Added tests for CollectionChangedEventManager.

Including a failing test: `Receives_Events_From_Wrapped_Collection`.

tests/Avalonia.Controls.UnitTests/Utils/CollectionChangedEventManagerTests.cs

@@ -0,0 +1,89 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using System.Text;
+using Avalonia.Collections;
+using Avalonia.Controls.Utils;
+using Xunit;
+using CollectionChangedEventManager = Avalonia.Controls.Utils.CollectionChangedEventManager;
+
+namespace Avalonia.Controls.UnitTests.Utils
+{
+    public class CollectionChangedEventManagerTests
+    {
+        [Fact]
+        public void AddListener_Listens_To_Events()
+        {
+            var source = new AvaloniaList<string>();
+            var listener = new Listener();
+
+            CollectionChangedEventManager.Instance.AddListener(source, listener);
+
+            Assert.Empty(listener.Received);
+
+            source.Add("foo");
+
+            Assert.Equal(1, listener.Received.Count);
+        }
+
+        [Fact]
+        public void RemoveListener_Stops_Listening_To_Events()
+        {
+            var source = new AvaloniaList<string>();
+            var listener = new Listener();
+
+            CollectionChangedEventManager.Instance.AddListener(source, listener);
+            CollectionChangedEventManager.Instance.RemoveListener(source, listener);
+
+            source.Add("foo");
+
+            Assert.Empty(listener.Received);
+        }
+
+        [Fact]
+        public void Receives_Events_From_Wrapped_Collection()
+        {
+            var source = new WrappingCollection();
+            var listener = new Listener();
+
+            CollectionChangedEventManager.Instance.AddListener(source, listener);
+
+            Assert.Empty(listener.Received);
+
+            source.Add("foo");
+
+            Assert.Equal(1, listener.Received.Count);
+        }
+
+        private class Listener : ICollectionChangedListener
+        {
+            public List<NotifyCollectionChangedEventArgs> Received { get; } = new List<NotifyCollectionChangedEventArgs>();
+
+            public void Changed(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
+            {
+                Received.Add(e);
+            }
+
+            public void PostChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
+            {
+            }
+
+            public void PreChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
+            {
+            }
+        }
+
+        private class WrappingCollection : INotifyCollectionChanged
+        {
+            private AvaloniaList<string> _inner = new AvaloniaList<string>();
+
+            public void Add(string s) => _inner.Add(s);
+
+            public event NotifyCollectionChangedEventHandler CollectionChanged
+            {
+                add => _inner.CollectionChanged += value;
+                remove => _inner.CollectionChanged -= value;
+            }
+        }
+    }
+}