Add and consume user cache class

osu.Game/Database/UserLookupCache.cs

@@ -0,0 +1,118 @@
+// 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.Concurrent;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using osu.Framework.Allocation;
+using osu.Game.Online.API;
+using osu.Game.Online.API.Requests;
+using osu.Game.Users;
+
+namespace osu.Game.Database
+{
+    public class UserLookupCache : MemoryCachingComponent<int, User>
+    {
+        private readonly ConcurrentBag<int> nextTaskIDs = new ConcurrentBag<int>();
+
+        [Resolved]
+        private IAPIProvider api { get; set; }
+
+        private readonly object taskAssignmentLock = new object();
+
+        private Task<List<User>> pendingRequest;
+
+        /// <summary>
+        /// Whether <see cref="pendingRequest"/> has already grabbed its IDs.
+        /// </summary>
+        private bool pendingRequestConsumedIDs;
+
+        public Task<User> GetUser(int userId, CancellationToken token = default) => GetAsync(userId, token);
+
+        protected override async Task<User> ComputeValueAsync(int lookup, CancellationToken token = default)
+        {
+            var users = await getQueryTaskForUser(lookup);
+            return users.FirstOrDefault(u => u.Id == lookup);
+        }
+
+        /// <summary>
+        /// Return the task responsible for fetching the provided user.
+        /// This may be part of a larger batch lookup to reduce web requests.
+        /// </summary>
+        /// <param name="userId">The user to lookup.</param>
+        /// <returns>The task responsible for the lookup.</returns>
+        private Task<List<User>> getQueryTaskForUser(int userId)
+        {
+            lock (taskAssignmentLock)
+            {
+                nextTaskIDs.Add(userId);
+
+                // if there's a pending request which hasn't been started yet (and is not yet full), we can wait on it.
+                if (pendingRequest != null && !pendingRequestConsumedIDs && nextTaskIDs.Count < 50)
+                    return pendingRequest;
+
+                return queueNextTask(nextLookup);
+            }
+
+            List<User> nextLookup()
+            {
+                int[] lookupItems;
+
+                lock (taskAssignmentLock)
+                {
+                    pendingRequestConsumedIDs = true;
+                    lookupItems = nextTaskIDs.ToArray();
+                    nextTaskIDs.Clear();
+
+                    if (lookupItems.Length == 0)
+                    {
+                        queueNextTask(null);
+                        return new List<User>();
+                    }
+                }
+
+                var request = new GetUsersRequest(lookupItems);
+
+                // rather than queueing, we maintain our own single-threaded request stream.
+                request.Perform(api);
+
+                return request.Result.Users;
+            }
+        }
+
+        /// <summary>
+        /// Queues new work at the end of the current work tasks.
+        /// Ensures the provided work is eventually run.
+        /// </summary>
+        /// <param name="work">The work to run. Can be null to signify the end of available work.</param>
+        /// <returns>The task tracking this work.</returns>
+        private Task<List<User>> queueNextTask(Func<List<User>> work)
+        {
+            lock (taskAssignmentLock)
+            {
+                if (work == null)
+                {
+                    pendingRequest = null;
+                    pendingRequestConsumedIDs = false;
+                }
+                else if (pendingRequest == null)
+                {
+                    // special case for the first request ever.
+                    pendingRequest = Task.Run(work);
+                    pendingRequestConsumedIDs = false;
+                }
+                else
+                {
+                    // append the new request on to the last to be executed.
+                    pendingRequest = pendingRequest.ContinueWith(_ => work());
+                    pendingRequestConsumedIDs = false;
+                }
+
+                return pendingRequest;
+            }
+        }
+    }
+}

osu.Game/OsuGameBase.cs

@@ -61,6 +61,8 @@ namespace osu.Game
 
         protected BeatmapDifficultyCache DifficultyCache;
 
+        protected UserLookupCache UserCache;
+
         protected SkinManager SkinManager;
 
         protected RulesetStore RulesetStore;
@@ -229,6 +231,9 @@ namespace osu.Game
             dependencies.Cache(DifficultyCache = new BeatmapDifficultyCache());
             AddInternal(DifficultyCache);
 
+            dependencies.Cache(UserCache = new UserLookupCache());
+            AddInternal(UserCache);
+
             var scorePerformanceManager = new ScorePerformanceCache();
             dependencies.Cache(scorePerformanceManager);
             AddInternal(scorePerformanceManager);

osu.Game/Overlays/Dashboard/CurrentlyPlayingDisplay.cs

@@ -8,8 +8,8 @@ using osu.Framework.Bindables;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Screens;
+using osu.Game.Database;
 using osu.Game.Online.API;
-using osu.Game.Online.API.Requests;
 using osu.Game.Online.Spectator;
 using osu.Game.Screens.Multi.Match.Components;
 using osu.Game.Screens.Play;
@@ -45,6 +45,9 @@ namespace osu.Game.Overlays.Dashboard
         [Resolved]
         private IAPIProvider api { get; set; }
 
+        [Resolved]
+        private UserLookupCache users { get; set; }
+
         protected override void LoadComplete()
         {
             base.LoadComplete();
@@ -55,18 +58,19 @@ namespace osu.Game.Overlays.Dashboard
                 switch (e.Action)
                 {
                     case NotifyCollectionChangedAction.Add:
-                        var request = new GetUsersRequest(e.NewItems.OfType<int>().ToArray());
 
-                        request.Success += users => Schedule(() =>
+                        foreach (var id in e.NewItems.OfType<int>().ToArray())
                         {
-                            foreach (var user in users.Users)
+                            users.GetUser(id).ContinueWith(u =>
                             {
-                                if (playingUsers.Contains(user.Id))
-                                    userFlow.Add(createUserPanel(user));
-                            }
-                        });
+                                Schedule(() =>
+                                {
+                                    if (playingUsers.Contains(u.Result.Id))
+                                        userFlow.Add(createUserPanel(u.Result));
+                                });
+                            });
+                        }
 
-                        api.Queue(request);
                         break;
 
                     case NotifyCollectionChangedAction.Remove: