        {
+            start(new[] { PLAYER_1_ID, PLAYER_2_ID });
+            loadSpectateScreen();
+
+            assertMuted(PLAYER_1_ID, true);
+            assertMuted(PLAYER_2_ID, true);
+
+            sendFrames(PLAYER_1_ID, 10);
+            sendFrames(PLAYER_2_ID, 20);
+            assertMuted(PLAYER_1_ID, false);
+            assertMuted(PLAYER_2_ID, true);
+
+            checkPaused(PLAYER_1_ID, true);
+            assertMuted(PLAYER_1_ID, true);
+            assertMuted(PLAYER_2_ID, false);
+
+            sendFrames(PLAYER_1_ID, 100);
+            waitForCatchup(PLAYER_1_ID);
+            checkPaused(PLAYER_2_ID, true);
+            assertMuted(PLAYER_1_ID, false);
+            assertMuted(PLAYER_2_ID, true);
+
+            sendFrames(PLAYER_2_ID, 100);
+            waitForCatchup(PLAYER_2_ID);
+            assertMuted(PLAYER_1_ID, false);
+            assertMuted(PLAYER_2_ID, true);
+        }
+
+        private void loadSpectateScreen(bool waitForPlayerLoad = true)
+        {
+            AddStep("load screen", () =>
+            {
+                Beatmap.Value = beatmapManager.GetWorkingBeatmap(importedBeatmap);
+                Ruleset.Value = importedBeatmap.Ruleset;
+
+                LoadScreen(spectatorScreen = new MultiSpectatorScreen(playingUserIds.ToArray()));
+            });
+
+            AddUntilStep("wait for screen load", () => spectatorScreen.LoadState == LoadState.Loaded && (!waitForPlayerLoad || spectatorScreen.AllPlayersLoaded));
+        }
+
+        private void start(int userId, int? beatmapId = null) => start(new[] { userId }, beatmapId);
+
+        private void start(int[] userIds, int? beatmapId = null)
+        {
+            AddStep("start play", () =>
+            {
+                foreach (int id in userIds)
+                {
+                    Client.CurrentMatchPlayingUserIds.Add(id);
+                    streamingClient.StartPlay(id, beatmapId ?? importedBeatmapId);
+                    playingUserIds.Add(id);
+                    nextFrame[id] = 0;
+                }
+            });
+        }
+
+        private void finish(int userId, int? beatmapId = null)
+        {
+            AddStep("end play", () =>
+            {
+                streamingClient.EndPlay(userId, beatmapId ?? importedBeatmapId);
+                playingUserIds.Remove(userId);
+                nextFrame.Remove(userId);
+            });
+        }
+
+        private void sendFrames(int userId, int count = 10) => sendFrames(new[] { userId }, count);
+
+        private void sendFrames(int[] userIds, int count = 10)
+        {
+            AddStep("send frames", () =>
+            {
+                foreach (int id in userIds)
+                {
+                    streamingClient.SendFrames(id, nextFrame[id], count);
+                    nextFrame[id] += count;
+                }
+            });
+        }
+
+        private void checkPaused(int userId, bool state)
+            => AddUntilStep($"{userId} is {(state ? "paused" : "playing")}", () => getPlayer(userId).ChildrenOfType<GameplayClockContainer>().First().GameplayClock.IsRunning != state);
+
+        private void checkPausedInstant(int userId, bool state)
+            => AddAssert($"{userId} is {(state ? "paused" : "playing")}", () => getPlayer(userId).ChildrenOfType<GameplayClockContainer>().First().GameplayClock.IsRunning != state);
+
+        private void assertMuted(int userId, bool muted)
+            => AddAssert($"{userId} {(muted ? "is" : "is not")} muted", () => getInstance(userId).Mute == muted);
+
+        private void waitForCatchup(int userId)
+            => AddUntilStep($"{userId} not catching up", () => !getInstance(userId).GameplayClock.IsCatchingUp);
+
+        private Player getPlayer(int userId) => getInstance(userId).ChildrenOfType<Player>().Single();
+
+        private PlayerArea getInstance(int userId) => spectatorScreen.ChildrenOfType<PlayerArea>().Single(p => p.UserId == userId);
+
+        internal class TestUserLookupCache : UserLookupCache
+        {
+            protected override Task<User> ComputeValueAsync(int lookup, CancellationToken token = default)
+            {
+                return Task.FromResult(new User
+                {
+                    Id = lookup,
+                    Username = $"User {lookup}"
+                });
+            }
+        }
+    }
+}

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiplayer.cs

@@ -1,9 +1,25 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using System;
+using System.Linq;
+using NUnit.Framework;
 using osu.Framework.Allocation;
+using osu.Framework.Audio;
+using osu.Framework.Platform;
+using osu.Framework.Screens;
+using osu.Framework.Testing;
+using osu.Game.Beatmaps;
 using osu.Game.Online.Multiplayer;
+using osu.Game.Online.Rooms;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Osu;
 using osu.Game.Screens.OnlinePlay.Components;
+using osu.Game.Screens.OnlinePlay.Multiplayer;
+using osu.Game.Screens.OnlinePlay.Multiplayer.Match;
+using osu.Game.Tests.Resources;
+using osu.Game.Users;
+using osuTK.Input;
 
 namespace osu.Game.Tests.Visual.Multiplayer
 {
@@ -11,7 +27,158 @@ namespace osu.Game.Tests.Visual.Multiplayer
     {
         private TestMultiplayer multiplayerScreen;
 
+        private BeatmapManager beatmaps;
+        private RulesetStore rulesets;
+        private BeatmapSetInfo importedSet;
+
+        private TestMultiplayerClient client => multiplayerScreen.Client;
+        private Room room => client.APIRoom;
+
         public TestSceneMultiplayer()
+        {
+            loadMultiplayer();
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(GameHost host, AudioManager audio)
+        {
+            Dependencies.Cache(rulesets = new RulesetStore(ContextFactory));
+            Dependencies.Cache(beatmaps = new BeatmapManager(LocalStorage, ContextFactory, rulesets, null, audio, host, Beatmap.Default));
+        }
+
+        [SetUp]
+        public void Setup() => Schedule(() =>
+        {
+            beatmaps.Import(TestResources.GetQuickTestBeatmapForImport()).Wait();
+            importedSet = beatmaps.GetAllUsableBeatmapSetsEnumerable(IncludedDetails.All).First();
+        });
+
+        [Test]
+        public void TestUserSetToIdleWhenBeatmapDeleted()
+        {
+            loadMultiplayer();
+
+            createRoom(() => new Room
+            {
+                Name = { Value = "Test Room" },
+                Playlist =
+                {
+                    new PlaylistItem
+                    {
+                        Beatmap = { Value = beatmaps.GetWorkingBeatmap(importedSet.Beatmaps.First(b => b.RulesetID == 0)).BeatmapInfo },
+                        Ruleset = { Value = new OsuRuleset().RulesetInfo },
+                    }
+                }
+            });
+
+            AddStep("set user ready", () => client.ChangeState(MultiplayerUserState.Ready));
+            AddStep("delete beatmap", () => beatmaps.Delete(importedSet));
+
+            AddUntilStep("user state is idle", () => client.LocalUser?.State == MultiplayerUserState.Idle);
+        }
+
+        [Test]
+        public void TestLocalPlayDoesNotStartWhileSpectatingWithNoBeatmap()
+        {
+            loadMultiplayer();
+
+            createRoom(() => new Room
+            {
+                Name = { Value = "Test Room" },
+                Playlist =
+                {
+                    new PlaylistItem
+                    {
+                        Beatmap = { Value = beatmaps.GetWorkingBeatmap(importedSet.Beatmaps.First(b => b.RulesetID == 0)).BeatmapInfo },
+                        Ruleset = { Value = new OsuRuleset().RulesetInfo },
+                    }
+                }
+            });
+
+            AddStep("join other user (ready, host)", () =>
+            {
+                client.AddUser(new User { Id = MultiplayerTestScene.PLAYER_1_ID, Username = "Other" });
+                client.TransferHost(MultiplayerTestScene.PLAYER_1_ID);
+                client.ChangeUserState(MultiplayerTestScene.PLAYER_1_ID, MultiplayerUserState.Ready);
+            });
+
+            AddStep("delete beatmap", () => beatmaps.Delete(importedSet));
+
+            AddStep("click spectate button", () =>
+            {
+                InputManager.MoveMouseTo(this.ChildrenOfType<MultiplayerSpectateButton>().Single());
+                InputManager.Click(MouseButton.Left);
+            });
+
+            AddStep("start match externally", () => client.StartMatch());
+
+            AddAssert("play not started", () => multiplayerScreen.IsCurrentScreen());
+        }
+
+        [Test]
+        public void TestLocalPlayStartsWhileSpectatingWhenBeatmapBecomesAvailable()
+        {
+            loadMultiplayer();
+
+            createRoom(() => new Room
+            {
+                Name = { Value = "Test Room" },
+                Playlist =
+                {
+                    new PlaylistItem
+                    {
+                        Beatmap = { Value = beatmaps.GetWorkingBeatmap(importedSet.Beatmaps.First(b => b.RulesetID == 0)).BeatmapInfo },
+                        Ruleset = { Value = new OsuRuleset().RulesetInfo },
+                    }
+                }
+            });
+
+            AddStep("delete beatmap", () => beatmaps.Delete(importedSet));
+
+            AddStep("join other user (ready, host)", () =>
+            {
+                client.AddUser(new User { Id = MultiplayerTestScene.PLAYER_1_ID, Username = "Other" });
+                client.TransferHost(MultiplayerTestScene.PLAYER_1_ID);
+                client.ChangeUserState(MultiplayerTestScene.PLAYER_1_ID, MultiplayerUserState.Ready);
+            });
+
+            AddStep("click spectate button", () =>
+            {
+                InputManager.MoveMouseTo(this.ChildrenOfType<MultiplayerSpectateButton>().Single());
+                InputManager.Click(MouseButton.Left);
+            });
+
+            AddStep("start match externally", () => client.StartMatch());
+
+            AddStep("restore beatmap", () =>
+            {
+                beatmaps.Import(TestResources.GetQuickTestBeatmapForImport()).Wait();
+                importedSet = beatmaps.GetAllUsableBeatmapSetsEnumerable(IncludedDetails.All).First();
+            });
+
+            AddUntilStep("play started", () => !multiplayerScreen.IsCurrentScreen());
+        }
+
+        private void createRoom(Func<Room> room)
+        {
+            AddStep("open room", () =>
+            {
+                multiplayerScreen.OpenNewRoom(room());
+            });
+
+            AddUntilStep("wait for room open", () => this.ChildrenOfType<MultiplayerMatchSubScreen>().FirstOrDefault()?.IsLoaded == true);
+            AddWaitStep("wait for transition", 2);
+
+            AddStep("create room", () =>
+            {
+                InputManager.MoveMouseTo(this.ChildrenOfType<MultiplayerMatchSettingsOverlay.CreateOrUpdateButton>().Single());
+                InputManager.Click(MouseButton.Left);
+            });
+
+            AddUntilStep("wait for join", () => client.Room != null);
+        }
+
+        private void loadMultiplayer()
         {
             AddStep("show", () =>
             {

