add OsuNumberBox with basic tests

osu.Game.Tests/Visual/UserInterface/TestSceneNumberBox.cs

@@ -0,0 +1,55 @@
+// 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 NUnit.Framework;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Game.Graphics.UserInterface;
+
+namespace osu.Game.Tests.Visual.UserInterface
+{
+    [TestFixture]
+    public class TestSceneNumberBox : OsuTestScene
+    {
+        public override IReadOnlyList<Type> RequiredTypes => new[]
+        {
+            typeof(OsuNumberBox),
+        };
+
+        private OsuNumberBox numberBox;
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            Child = new Container
+            {
+                Anchor = Anchor.Centre,
+                Origin = Anchor.Centre,
+                RelativeSizeAxes = Axes.X,
+                Padding = new MarginPadding { Horizontal = 250 },
+                Child = numberBox = new OsuNumberBox
+                {
+                    Anchor = Anchor.Centre,
+                    Origin = Anchor.Centre,
+                    RelativeSizeAxes = Axes.X,
+                    PlaceholderText = "Insert numbers here"
+                }
+            };
+
+            clearInput();
+            AddStep("enter numbers", () => numberBox.Text = "987654321");
+            expectedValue("987654321");
+            clearInput();
+            AddStep("enter text + single number", () => numberBox.Text = "1 hello 2 world 3");
+            expectedValue("123");
+            clearInput();
+        }
+
+        private void clearInput() => AddStep("clear input", () => numberBox.Text = null);
+
+        private void expectedValue(string value) => AddAssert("expect number", () => numberBox.Text == value);
+    }
+}

osu.Game/Graphics/UserInterface/OsuNumberBox.cs

@@ -0,0 +1,10 @@
+// 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.
+
+namespace osu.Game.Graphics.UserInterface
+{
+    public class OsuNumberBox : OsuTextBox
+    {
+        protected override bool CanAddCharacter(char character) => char.IsNumber(character);
+    }
+}