Merge pull request #2895 from AvaloniaUI/animations-alloc

Animations subsystem perf fixes.

src/Avalonia.Animation/Animatable.cs

@@ -45,16 +45,17 @@ namespace Avalonia.Animation
         {
             get
             {
-                if (_transitions == null)
+                if (_transitions is null)
                     _transitions = new Transitions();
 
-                if (_previousTransitions == null)
+                if (_previousTransitions is null)
                     _previousTransitions = new Dictionary<AvaloniaProperty, IDisposable>();
 
                 return _transitions;
             }
             set
             {
+
                 SetAndRaise(TransitionsProperty, ref _transitions, value);
             }
         }
@@ -66,18 +67,20 @@ namespace Avalonia.Animation
         /// <param name="e">The event args.</param>
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            if (e.Priority != BindingPriority.Animation && Transitions != null && _previousTransitions != null)
-            {
-                var match = Transitions.FirstOrDefault(x => x.Property == e.Property);
+            if (_transitions is null || _previousTransitions is null || e.Priority == BindingPriority.Animation) return;
 
-                if (match != null)
+            // PERF-SENSITIVE: Called on every property change. Don't use LINQ here (too many allocations).
+            foreach (var transition in Transitions)
+            {
+                if (transition.Property == e.Property)
                 {
                     if (_previousTransitions.TryGetValue(e.Property, out var dispose))
                         dispose.Dispose();
 
-                    var instance = match.Apply(this, Clock ?? Avalonia.Animation.Clock.GlobalClock, e.OldValue, e.NewValue);
+                    var instance = transition.Apply(this, Clock ?? Avalonia.Animation.Clock.GlobalClock, e.OldValue, e.NewValue);
 
                     _previousTransitions[e.Property] = instance;
+                    return;
                 }
             }
         }