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiplayerGameplayLeaderboard.cs

@@ -6,14 +6,12 @@ using System.Collections.Generic;
 using System.Linq;
 using NUnit.Framework;
 using osu.Framework.Allocation;
-using osu.Framework.Bindables;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Testing;
 using osu.Framework.Utils;
 using osu.Game.Configuration;
 using osu.Game.Database;
-using osu.Game.Online;
 using osu.Game.Online.API;
 using osu.Game.Online.Spectator;
 using osu.Game.Replays.Legacy;
@@ -22,6 +20,7 @@ using osu.Game.Rulesets.Scoring;
 using osu.Game.Scoring;
 using osu.Game.Screens.Play.HUD;
 using osu.Game.Tests.Visual.Online;
+using osu.Game.Tests.Visual.Spectator;
 
 namespace osu.Game.Tests.Visual.Multiplayer
 {
@@ -30,7 +29,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
         private const int users = 16;
 
         [Cached(typeof(SpectatorStreamingClient))]
-        private TestMultiplayerStreaming streamingClient = new TestMultiplayerStreaming(users);
+        private TestMultiplayerStreaming streamingClient = new TestMultiplayerStreaming();
 
         [Cached(typeof(UserLookupCache))]
         private UserLookupCache lookupCache = new TestSceneCurrentlyPlayingDisplay.TestUserLookupCache();
@@ -71,7 +70,8 @@ namespace osu.Game.Tests.Visual.Multiplayer
 
                 var playable = Beatmap.Value.GetPlayableBeatmap(Ruleset.Value);
 
-                streamingClient.Start(Beatmap.Value.BeatmapInfo.OnlineBeatmapID ?? 0);
+                for (int i = 0; i < users; i++)
+                    streamingClient.StartPlay(i, Beatmap.Value.BeatmapInfo.OnlineBeatmapID ?? 0);
 
                 Client.CurrentMatchPlayingUserIds.Clear();
                 Client.CurrentMatchPlayingUserIds.AddRange(streamingClient.PlayingUsers);
@@ -114,30 +114,8 @@ namespace osu.Game.Tests.Visual.Multiplayer
             AddStep("change to standardised", () => config.SetValue(OsuSetting.ScoreDisplayMode, ScoringMode.Standardised));
         }
 
-        public class TestMultiplayerStreaming : SpectatorStreamingClient
+        public class TestMultiplayerStreaming : TestSpectatorStreamingClient
         {
-            public new BindableList<int> PlayingUsers => (BindableList<int>)base.PlayingUsers;
-
-            private readonly int totalUsers;
-
-            public TestMultiplayerStreaming(int totalUsers)
-                : base(new DevelopmentEndpointConfiguration())
-            {
-                this.totalUsers = totalUsers;
-            }
-
-            public void Start(int beatmapId)
-            {
-                for (int i = 0; i < totalUsers; i++)
-                {
-                    ((ISpectatorClient)this).UserBeganPlaying(i, new SpectatorState
-                    {
-                        BeatmapID = beatmapId,
-                        RulesetID = 0,
-                    });
-                }
-            }
-
             private readonly Dictionary<int, FrameHeader> lastHeaders = new Dictionary<int, FrameHeader>();
 
             public void RandomlyUpdateState()

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiplayerMatchSubScreen.cs

@@ -119,8 +119,8 @@ namespace osu.Game.Tests.Visual.Multiplayer
 
             AddStep("join other user (ready)", () =>
             {
-                Client.AddUser(new User { Id = 55 });
-                Client.ChangeUserState(55, MultiplayerUserState.Ready);
+                Client.AddUser(new User { Id = PLAYER_1_ID });
+                Client.ChangeUserState(PLAYER_1_ID, MultiplayerUserState.Ready);
             });
 
             AddStep("click spectate button", () =>

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiplayerSpectateButton.cs

@@ -120,9 +120,12 @@ namespace osu.Game.Tests.Visual.Multiplayer
             };
         });
 
-        [Test]
-        public void TestEnabledWhenRoomOpen()
+        [TestCase(MultiplayerRoomState.Open)]
+        [TestCase(MultiplayerRoomState.WaitingForLoad)]
+        [TestCase(MultiplayerRoomState.Playing)]
+        public void TestEnabledWhenRoomOpenOrInGameplay(MultiplayerRoomState roomState)
         {
+            AddStep($"change room to {roomState}", () => Client.ChangeRoomState(roomState));
             assertSpectateButtonEnablement(true);
         }
 
@@ -137,12 +140,10 @@ namespace osu.Game.Tests.Visual.Multiplayer
             AddAssert("user is idle", () => Client.Room?.Users[0].State == MultiplayerUserState.Idle);
         }
 
-        [TestCase(MultiplayerRoomState.WaitingForLoad)]
-        [TestCase(MultiplayerRoomState.Playing)]
         [TestCase(MultiplayerRoomState.Closed)]
-        public void TestDisabledDuringGameplayOrClosed(MultiplayerRoomState roomState)
+        public void TestDisabledWhenClosed(MultiplayerRoomState roomState)
         {
-            AddStep($"change user to {roomState}", () => Client.ChangeRoomState(roomState));
+            AddStep($"change room to {roomState}", () => Client.ChangeRoomState(roomState));
             assertSpectateButtonEnablement(false);
         }
 
@@ -156,8 +157,8 @@ namespace osu.Game.Tests.Visual.Multiplayer
         [Test]
         public void TestReadyButtonEnabledWhenHostAndUsersReady()
         {
-            AddStep("add user", () => Client.AddUser(new User { Id = 55 }));
-            AddStep("set user ready", () => Client.ChangeUserState(55, MultiplayerUserState.Ready));
+            AddStep("add user", () => Client.AddUser(new User { Id = PLAYER_1_ID }));
+            AddStep("set user ready", () => Client.ChangeUserState(PLAYER_1_ID, MultiplayerUserState.Ready));
 
             addClickSpectateButtonStep();
             assertReadyButtonEnablement(true);
@@ -168,11 +169,11 @@ namespace osu.Game.Tests.Visual.Multiplayer
         {
             AddStep("add user and transfer host", () =>
             {
-                Client.AddUser(new User { Id = 55 });
-                Client.TransferHost(55);
+                Client.AddUser(new User { Id = PLAYER_1_ID });
+                Client.TransferHost(PLAYER_1_ID);
             });
 
-            AddStep("set user ready", () => Client.ChangeUserState(55, MultiplayerUserState.Ready));
+            AddStep("set user ready", () => Client.ChangeUserState(PLAYER_1_ID, MultiplayerUserState.Ready));
 
             addClickSpectateButtonStep();
             assertReadyButtonEnablement(false);

osu.Game.Tests/Visual/Online/TestSceneCurrentlyPlayingDisplay.cs

@@ -12,7 +12,7 @@ using osu.Framework.Testing;
 using osu.Game.Database;
 using osu.Game.Online.Spectator;
 using osu.Game.Overlays.Dashboard;
-using osu.Game.Tests.Visual.Gameplay;
+using osu.Game.Tests.Visual.Spectator;
 using osu.Game.Users;
 
 namespace osu.Game.Tests.Visual.Online
@@ -20,7 +20,7 @@ namespace osu.Game.Tests.Visual.Online
     public class TestSceneCurrentlyPlayingDisplay : OsuTestScene
     {
         [Cached(typeof(SpectatorStreamingClient))]
-        private TestSceneSpectator.TestSpectatorStreamingClient testSpectatorStreamingClient = new TestSceneSpectator.TestSpectatorStreamingClient();
+        private TestSpectatorStreamingClient testSpectatorStreamingClient = new TestSpectatorStreamingClient();
 
         private CurrentlyPlayingDisplay currentlyPlaying;
 

osu.Game/Extensions/TypeExtensions.cs

@@ -0,0 +1,31 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Linq;
+
+namespace osu.Game.Extensions
+{
+    internal static class TypeExtensions
+    {
+        /// <summary>
+        /// Returns <paramref name="type"/>'s <see cref="Type.AssemblyQualifiedName"/>
+        /// with the assembly version, culture and public key token values removed.
+        /// </summary>
+        /// <remarks>
+        /// This method is usually used in extensibility scenarios (i.e. for custom rulesets or skins)
+        /// when a version-agnostic identifier associated with a C# class - potentially originating from
+        /// an external assembly - is needed.
+        /// Leaving only the type and assembly names in such a scenario allows to preserve compatibility
+        /// across assembly versions.
+        /// </remarks>
+        internal static string GetInvariantInstantiationInfo(this Type type)
+        {
+            string assemblyQualifiedName = type.AssemblyQualifiedName;
+            if (assemblyQualifiedName == null)
+                throw new ArgumentException($"{type}'s assembly-qualified name is null. Ensure that it is a concrete type and not a generic type parameter.", nameof(type));
+
+            return string.Join(',', assemblyQualifiedName.Split(',').Take(2));
+        }
+    }
+}

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -96,9 +96,6 @@ namespace osu.Game.Online.Multiplayer
             if (!IsConnected.Value)
                 return Task.CompletedTask;
 
-            if (newState == MultiplayerUserState.Spectating)
-                return Task.CompletedTask; // Not supported yet.
-
             return connection.InvokeAsync(nameof(IMultiplayerServer.ChangeState), newState);
         }
 

osu.Game/Online/Spectator/SpectatorStreamingClient.cs

@@ -142,6 +142,10 @@ namespace osu.Game.Online.Spectator
                 if (!playingUsers.Contains(userId))
                     playingUsers.Add(userId);
 
+                // UserBeganPlaying() is called by the server regardless of whether the local user is watching the remote user, and is called a further time when the remote user is watched.
+                // This may be a temporary thing (see: https://github.com/ppy/osu-server-spectator/blob/2273778e02cfdb4a9c6a934f2a46a8459cb5d29c/osu.Server.Spectator/Hubs/SpectatorHub.cs#L28-L29).
+                // We don't want the user states to update unless the player is being watched, otherwise calling BindUserBeganPlaying() can lead to double invocations.
+                if (watchingUsers.Contains(userId))
                     playingUserStates[userId] = state;
             }
 
@@ -230,7 +234,7 @@ namespace osu.Game.Online.Spectator
             connection.SendAsync(nameof(ISpectatorServer.StartWatchingUser), userId);
         }
 
-        public void StopWatchingUser(int userId)
+        public virtual void StopWatchingUser(int userId)
         {
             lock (userLock)
             {

osu.Game/Rulesets/Ruleset.cs

@@ -26,6 +26,7 @@ using JetBrains.Annotations;
 using osu.Framework.Extensions;
 using osu.Framework.Extensions.EnumExtensions;
 using osu.Framework.Testing;
+using osu.Game.Extensions;
 using osu.Game.Rulesets.Filter;
 using osu.Game.Screens.Ranking.Statistics;
 
@@ -135,7 +136,7 @@ namespace osu.Game.Rulesets
                 Name = Description,
                 ShortName = ShortName,
                 ID = (this as ILegacyRuleset)?.LegacyID,
-                InstantiationInfo = GetType().AssemblyQualifiedName,
+                InstantiationInfo = GetType().GetInvariantInstantiationInfo(),
                 Available = true,
             };
         }

osu.Game/Rulesets/RulesetInfo.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Diagnostics.CodeAnalysis;
-using System.Linq;
 using Newtonsoft.Json;
 using osu.Framework.Testing;
 
@@ -18,20 +17,7 @@ namespace osu.Game.Rulesets
 
         public string ShortName { get; set; }
 
-        private string instantiationInfo;
-
-        public string InstantiationInfo
-        {
-            get => instantiationInfo;
-            set => instantiationInfo = abbreviateInstantiationInfo(value);
-        }
-
-        private string abbreviateInstantiationInfo(string value)
-        {
-            // exclude version onwards, matching only on namespace and type.
-            // this is mainly to allow for new versions of already loaded rulesets to "upgrade" from old.
-            return string.Join(',', value.Split(',').Take(2));
-        }
+        public string InstantiationInfo { get; set; }
 
         [JsonIgnore]
         public bool Available { get; set; }

osu.Game/Screens/OnlinePlay/Match/RoomSubScreen.cs

@@ -17,7 +17,6 @@ using osu.Game.Online.Rooms;
 using osu.Game.Overlays;
 using osu.Game.Overlays.Mods;
 using osu.Game.Rulesets.Mods;
-using osu.Game.Screens.Play;
 
 namespace osu.Game.Screens.OnlinePlay.Match
 {
@@ -148,12 +147,18 @@ namespace osu.Game.Screens.OnlinePlay.Match
             return base.OnExiting(next);
         }
 
-        protected void StartPlay(Func<Player> player)
+        protected void StartPlay()
         {
             sampleStart?.Play();
-            ParentScreen?.Push(new PlayerLoader(player));
+            ParentScreen?.Push(CreateGameplayScreen());
         }
 
+        /// <summary>
+        /// Creates the gameplay screen to be entered.
+        /// </summary>
+        /// <returns>The screen to enter.</returns>
+        protected abstract Screen CreateGameplayScreen();
+
         private void selectedItemChanged()
         {
             updateWorkingBeatmap();

osu.Game/Screens/OnlinePlay/Multiplayer/Match/MultiplayerSpectateButton.cs

@@ -81,7 +81,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Match
                     break;
             }
 
-            button.Enabled.Value = Client.Room?.State == MultiplayerRoomState.Open && !operationInProgress.Value;
+            button.Enabled.Value = Client.Room?.State != MultiplayerRoomState.Closed && !operationInProgress.Value;
         }
 
         private class ButtonWithTrianglesExposed : TriangleButton

osu.Game/Screens/OnlinePlay/Multiplayer/Multiplayer.cs

@@ -21,7 +21,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
         {
             base.OnResuming(last);
 
