perf(timeline): Remove the use of events and disposables

Removes the use of events and disposable to reduce the memory footprint.

src/Uno.UI/UI/Xaml/Media/Animation/ITimeline.cs

@@ -19,7 +19,7 @@ internal interface ITimeline : IDisposable
 		void SeekAlignedToLastTick(TimeSpan offset);
 		void SkipToFill();
 		void Deactivate();
-		event EventHandler<object> Completed;
-		event EventHandler<object> Failed;
+		void RegisterListener(ITimelineListener storyboard);
+		void UnregisterListener(ITimelineListener storyboard);
 	}
 }

src/Uno.UI/UI/Xaml/Media/Animation/ITimelineListener.cs

@@ -0,0 +1,8 @@
+namespace Windows.UI.Xaml.Media.Animation
+{
+	internal interface ITimelineListener
+	{
+		void ChildCompleted(Timeline timeline);
+		void ChildFailed(Timeline timeline);
+	}
+}

src/Uno.UI/UI/Xaml/Media/Animation/Storyboard.cs

@@ -15,7 +15,7 @@
 namespace Windows.UI.Xaml.Media.Animation
 {
 	[ContentProperty(Name = "Children")]
-	public sealed partial class Storyboard : Timeline, ITimeline, IAdditionalChildrenProvider, IThemeChangeAware
+	public sealed partial class Storyboard : Timeline, ITimeline, IAdditionalChildrenProvider, IThemeChangeAware, ITimelineListener
 	{
 		private static readonly IEventProvider _trace = Tracing.Get(TraceProvider.Id);
 		private EventActivity _traceActivity;
@@ -34,7 +34,6 @@ public static class TraceProvider
 		private int _replayCount = 1;
 		private int _runningChildren = 0;
 		private bool _hasFillingChildren = false;
-		private Dictionary<ITimeline, IDisposable> _childrenSubscriptions = new Dictionary<ITimeline, IDisposable>();
 
 		public Storyboard()
 		{
@@ -77,11 +76,7 @@ public Storyboard()
 		/// <param name="child"></param>
 		private void DisposeChildRegistrations(ITimeline child)
 		{
-			if(_childrenSubscriptions.TryGetValue(child, out var disposable))
-			{
-				disposable.Dispose();
-				_childrenSubscriptions.Remove(child);
-			}
+			child.UnregisterListener(this);
 		}
 
 		/// <summary>
@@ -127,16 +122,7 @@ private void Play()
 					DisposeChildRegistrations(child);
 
 					_runningChildren++;
-					child.Completed += Child_Completed;
-					child.Failed += Child_Failed;
-
-					_childrenSubscriptions.Add(
-						child,
-						Disposable.Create(() => {
-							child.Completed -= Child_Completed;
-							child.Failed -= Child_Failed;
-						})
-					);
+					child.RegisterListener(this);
 
 					child.Begin();
 				}
@@ -330,13 +316,8 @@ public TimeSpan GetCurrentTime()
 			throw new NotImplementedException();
 		}
 
-		private void Child_Failed(object sender, object e)
+		void ITimelineListener.ChildFailed(Timeline child)
 		{
-			if (!(sender is Timeline child))
-			{
-				return;
-			}
-
 			DisposeChildRegistrations(child);
 
 			Interlocked.Decrement(ref _runningChildren);
@@ -345,13 +326,8 @@ private void Child_Failed(object sender, object e)
 			// child, where completed relates to all children being completed.
 		}
 
-		private void Child_Completed(object sender, object e)
+		void ITimelineListener.ChildCompleted(Timeline child)
 		{
-			if (!(sender is Timeline child))
-			{
-				return;
-			}
-
 			DisposeChildRegistrations(child);
 
 			Interlocked.Decrement(ref _runningChildren);

src/Uno.UI/UI/Xaml/Media/Animation/Timeline.cs

@@ -15,6 +15,7 @@ public partial class Timeline : DependencyObject, ITimeline
 	{
 		private WeakReference<DependencyObject> _targetElement;
 		private BindingPath _propertyInfo;
+		private List<ITimelineListener> _timelineListeners = new();
 
 		public Timeline()
 		{
@@ -87,16 +88,30 @@ public RepeatBehavior RepeatBehavior
 
 
 		public event EventHandler<object> Completed;
-		internal event EventHandler<object> Failed;
 
-		event EventHandler<object> ITimeline.Failed
+		void ITimeline.RegisterListener(ITimelineListener listener)
+			=> _timelineListeners.Add(listener);
+
+		void ITimeline.UnregisterListener(ITimelineListener listener)
+			=> _timelineListeners.Remove(listener);
+
+		protected void OnCompleted()
 		{
-			add => Failed += value;
-			remove => Failed += value;
+			Completed?.Invoke(this, null);
+
+			for (var i = 0; i < _timelineListeners.Count; i++)
+			{
+				_timelineListeners[i].ChildCompleted(this);
+			}
 		}
 
-		protected void OnCompleted() => Completed?.Invoke(this, null);
-		protected void OnFailed() => Failed?.Invoke(this, null);
+		protected void OnFailed()
+		{
+			for (var i = 0; i < _timelineListeners.Count; i++)
+			{
+				_timelineListeners[i].ChildFailed(this);
+			}
+		}
 
 		/// <summary>
 		/// Compute duration of the Timeline. Sometimes it's define by components.