Removed unnecessary INode interface

Author: Sergio0694

Microsoft.Toolkit.Uwp.UI.Animations/Xaml/AnimationSet.cs

@@ -36,13 +36,6 @@ public sealed class AnimationSet : DependencyObjectCollection
         /// </summary>
         public event EventHandler? Completed;
 
-        /// <summary>
-        /// An interface representing a node in an <see cref="AnimationSet"/> instance.
-        /// </summary>
-        public interface INode
-        {
-        }
-
         /// <summary>
         /// Gets or sets a value indicating whether top level animation nodes in this collection are invoked
         /// sequentially. This applies to both <see cref="AnimationScope"/> nodes (which will still trigger
@@ -124,7 +117,7 @@ public async Task StartAsync(UIElement element, CancellationToken token)
 
             if (IsSequential)
             {
-                foreach (INode node in this)
+                foreach (object node in this)
                 {
                     if (node is ITimeline timeline)
                     {
@@ -151,6 +144,10 @@ public async Task StartAsync(UIElement element, CancellationToken token)
                             break;
                         }
                     }
+                    else
+                    {
+                        ThrowArgumentException();
+                    }
 
                     // This should in theory only be necessary in the timeline branch, but doing this check
                     // after running activities too help guard against 3rd party activities that might not
@@ -165,7 +162,7 @@ public async Task StartAsync(UIElement element, CancellationToken token)
             {
                 var builder = AnimationBuilder.Create();
 
-                foreach (INode node in this)
+                foreach (object node in this)
                 {
                     switch (node)
                     {
@@ -175,13 +172,18 @@ public async Task StartAsync(UIElement element, CancellationToken token)
                         case IActivity activity:
                             _ = activity.InvokeAsync(element, token);
                             break;
+                        default:
+                            ThrowArgumentException();
+                            break;
                     }
                 }
 
                 await builder.StartAsync(element, token);
             }
 
             Completed?.Invoke(this, EventArgs.Empty);
+
+            static void ThrowArgumentException() => throw new ArgumentException($"An animation set can only contain nodes implementing either ITimeline or IActivity");
         }
 
         /// <summary>

Microsoft.Toolkit.Uwp.UI.Animations/Xaml/Interfaces/IActivity.cs

@@ -11,7 +11,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Animations
     /// <summary>
     /// An interface representing a XAML model for a custom activity or action within an <see cref="AnimationSet"/>.
     /// </summary>
-    public interface IActivity : AnimationSet.INode
+    public interface IActivity
     {
         /// <summary>
         /// Invokes the current activity.

Microsoft.Toolkit.Uwp.UI.Animations/Xaml/Interfaces/ITimeline.cs

@@ -10,7 +10,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Animations
     /// <summary>
     /// An interface representing a XAML model for a custom animation.
     /// </summary>
-    public interface ITimeline : AnimationSet.INode
+    public interface ITimeline
     {
         /// <summary>
         /// Appens the current animation to a target <see cref="AnimationBuilder"/> instance.