-            if (client.Room != null)
+            if (client.Room != null && client.LocalUser?.State != MultiplayerUserState.Spectating)
                 client.ChangeState(MultiplayerUserState.Idle);
         }
 

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerMatchSubScreen.cs

@@ -14,6 +14,7 @@ using osu.Framework.Screens;
 using osu.Framework.Threading;
 using osu.Game.Beatmaps;
 using osu.Game.Configuration;
+using osu.Game.Online;
 using osu.Game.Online.Multiplayer;
 using osu.Game.Online.Rooms;
 using osu.Game.Overlays;
@@ -25,6 +26,7 @@ using osu.Game.Screens.OnlinePlay.Match;
 using osu.Game.Screens.OnlinePlay.Match.Components;
 using osu.Game.Screens.OnlinePlay.Multiplayer.Match;
 using osu.Game.Screens.OnlinePlay.Multiplayer.Participants;
+using osu.Game.Screens.OnlinePlay.Multiplayer.Spectate;
 using osu.Game.Screens.Play.HUD;
 using osu.Game.Users;
 using osuTK;
@@ -353,11 +355,18 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
 
             client.ChangeBeatmapAvailability(availability.NewValue);
 
+            if (availability.NewValue.State != DownloadState.LocallyAvailable)
+            {
                 // while this flow is handled server-side, this covers the edge case of the local user being in a ready state and then deleting the current beatmap.
-            if (availability.NewValue != Online.Rooms.BeatmapAvailability.LocallyAvailable()
-                && client.LocalUser?.State == MultiplayerUserState.Ready)
+                if (client.LocalUser?.State == MultiplayerUserState.Ready)
                     client.ChangeState(MultiplayerUserState.Idle);
             }
+            else
+            {
+                if (client.LocalUser?.State == MultiplayerUserState.Spectating && (client.Room?.State == MultiplayerRoomState.WaitingForLoad || client.Room?.State == MultiplayerRoomState.Playing))
+                    onLoadRequested();
+            }
+        }
 
         private void onReadyClick()
         {
@@ -407,22 +416,46 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
 
         private void onRoomUpdated()
         {
-            // user mods may have changed.
             Scheduler.AddOnce(UpdateMods);
         }
 
         private void onLoadRequested()
         {
-            Debug.Assert(client.Room != null);
+            if (BeatmapAvailability.Value.State != DownloadState.LocallyAvailable)
+                return;
 
-            int[] userIds = client.CurrentMatchPlayingUserIds.ToArray();
+            // In the case of spectating, IMultiplayerClient.LoadRequested can be fired while the game is still spectating a previous session.
+            // For now, we want to game to switch to the new game so need to request exiting from the play screen.
+            if (!ParentScreen.IsCurrentScreen())
+            {
+                ParentScreen.MakeCurrent();
 
-            StartPlay(() => new MultiplayerPlayer(SelectedItem.Value, userIds));
+                Schedule(onLoadRequested);
+                return;
+            }
+
+            StartPlay();
 
             readyClickOperation?.Dispose();
             readyClickOperation = null;
         }
 
+        protected override Screen CreateGameplayScreen()
+        {
+            Debug.Assert(client.LocalUser != null);
+
+            int[] userIds = client.CurrentMatchPlayingUserIds.ToArray();
+
+            switch (client.LocalUser.State)
+            {
+                case MultiplayerUserState.Spectating:
+                    return new MultiSpectatorScreen(userIds);
+
+                default:
+                    return new MultiplayerPlayer(SelectedItem.Value, userIds);
+            }
+        }
+
         protected override void Dispose(bool isDisposing)
         {
             base.Dispose(isDisposing);

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/CatchUpSpectatorPlayerClock.cs

@@ -0,0 +1,84 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+#nullable enable
+
+using System;
+using osu.Framework.Bindables;
+using osu.Framework.Timing;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// A <see cref="ISpectatorPlayerClock"/> which catches up using rate adjustment.
+    /// </summary>
+    public class CatchUpSpectatorPlayerClock : ISpectatorPlayerClock
+    {
+        /// <summary>
+        /// The catch up rate.
+        /// </summary>
+        public const double CATCHUP_RATE = 2;
+
+        /// <summary>
+        /// The source clock.
+        /// </summary>
+        public IFrameBasedClock? Source { get; set; }
+
+        public double CurrentTime { get; private set; }
+
+        public bool IsRunning { get; private set; }
+
+        public void Reset() => CurrentTime = 0;
+
+        public void Start() => IsRunning = true;
+
+        public void Stop() => IsRunning = false;
+
+        public bool Seek(double position) => true;
+
+        public void ResetSpeedAdjustments()
+        {
+        }
+
+        public double Rate => IsCatchingUp ? CATCHUP_RATE : 1;
+
+        double IAdjustableClock.Rate
+        {
+            get => Rate;
+            set => throw new NotSupportedException();
+        }
+
+        double IClock.Rate => Rate;
+
+        public void ProcessFrame()
+        {
+            ElapsedFrameTime = 0;
+            FramesPerSecond = 0;
+
+            if (Source == null)
+                return;
+
+            Source.ProcessFrame();
+
+            if (IsRunning)
+            {
+                double elapsedSource = Source.ElapsedFrameTime;
+                double elapsed = elapsedSource * Rate;
+
+                CurrentTime += elapsed;
+                ElapsedFrameTime = elapsed;
+                FramesPerSecond = Source.FramesPerSecond;
+            }
+        }
+
+        public double ElapsedFrameTime { get; private set; }
+
+        public double FramesPerSecond { get; private set; }
+
+        public FrameTimeInfo TimeInfo => new FrameTimeInfo { Elapsed = ElapsedFrameTime, Current = CurrentTime };
+
+        public Bindable<bool> WaitingOnFrames { get; } = new Bindable<bool>(true);
+
+        public bool IsCatchingUp { get; set; }
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/CatchUpSyncManager.cs

@@ -0,0 +1,153 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+using System.Linq;
+using osu.Framework.Graphics;
+using osu.Framework.Timing;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// A <see cref="ISyncManager"/> which synchronises de-synced player clocks through catchup.
+    /// </summary>
+    public class CatchUpSyncManager : Component, ISyncManager
+    {
+        /// <summary>
+        /// The offset from the master clock to which player clocks should remain within to be considered in-sync.
+        /// </summary>
+        public const double SYNC_TARGET = 16;
+
+        /// <summary>
+        /// The offset from the master clock at which player clocks begin resynchronising.
+        /// </summary>
+        public const double MAX_SYNC_OFFSET = 50;
+
+        /// <summary>
+        /// The maximum delay to start gameplay, if any (but not all) player clocks are ready.
+        /// </summary>
+        public const double MAXIMUM_START_DELAY = 15000;
+
+        /// <summary>
+        /// The master clock which is used to control the timing of all player clocks clocks.
+        /// </summary>
+        public IAdjustableClock MasterClock { get; }
+
+        /// <summary>
+        /// The player clocks.
+        /// </summary>
+        private readonly List<ISpectatorPlayerClock> playerClocks = new List<ISpectatorPlayerClock>();
+
+        private bool hasStarted;
+        private double? firstStartAttemptTime;
+
+        public CatchUpSyncManager(IAdjustableClock master)
+        {
+            MasterClock = master;
+        }
+
+        public void AddPlayerClock(ISpectatorPlayerClock clock) => playerClocks.Add(clock);
+
+        public void RemovePlayerClock(ISpectatorPlayerClock clock) => playerClocks.Remove(clock);
+
+        protected override void Update()
+        {
+            base.Update();
+
+            if (!attemptStart())
+            {
+                // Ensure all player clocks are stopped until the start succeeds.
+                foreach (var clock in playerClocks)
+                    clock.Stop();
+                return;
+            }
+
+            updateCatchup();
+            updateMasterClock();
+        }
+
+        /// <summary>
+        /// Attempts to start playback. Waits for all player clocks to have available frames for up to <see cref="MAXIMUM_START_DELAY"/> milliseconds.
+        /// </summary>
+        /// <returns>Whether playback was started and syncing should occur.</returns>
+        private bool attemptStart()
+        {
+            if (hasStarted)
+                return true;
+
+            if (playerClocks.Count == 0)
+                return false;
+
+            int readyCount = playerClocks.Count(s => !s.WaitingOnFrames.Value);
+
+            if (readyCount == playerClocks.Count)
+                return hasStarted = true;
+
+            if (readyCount > 0)
+            {
+                firstStartAttemptTime ??= Time.Current;
+
+                if (Time.Current - firstStartAttemptTime > MAXIMUM_START_DELAY)
+                    return hasStarted = true;
+            }
+
+            return false;
+        }
+
+        /// <summary>
+        /// Updates the catchup states of all player clocks clocks.
+        /// </summary>
+        private void updateCatchup()
+        {
+            for (int i = 0; i < playerClocks.Count; i++)
+            {
+                var clock = playerClocks[i];
+
+                // How far this player's clock is out of sync, compared to the master clock.
+                // A negative value means the player is running fast (ahead); a positive value means the player is running behind (catching up).
+                double timeDelta = MasterClock.CurrentTime - clock.CurrentTime;
+
+                // Check that the player clock isn't too far ahead.
+                // This is a quiet case in which the catchup is done by the master clock, so IsCatchingUp is not set on the player clock.
+                if (timeDelta < -SYNC_TARGET)
+                {
+                    clock.Stop();
+                    continue;
+                }
+
+                // Make sure the player clock is running if it can.
+                if (!clock.WaitingOnFrames.Value)
+                    clock.Start();
+
+                if (clock.IsCatchingUp)
+                {
+                    // Stop the player clock from catching up if it's within the sync target.
+                    if (timeDelta <= SYNC_TARGET)
+                        clock.IsCatchingUp = false;
+                }
+                else
+                {
+                    // Make the player clock start catching up if it's exceeded the maximum allowable sync offset.
+                    if (timeDelta > MAX_SYNC_OFFSET)
+                        clock.IsCatchingUp = true;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Updates the master clock's running state.
+        /// </summary>
+        private void updateMasterClock()
+        {
+            bool anyInSync = playerClocks.Any(s => !s.IsCatchingUp);
+
+            if (MasterClock.IsRunning != anyInSync)
+            {
+                if (anyInSync)
+                    MasterClock.Start();
+                else
+                    MasterClock.Stop();
+            }
+        }
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/ISpectatorPlayerClock.cs

