Add an ad-hoc way to provide dependency to children

osu.Game/Tests/Visual/DependencyProvidingContainer.cs

@@ -0,0 +1,50 @@
+// 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 osu.Framework.Allocation;
+using osu.Framework.Graphics.Containers;
+
+namespace osu.Game.Tests.Visual
+{
+    /// <summary>
+    /// A <see cref="Container"/> which providing ad-hoc dependencies to the child drawables.
+    /// <para>
+    /// To provide a dependency, specify the dependency type to <see cref="Types"/>, then specify the dependency value to either <see cref="Values"/> or <see cref="Container{Drawable}.Children"/>.
+    /// For each type specified in <see cref="Types"/>, the first value compatible with the type is selected and provided to the children.
+    /// </para>
+    /// </summary>
+    /// <remarks>
+    /// The <see cref="Types"/> and values of the dependencies must be set while this <see cref="DependencyProvidingContainer"/> is not loaded.
+    /// </remarks>
+    public class DependencyProvidingContainer : Container
+    {
+        /// <summary>
+        /// The types of the dependencies provided to the children.
+        /// </summary>
+        // TODO: should be an init-only property when C# 9
+        public Type[] Types { get; set; } = Array.Empty<Type>();
+
+        /// <summary>
+        /// The dependency values provided to the children.
+        /// </summary>
+        public object[] Values { get; set; } = Array.Empty<object>();
+
+        protected override IReadOnlyDependencyContainer CreateChildDependencies(IReadOnlyDependencyContainer parent)
+        {
+            var dependencyContainer = new DependencyContainer(base.CreateChildDependencies(parent));
+
+            foreach (var type in Types)
+            {
+                object value = Values.FirstOrDefault(v => type.IsInstanceOfType(v)) ??
+                               Children.FirstOrDefault(d => type.IsInstanceOfType(d)) ??
+                               throw new InvalidOperationException($"The type {type} is specified in this {nameof(DependencyProvidingContainer)}, but no corresponding value is provided.");
+
+                dependencyContainer.CacheAs(type, value);
+            }
+
+            return dependencyContainer;
+        }
+    }
+}