@@ -0,0 +1,29 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Bindables;
+using osu.Framework.Timing;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// A clock which is used by <see cref="MultiSpectatorPlayer"/>s and managed by an <see cref="ISyncManager"/>.
+    /// </summary>
+    public interface ISpectatorPlayerClock : IFrameBasedClock, IAdjustableClock
+    {
+        /// <summary>
+        /// Whether this clock is waiting on frames to continue playback.
+        /// </summary>
+        Bindable<bool> WaitingOnFrames { get; }
+
+        /// <summary>
+        /// Whether this clock is resynchronising to the master clock.
+        /// </summary>
+        bool IsCatchingUp { get; set; }
+
+        /// <summary>
+        /// The source clock
+        /// </summary>
+        IFrameBasedClock Source { set; }
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/ISyncManager.cs

@@ -0,0 +1,30 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Timing;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// Manages the synchronisation between one or more <see cref="ISpectatorPlayerClock"/>s in relation to a master clock.
+    /// </summary>
+    public interface ISyncManager
+    {
+        /// <summary>
+        /// The master clock which player clocks should synchronise to.
+        /// </summary>
+        IAdjustableClock MasterClock { get; }
+
+        /// <summary>
+        /// Adds an <see cref="ISpectatorPlayerClock"/> to manage.
+        /// </summary>
+        /// <param name="clock">The <see cref="ISpectatorPlayerClock"/> to add.</param>
+        void AddPlayerClock(ISpectatorPlayerClock clock);
+
+        /// <summary>
+        /// Removes an <see cref="ISpectatorPlayerClock"/>, stopping it from being managed by this <see cref="ISyncManager"/>.
+        /// </summary>
+        /// <param name="clock">The <see cref="ISpectatorPlayerClock"/> to remove.</param>
+        void RemovePlayerClock(ISpectatorPlayerClock clock);
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/MultiSpectatorLeaderboard.cs

@@ -9,9 +9,9 @@ using osu.Game.Screens.Play.HUD;
 
 namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
 {
-    public class MultiplayerSpectatorLeaderboard : MultiplayerGameplayLeaderboard
+    public class MultiSpectatorLeaderboard : MultiplayerGameplayLeaderboard
     {
-        public MultiplayerSpectatorLeaderboard(ScoreProcessor scoreProcessor, int[] userIds)
+        public MultiSpectatorLeaderboard([NotNull] ScoreProcessor scoreProcessor, int[] userIds)
             : base(scoreProcessor, userIds)
         {
         }

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/MultiSpectatorPlayer.cs

@@ -0,0 +1,73 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using JetBrains.Annotations;
+using osu.Framework.Allocation;
+using osu.Framework.Bindables;
+using osu.Framework.Timing;
+using osu.Game.Beatmaps;
+using osu.Game.Scoring;
+using osu.Game.Screens.Play;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// A single spectated player within a <see cref="MultiSpectatorScreen"/>.
+    /// </summary>
+    public class MultiSpectatorPlayer : SpectatorPlayer
+    {
+        private readonly Bindable<bool> waitingOnFrames = new Bindable<bool>(true);
+        private readonly Score score;
+        private readonly ISpectatorPlayerClock spectatorPlayerClock;
+
+        /// <summary>
+        /// Creates a new <see cref="MultiSpectatorPlayer"/>.
+        /// </summary>
+        /// <param name="score">The score containing the player's replay.</param>
+        /// <param name="spectatorPlayerClock">The clock controlling the gameplay running state.</param>
+        public MultiSpectatorPlayer([NotNull] Score score, [NotNull] ISpectatorPlayerClock spectatorPlayerClock)
+            : base(score)
+        {
+            this.score = score;
+            this.spectatorPlayerClock = spectatorPlayerClock;
+        }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            spectatorPlayerClock.WaitingOnFrames.BindTo(waitingOnFrames);
+        }
+
+        protected override void UpdateAfterChildren()
+        {
+            base.UpdateAfterChildren();
+
+            // This is required because the frame stable clock is set to WaitingOnFrames = false for one frame.
+            waitingOnFrames.Value = DrawableRuleset.FrameStableClock.WaitingOnFrames.Value || score.Replay.Frames.Count == 0;
+        }
+
+        protected override GameplayClockContainer CreateGameplayClockContainer(WorkingBeatmap beatmap, double gameplayStart)
+            => new SpectatorGameplayClockContainer(spectatorPlayerClock);
+
+        private class SpectatorGameplayClockContainer : GameplayClockContainer
+        {
+            public SpectatorGameplayClockContainer([NotNull] IClock sourceClock)
+                : base(sourceClock)
+            {
+            }
+
+            protected override void Update()
+            {
+                // The player clock's running state is controlled externally, but the local pausing state needs to be updated to stop gameplay.
+                if (SourceClock.IsRunning)
+                    Start();
+                else
+                    Stop();
+
+                base.Update();
+            }
+
+            protected override GameplayClock CreateGameplayClock(IFrameBasedClock source) => new GameplayClock(source);
+        }
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/MultiSpectatorPlayerLoader.cs

@@ -0,0 +1,30 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using JetBrains.Annotations;
+using osu.Game.Scoring;
+using osu.Game.Screens.Menu;
+using osu.Game.Screens.Play;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// Used to load a single <see cref="MultiSpectatorPlayer"/> in a <see cref="MultiSpectatorScreen"/>.
+    /// </summary>
+    public class MultiSpectatorPlayerLoader : SpectatorPlayerLoader
+    {
+        public MultiSpectatorPlayerLoader([NotNull] Score score, [NotNull] Func<MultiSpectatorPlayer> createPlayer)
+            : base(score, createPlayer)
+        {
+        }
+
+        protected override void LogoArriving(OsuLogo logo, bool resuming)
+        {
+        }
+
+        protected override void LogoExiting(OsuLogo logo)
+        {
+        }
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/MultiSpectatorScreen.cs

@@ -0,0 +1,156 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Linq;
+using JetBrains.Annotations;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Game.Online.Multiplayer;
+using osu.Game.Online.Spectator;
+using osu.Game.Screens.Play;
+using osu.Game.Screens.Spectate;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// A <see cref="SpectatorScreen"/> that spectates multiple users in a match.
+    /// </summary>
+    public class MultiSpectatorScreen : SpectatorScreen
+    {
+        // Isolates beatmap/ruleset to this screen.
+        public override bool DisallowExternalBeatmapRulesetChanges => true;
+
+        /// <summary>
+        /// Whether all spectating players have finished loading.
+        /// </summary>
+        public bool AllPlayersLoaded => instances.All(p => p?.PlayerLoaded == true);
+
+        [Resolved]
+        private SpectatorStreamingClient spectatorClient { get; set; }
+
+        [Resolved]
+        private StatefulMultiplayerClient multiplayerClient { get; set; }
+
+        private readonly PlayerArea[] instances;
+        private MasterGameplayClockContainer masterClockContainer;
+        private ISyncManager syncManager;
+        private PlayerGrid grid;
+        private MultiSpectatorLeaderboard leaderboard;
+        private PlayerArea currentAudioSource;
+
+        /// <summary>
+        /// Creates a new <see cref="MultiSpectatorScreen"/>.
+        /// </summary>
+        /// <param name="userIds">The players to spectate.</param>
+        public MultiSpectatorScreen(int[] userIds)
+            : base(userIds.Take(PlayerGrid.MAX_PLAYERS).ToArray())
+        {
+            instances = new PlayerArea[UserIds.Count];
+        }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            Container leaderboardContainer;
+            masterClockContainer = new MasterGameplayClockContainer(Beatmap.Value, 0);
+
+            InternalChildren = new[]
+            {
+                (Drawable)(syncManager = new CatchUpSyncManager(masterClockContainer)),
+                masterClockContainer.WithChild(new GridContainer
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    ColumnDimensions = new[]
+                    {
+                        new Dimension(GridSizeMode.AutoSize)
+                    },
+                    Content = new[]
+                    {
+                        new Drawable[]
+                        {
+                            leaderboardContainer = new Container
+                            {
+                                RelativeSizeAxes = Axes.Y,
+                                AutoSizeAxes = Axes.X
+                            },
+                            grid = new PlayerGrid { RelativeSizeAxes = Axes.Both }
+                        }
+                    }
+                })
+            };
+
+            for (int i = 0; i < UserIds.Count; i++)
+            {
+                grid.Add(instances[i] = new PlayerArea(UserIds[i], masterClockContainer.GameplayClock));
+                syncManager.AddPlayerClock(instances[i].GameplayClock);
+            }
+
+            // Todo: This is not quite correct - it should be per-user to adjust for other mod combinations.
+            var playableBeatmap = Beatmap.Value.GetPlayableBeatmap(Ruleset.Value);
+            var scoreProcessor = Ruleset.Value.CreateInstance().CreateScoreProcessor();
+            scoreProcessor.ApplyBeatmap(playableBeatmap);
+
+            LoadComponentAsync(leaderboard = new MultiSpectatorLeaderboard(scoreProcessor, UserIds.ToArray())
+            {
+                Expanded = { Value = true },
+                Anchor = Anchor.CentreLeft,
+                Origin = Anchor.CentreLeft,
+            }, leaderboardContainer.Add);
+        }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+
+            masterClockContainer.Stop();
+            masterClockContainer.Reset();
+        }
+
+        protected override void Update()
+        {
+            base.Update();
+
+            if (!isCandidateAudioSource(currentAudioSource?.GameplayClock))
+            {
+                currentAudioSource = instances.Where(i => isCandidateAudioSource(i.GameplayClock))
+                                              .OrderBy(i => Math.Abs(i.GameplayClock.CurrentTime - syncManager.MasterClock.CurrentTime))
+                                              .FirstOrDefault();
+
+                foreach (var instance in instances)
+                    instance.Mute = instance != currentAudioSource;
+            }
+        }
+
+        private bool isCandidateAudioSource([CanBeNull] ISpectatorPlayerClock clock)
+            => clock?.IsRunning == true && !clock.IsCatchingUp && !clock.WaitingOnFrames.Value;
+
+        protected override void OnUserStateChanged(int userId, SpectatorState spectatorState)
+        {
+        }
+
+        protected override void StartGameplay(int userId, GameplayState gameplayState)
+        {
+            var instance = instances.Single(i => i.UserId == userId);
+
+            instance.LoadScore(gameplayState.Score);
+
+            syncManager.AddPlayerClock(instance.GameplayClock);
+            leaderboard.AddClock(instance.UserId, instance.GameplayClock);
+        }
+
+        protected override void EndGameplay(int userId)
+        {
+            RemoveUser(userId);
+            leaderboard.RemoveClock(userId);
+        }
+
+        public override bool OnBackButton()
+        {
+            // On a manual exit, set the player state back to idle.
+            multiplayerClient.ChangeState(MultiplayerUserState.Idle);
+            return base.OnBackButton();
+        }
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/PlayerArea.cs

@@ -0,0 +1,144 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using JetBrains.Annotations;
+using osu.Framework.Allocation;
+using osu.Framework.Audio;
+using osu.Framework.Bindables;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Timing;
+using osu.Game.Beatmaps;
+using osu.Game.Graphics.UserInterface;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Scoring;
+using osu.Game.Screens.Play;
+
+namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
+{
+    /// <summary>
+    /// Provides an area for and manages the hierarchy of a spectated player within a <see cref="MultiSpectatorScreen"/>.
+    /// </summary>
+    public class PlayerArea : CompositeDrawable
+    {
+        /// <summary>
+        /// Whether a <see cref="Player"/> is loaded in the area.
+        /// </summary>
+        public bool PlayerLoaded => stack?.CurrentScreen is Player;
+
+        /// <summary>
+        /// The user id this <see cref="PlayerArea"/> corresponds to.
+        /// </summary>
+        public readonly int UserId;
+
+        /// <summary>
+        /// The <see cref="ISpectatorPlayerClock"/> used to control the gameplay running state of a loaded <see cref="Player"/>.
+        /// </summary>
+        [NotNull]
+        public readonly ISpectatorPlayerClock GameplayClock = new CatchUpSpectatorPlayerClock();
+
+        /// <summary>
+        /// The currently-loaded score.
+        /// </summary>
+        [CanBeNull]
+        public Score Score { get; private set; }
+
+        [Resolved]
+        private BeatmapManager beatmapManager { get; set; }
+
+        private readonly BindableDouble volumeAdjustment = new BindableDouble();
+        private readonly Container gameplayContent;
+        private readonly LoadingLayer loadingLayer;
+        private OsuScreenStack stack;
+
+        public PlayerArea(int userId, IFrameBasedClock masterClock)
+        {
+            UserId = userId;
+
+            RelativeSizeAxes = Axes.Both;
+            Masking = true;
+
+            AudioContainer audioContainer;
+            InternalChildren = new Drawable[]
+            {
+                audioContainer = new AudioContainer
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Child = gameplayContent = new DrawSizePreservingFillContainer { RelativeSizeAxes = Axes.Both },
+                },
+                loadingLayer = new LoadingLayer(true) { State = { Value = Visibility.Visible } }
+            };
+
+            audioContainer.AddAdjustment(AdjustableProperty.Volume, volumeAdjustment);
+
+            GameplayClock.Source = masterClock;
+        }
+
+        public void LoadScore([NotNull] Score score)
+        {
+            if (Score != null)
+                throw new InvalidOperationException($"Cannot load a new score on a {nameof(PlayerArea)} that has an existing score.");
+
+            Score = score;
+
+            gameplayContent.Child = new PlayerIsolationContainer(beatmapManager.GetWorkingBeatmap(Score.ScoreInfo.Beatmap), Score.ScoreInfo.Ruleset, Score.ScoreInfo.Mods)
+            {
+                RelativeSizeAxes = Axes.Both,
+                Child = stack = new OsuScreenStack()
+            };
+
+            stack.Push(new MultiSpectatorPlayerLoader(Score, () => new MultiSpectatorPlayer(Score, GameplayClock)));
+            loadingLayer.Hide();
+        }
+
+        private bool mute = true;
+
+        public bool Mute
+        {
+            get => mute;
+            set
+            {
+                mute = value;
+                volumeAdjustment.Value = value ? 0 : 1;
+            }
+        }
+
+        // Player interferes with global input, so disable input for now.
+        public override bool PropagatePositionalInputSubTree => false;
+        public override bool PropagateNonPositionalInputSubTree => false;
+
+        /// <summary>
+        /// Isolates each player instance from the game-wide ruleset/beatmap/mods (to allow for different players having different settings).
+        /// </summary>
+        private class PlayerIsolationContainer : Container
+        {
+            [Cached]
+            private readonly Bindable<RulesetInfo> ruleset = new Bindable<RulesetInfo>();
+
+            [Cached]
+            private readonly Bindable<WorkingBeatmap> beatmap = new Bindable<WorkingBeatmap>();
+
+            [Cached]
+            private readonly Bindable<IReadOnlyList<Mod>> mods = new Bindable<IReadOnlyList<Mod>>();
+
+            public PlayerIsolationContainer(WorkingBeatmap beatmap, RulesetInfo ruleset, IReadOnlyList<Mod> mods)
+            {
+                this.beatmap.Value = beatmap;
+                this.ruleset.Value = ruleset;
+                this.mods.Value = mods;
+            }
+
+            protected override IReadOnlyDependencyContainer CreateChildDependencies(IReadOnlyDependencyContainer parent)
+            {
+                var dependencies = new DependencyContainer(base.CreateChildDependencies(parent));
+                dependencies.CacheAs(ruleset.BeginLease(false));
+                dependencies.CacheAs(beatmap.BeginLease(false));
+                dependencies.CacheAs(mods.BeginLease(false));
+                return dependencies;
+            }
+        }
+    }
+}

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/PlayerGrid.cs

@@ -15,6 +15,12 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
     /// </summary>
     public partial class PlayerGrid : CompositeDrawable
     {
+        /// <summary>
+        /// A temporary limitation on the number of players, because only layouts up to 16 players are supported for a single screen.
+        /// Todo: Can be removed in the future with scrolling support + performance improvements.
+        /// </summary>
+        public const int MAX_PLAYERS = 16;
+
         private const float player_spacing = 5;
 
         /// <summary>
@@ -58,11 +64,11 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
         /// Adds a new cell with content to this grid.
         /// </summary>
         /// <param name="content">The content the cell should contain.</param>
-        /// <exception cref="InvalidOperationException">If more than 16 cells are added.</exception>
+        /// <exception cref="InvalidOperationException">If more than <see cref="MAX_PLAYERS"/> cells are added.</exception>
         public void Add(Drawable content)
         {
-            if (cellContainer.Count == 16)
-                throw new InvalidOperationException("Only 16 cells are supported.");
+            if (cellContainer.Count == MAX_PLAYERS)
+                throw new InvalidOperationException($"Only {MAX_PLAYERS} cells are supported.");
 
             int index = cellContainer.Count;
 

osu.Game/Screens/OnlinePlay/Playlists/PlaylistsRoomSubScreen.cs

@@ -218,10 +218,7 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
                         },
                         new Drawable[]
                         {
-                            new Footer
-                            {
-                                OnStart = onStart,
-                            }
+                            new Footer { OnStart = StartPlay }
                         }
                     },
                     RowDimensions = new[]
@@ -274,9 +271,9 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
             }, true);
         }
 
-        private void onStart() => StartPlay(() => new PlaylistsPlayer(SelectedItem.Value)
+        protected override Screen CreateGameplayScreen() => new PlaylistsPlayer(SelectedItem.Value)
         {
             Exited = () => leaderboard.RefreshScores()
-        });
+        };
     }
 }

osu.Game/Screens/Play/GameplayClockContainer.cs

@@ -1,6 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using System;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
@@ -12,7 +13,7 @@ namespace osu.Game.Screens.Play
     /// <summary>
     /// Encapsulates gameplay timing logic and provides a <see cref="GameplayClock"/> via DI for gameplay components to use.
     /// </summary>
-    public abstract class GameplayClockContainer : Container
+    public abstract class GameplayClockContainer : Container, IAdjustableClock
     {
         /// <summary>
         /// The final clock which is exposed to gameplay components.
@@ -157,5 +158,33 @@ namespace osu.Game.Screens.Play
         /// <param name="source">The <see cref="IFrameBasedClock"/> providing the source time.</param>
         /// <returns>The final <see cref="GameplayClock"/>.</returns>
         protected abstract GameplayClock CreateGameplayClock(IFrameBasedClock source);
+
+        #region IAdjustableClock
+
+        bool IAdjustableClock.Seek(double position)
+        {
+            Seek(position);
+            return true;
+        }
+
+        void IAdjustableClock.Reset() => Reset();
+
+        public void ResetSpeedAdjustments()
+        {
+        }
+
+        double IAdjustableClock.Rate
+        {
+            get => GameplayClock.Rate;
+            set => throw new NotSupportedException();
+        }
+
+        double IClock.Rate => GameplayClock.Rate;
+
+        public double CurrentTime => GameplayClock.CurrentTime;
+
+        public bool IsRunning => GameplayClock.IsRunning;
+
+        #endregion
     }
 }

osu.Game/Screens/Play/SpectatorPlayerLoader.cs

@@ -12,7 +12,12 @@ namespace osu.Game.Screens.Play
         public readonly ScoreInfo Score;
 
         public SpectatorPlayerLoader(Score score)
-            : base(() => new SpectatorPlayer(score))
+            : this(score, () => new SpectatorPlayer(score))
+        {
+        }
+
+        public SpectatorPlayerLoader(Score score, Func<Player> createPlayer)
+            : base(createPlayer)
         {
             if (score.Replay == null)
                 throw new ArgumentException($"{nameof(score)} must have a non-null {nameof(score.Replay)}.", nameof(score));

osu.Game/Screens/Spectate/SpectatorScreen.cs

@@ -26,7 +26,9 @@ namespace osu.Game.Screens.Spectate
     /// </summary>
     public abstract class SpectatorScreen : OsuScreen
     {
-        private readonly int[] userIds;
+        protected IReadOnlyList<int> UserIds => userIds;
+
+        private readonly List<int> userIds = new List<int>();
 
         [Resolved]
         private BeatmapManager beatmaps { get; set; }
@@ -54,7 +56,7 @@ namespace osu.Game.Screens.Spectate
         /// <param name="userIds">The users to spectate.</param>
         protected SpectatorScreen(params int[] userIds)
         {
-            this.userIds = userIds;
+            this.userIds.AddRange(userIds);
         }
 
         protected override void LoadComplete()
@@ -80,20 +82,18 @@ namespace osu.Game.Screens.Spectate
 
         private Task populateAllUsers()
         {
-            var userLookupTasks = new Task[userIds.Length];
+            var userLookupTasks = new List<Task>();
 
-            for (int i = 0; i < userIds.Length; i++)
+            foreach (var u in userIds)
             {
-                var userId = userIds[i];
-
-                userLookupTasks[i] = userLookupCache.GetUserAsync(userId).ContinueWith(task =>
+                userLookupTasks.Add(userLookupCache.GetUserAsync(u).ContinueWith(task =>
                 {
                     if (!task.IsCompletedSuccessfully)
                         return;
 
                     lock (stateLock)
-                        userMap[userId] = task.Result;
-                });
+                        userMap[u] = task.Result;
+                }));
             }
 
             return Task.WhenAll(userLookupTasks);
@@ -239,6 +239,23 @@ namespace osu.Game.Screens.Spectate
         /// <param name="userId">The user to end gameplay for.</param>
         protected abstract void EndGameplay(int userId);
 
+        /// <summary>
+        /// Stops spectating a user.
+        /// </summary>
+        /// <param name="userId">The user to stop spectating.</param>
+        protected void RemoveUser(int userId)
+        {
+            lock (stateLock)
+            {
+                userFinishedPlaying(userId, null);
+
+                userIds.Remove(userId);
+                userMap.Remove(userId);
+
+                spectatorClient.StopWatchingUser(userId);
+            }
+        }
+
         protected override void Dispose(bool isDisposing)
         {
             base.Dispose(isDisposing);

osu.Game/Skinning/DefaultLegacySkin.cs

@@ -1,7 +1,9 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using JetBrains.Annotations;
 using osu.Framework.IO.Stores;
+using osu.Game.Extensions;
 using osu.Game.IO;
 using osuTK.Graphics;
 
@@ -14,6 +16,7 @@ namespace osu.Game.Skinning
         {
         }
 
+        [UsedImplicitly(ImplicitUseKindFlags.InstantiatedWithFixedConstructorSignature)]
         public DefaultLegacySkin(SkinInfo skin, IResourceStore<byte[]> storage, IStorageResourceProvider resources)
             : base(skin, storage, resources, string.Empty)
         {
@@ -33,7 +36,7 @@ namespace osu.Game.Skinning
             ID = SkinInfo.CLASSIC_SKIN, // this is temporary until database storage is decided upon.
             Name = "osu!classic",
             Creator = "team osu!",
-            InstantiationInfo = typeof(DefaultLegacySkin).AssemblyQualifiedName,
+            InstantiationInfo = typeof(DefaultLegacySkin).GetInvariantInstantiationInfo()
         };
     }
 }

osu.Game/Skinning/DefaultSkin.cs

@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Linq;
+using JetBrains.Annotations;
 using osu.Framework.Audio.Sample;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
@@ -24,6 +25,7 @@ namespace osu.Game.Skinning
         {
         }
 
+        [UsedImplicitly(ImplicitUseKindFlags.InstantiatedWithFixedConstructorSignature)]
         public DefaultSkin(SkinInfo skin, IStorageResourceProvider resources)
             : base(skin, resources)
         {

osu.Game/Skinning/LegacySkin.cs

@@ -52,6 +52,7 @@ namespace osu.Game.Skinning
 
         private readonly Dictionary<int, LegacyManiaSkinConfiguration> maniaConfigurations = new Dictionary<int, LegacyManiaSkinConfiguration>();
 
+        [UsedImplicitly(ImplicitUseKindFlags.InstantiatedWithFixedConstructorSignature)]
         public LegacySkin(SkinInfo skin, IStorageResourceProvider resources)
             : this(skin, new LegacySkinResourceStore<SkinFileInfo>(skin, resources.Files), resources, "skin.ini")
         {

osu.Game/Skinning/SkinInfo.cs

@@ -3,10 +3,10 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using osu.Framework.IO.Stores;
 using osu.Game.Configuration;
 using osu.Game.Database;
+using osu.Game.Extensions;
 using osu.Game.IO;
 
 namespace osu.Game.Skinning
@@ -25,20 +25,7 @@ namespace osu.Game.Skinning
 
         public string Creator { get; set; }
 
-        private string instantiationInfo;
-
-        public string InstantiationInfo
-        {
-            get => instantiationInfo;
-            set => instantiationInfo = abbreviateInstantiationInfo(value);
-        }
-
-        private string abbreviateInstantiationInfo(string value)
-        {
-            // exclude version onwards, matching only on namespace and type.
-            // this is mainly to allow for new versions of already loaded rulesets to "upgrade" from old.
-            return string.Join(',', value.Split(',').Take(2));
-        }
+        public string InstantiationInfo { get; set; }
 
         public virtual Skin CreateInstance(IResourceStore<byte[]> legacyDefaultResources, IStorageResourceProvider resources)
         {
@@ -64,7 +51,7 @@ namespace osu.Game.Skinning
             ID = DEFAULT_SKIN,
             Name = "osu!lazer",
             Creator = "team osu!",
-            InstantiationInfo = typeof(DefaultSkin).AssemblyQualifiedName,
+            InstantiationInfo = typeof(DefaultSkin).GetInvariantInstantiationInfo()
         };
 
         public bool Equals(SkinInfo other) => other != null && ID == other.ID;

osu.Game/Skinning/SkinManager.cs

@@ -25,6 +25,7 @@ using osu.Framework.Testing;
 using osu.Framework.Utils;
 using osu.Game.Audio;
 using osu.Game.Database;
+using osu.Game.Extensions;
 using osu.Game.IO;
 using osu.Game.IO.Archives;
 
@@ -127,7 +128,7 @@ namespace osu.Game.Skinning
 
             var instance = GetSkin(model);
 
-            model.InstantiationInfo ??= instance.GetType().AssemblyQualifiedName;
+            model.InstantiationInfo ??= instance.GetType().GetInvariantInstantiationInfo();
 
             if (model.Name?.Contains(".osk", StringComparison.OrdinalIgnoreCase) == true)
                 populateMetadata(model, instance);

osu.Game/Tests/Visual/Multiplayer/MultiplayerTestScene.cs

@@ -16,6 +16,9 @@ namespace osu.Game.Tests.Visual.Multiplayer
 {
     public abstract class MultiplayerTestScene : RoomTestScene
     {
+        public const int PLAYER_1_ID = 55;
+        public const int PLAYER_2_ID = 56;
+
         [Cached(typeof(StatefulMultiplayerClient))]
         public TestMultiplayerClient Client { get; }
 

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerClient.cs

@@ -25,6 +25,8 @@ namespace osu.Game.Tests.Visual.Multiplayer
         public override IBindable<bool> IsConnected => isConnected;
         private readonly Bindable<bool> isConnected = new Bindable<bool>(true);
 
+        public Room? APIRoom { get; private set; }
+
         public Action<MultiplayerRoom>? RoomSetupAction;
 
         [Resolved]
@@ -138,10 +140,16 @@ namespace osu.Game.Tests.Visual.Multiplayer
             RoomSetupAction?.Invoke(room);
             RoomSetupAction = null;
 
+            APIRoom = apiRoom;
+
             return Task.FromResult(room);
         }
 
-        protected override Task LeaveRoomInternal() => Task.CompletedTask;
+        protected override Task LeaveRoomInternal()
+        {
+            APIRoom = null;
+            return Task.CompletedTask;
+        }
 
         public override Task TransferHost(int userId) => ((IMultiplayerClient)this).HostChanged(userId);
 

osu.Game/Tests/Visual/Spectator/TestSpectatorStreamingClient.cs

@@ -0,0 +1,90 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using osu.Framework.Bindables;
+using osu.Framework.Utils;
+using osu.Game.Online;
+using osu.Game.Online.Spectator;
+using osu.Game.Replays.Legacy;
+using osu.Game.Scoring;
+
+namespace osu.Game.Tests.Visual.Spectator
+{
+    public class TestSpectatorStreamingClient : SpectatorStreamingClient
+    {
+        public new BindableList<int> PlayingUsers => (BindableList<int>)base.PlayingUsers;
+        private readonly ConcurrentDictionary<int, byte> watchingUsers = new ConcurrentDictionary<int, byte>();
+
+        private readonly Dictionary<int, int> userBeatmapDictionary = new Dictionary<int, int>();
+        private readonly Dictionary<int, bool> userSentStateDictionary = new Dictionary<int, bool>();
+
+        public TestSpectatorStreamingClient()
+            : base(new DevelopmentEndpointConfiguration())
+        {
+        }
+
+        public void StartPlay(int userId, int beatmapId)
+        {
+            userBeatmapDictionary[userId] = beatmapId;
+            sendState(userId, beatmapId);
+        }
+
+        public void EndPlay(int userId, int beatmapId)
+        {
+            ((ISpectatorClient)this).UserFinishedPlaying(userId, new SpectatorState
+            {
+                BeatmapID = beatmapId,
+                RulesetID = 0,
+            });
+
+            userBeatmapDictionary.Remove(userId);
+            userSentStateDictionary.Remove(userId);
+        }
+
+        public void SendFrames(int userId, int index, int count)
+        {
+            var frames = new List<LegacyReplayFrame>();
+
+            for (int i = index; i < index + count; i++)
+            {
+                var buttonState = i == index + count - 1 ? ReplayButtonState.None : ReplayButtonState.Left1;
+
+                frames.Add(new LegacyReplayFrame(i * 100, RNG.Next(0, 512), RNG.Next(0, 512), buttonState));
+            }
+
+            var bundle = new FrameDataBundle(new ScoreInfo { Combo = index + count }, frames);
+            ((ISpectatorClient)this).UserSentFrames(userId, bundle);
+
+            if (!userSentStateDictionary[userId])
+                sendState(userId, userBeatmapDictionary[userId]);
+        }
+
+        public override void WatchUser(int userId)
+        {
+            base.WatchUser(userId);
+
+            // When newly watching a user, the server sends the playing state immediately.
+            if (watchingUsers.TryAdd(userId, 0) && PlayingUsers.Contains(userId))
+                sendState(userId, userBeatmapDictionary[userId]);
+        }
+
+        public override void StopWatchingUser(int userId)
+        {
+            base.StopWatchingUser(userId);
+            watchingUsers.TryRemove(userId, out _);
+        }
+
+        private void sendState(int userId, int beatmapId)
+        {
+            ((ISpectatorClient)this).UserBeganPlaying(userId, new SpectatorState
+            {
+                BeatmapID = beatmapId,
+                RulesetID = 0,
+            });
+
+            userSentStateDictionary[userId] = true;
+        }
+    }
+}