Finally fixed the flickering heart bug!

Simplify the Keyframe selection and removed all LINQ code.
Use the Cue property on AnimatorKeyFrame as the time index and integrate the IsNeutral value as an internal property.

src/Avalonia.Animation/AnimatorKeyFrame.cs

@@ -29,6 +29,7 @@ namespace Avalonia.Animation
             Cue = cue;
         }
 
+        internal bool isNeutral;
         public Type AnimatorType { get; }
         public Cue Cue { get; }
         public AvaloniaProperty Property { get; private set; }

src/Avalonia.Animation/Animator`1.cs

@@ -19,7 +19,7 @@ namespace Avalonia.Animation
         /// <summary>
         /// List of type-converted keyframes.
         /// </summary>
-        private readonly SortedList<double, (AnimatorKeyFrame, bool isNeutral)> _convertedKeyframes = new SortedList<double, (AnimatorKeyFrame, bool)>();
+        private readonly List<AnimatorKeyFrame> _convertedKeyframes = new List<AnimatorKeyFrame>();
 
         private bool isVerfifiedAndConverted;
 
@@ -58,41 +58,49 @@ namespace Avalonia.Animation
         /// <param name="t">The time parameter, relative to the total animation time</param>
         protected (double IntraKFTime, KeyFramePair<T> KFPair) GetKFPairAndIntraKFTime(double t)
         {
-            KeyValuePair<double, (AnimatorKeyFrame frame, bool isNeutral)> firstCue, lastCue;
+            AnimatorKeyFrame firstCue, lastCue ;
             int kvCount = _convertedKeyframes.Count;
             if (kvCount > 2)
             {
                 if (DoubleUtils.AboutEqual(t, 0.0) || t < 0.0)
                 {
-                    firstCue = _convertedKeyframes.First();
-                    lastCue = _convertedKeyframes.Skip(1).First();
+                    firstCue = _convertedKeyframes[0];
+                    lastCue = _convertedKeyframes[1];
                 }
                 else if (DoubleUtils.AboutEqual(t, 1.0) || t > 1.0)
                 {
-                    firstCue = _convertedKeyframes.Skip(kvCount - 2).First();
-                    lastCue = _convertedKeyframes.Last();
+                    firstCue = _convertedKeyframes[_convertedKeyframes.Count - 2];
+                    lastCue = _convertedKeyframes[_convertedKeyframes.Count - 1];
                 }
                 else
                 {
-                    firstCue = _convertedKeyframes.Last(j => j.Key <= t);
-                    lastCue = _convertedKeyframes.First(j => j.Key >= t);
+                    (double time, int index) maxval = (0.0d, 0);
+                    for (int i = 0; i < _convertedKeyframes.Count; i++)
+                    {
+                        var comp = _convertedKeyframes[i].Cue.CueValue;
+                        if (t >= comp)
+                        {
+                            maxval = (comp, i);
+                        }
+                    }
+                    firstCue = _convertedKeyframes[maxval.index];
+                    lastCue = _convertedKeyframes[maxval.index + 1];
                 }
             }
             else
             {
-                firstCue = _convertedKeyframes.First();
-                lastCue = _convertedKeyframes.Last();
+                firstCue = _convertedKeyframes[0];
+                lastCue = _convertedKeyframes[1];
             }
 
-            double t0 = firstCue.Key;
-            double t1 = lastCue.Key;
+            double t0 = firstCue.Cue.CueValue;
+            double t1 = lastCue.Cue.CueValue;
             var intraframeTime = (t - t0) / (t1 - t0);
-            var firstFrameData = (firstCue.Value.frame.GetTypedValue<T>(), firstCue.Value.isNeutral);
-            var lastFrameData = (lastCue.Value.frame.GetTypedValue<T>(), lastCue.Value.isNeutral);
+            var firstFrameData = (firstCue.GetTypedValue<T>(), firstCue.isNeutral);
+            var lastFrameData = (lastCue.GetTypedValue<T>(), lastCue.isNeutral);
             return (intraframeTime, new KeyFramePair<T>(firstFrameData, lastFrameData));
         }
 
-
         /// <summary>
         /// Runs the KeyFrames Animation.
         /// </summary>
@@ -113,16 +121,25 @@ namespace Avalonia.Animation
         protected abstract T DoInterpolation(double time, T neutralValue);
 
         /// <summary>
-        /// Verifies and converts keyframe values according to this class's target type.
+        /// Verifies, converts and sorts keyframe values according to this class's target type.
         /// </summary>
         private void VerifyConvertKeyFrames()
         {
             foreach (AnimatorKeyFrame keyframe in this)
             {
-                _convertedKeyframes.Add(keyframe.Cue.CueValue, (keyframe, false));
+                _convertedKeyframes.Add(keyframe);
             }
 
             AddNeutralKeyFramesIfNeeded();
+
+            var copy = _convertedKeyframes.ToList().OrderBy(p => p.Cue.CueValue);
+            _convertedKeyframes.Clear();
+
+            foreach (AnimatorKeyFrame keyframe in copy)
+            {
+                _convertedKeyframes.Add(keyframe);
+            }
+
             isVerfifiedAndConverted = true;
 
         }
@@ -133,13 +150,13 @@ namespace Avalonia.Animation
             hasStartKey = hasEndKey = false;
 
             // Check if there's start and end keyframes.
-            foreach (var converted in _convertedKeyframes.Keys)
+            foreach (var frame in _convertedKeyframes)
             {
-                if (DoubleUtils.AboutEqual(converted, 0.0))
+                if (DoubleUtils.AboutEqual(frame.Cue.CueValue, 0.0))
                 {
                     hasStartKey = true;
                 }
-                else if (DoubleUtils.AboutEqual(converted, 1.0))
+                else if (DoubleUtils.AboutEqual(frame.Cue.CueValue, 1.0))
                 {
                     hasEndKey = true;
                 }
@@ -153,12 +170,12 @@ namespace Avalonia.Animation
         {
             if (!hasStartKey)
             {
-                _convertedKeyframes.Add(0.0d, (new AnimatorKeyFrame { Value = default(T) }, true));
+                _convertedKeyframes.Add(new AnimatorKeyFrame(null, new Cue(0.0d)) { Value = default(T), isNeutral = true });
             }
 
             if (!hasEndKey)
             {
-                _convertedKeyframes.Add(1.0d, (new AnimatorKeyFrame { Value = default(T) }, true));
+                _convertedKeyframes.Add(new AnimatorKeyFrame(null, new Cue(1.0d)) { Value = default(T), isNeutral = true });
             }
         }
     }

src/Avalonia.Animation/Cue.cs

@@ -7,7 +7,7 @@ using System.Text;
 namespace Avalonia.Animation
 {
     /// <summary>
-    /// A Cue object for <see cref="KeyFrame"/>. 
+    /// Determines the time index for a <see cref="KeyFrame"/>. 
     /// </summary>
     [TypeConverter(typeof(CueTypeConverter))]
     public readonly struct Cue : IEquatable<Cue>, IEquatable<double>
@@ -84,5 +84,4 @@ namespace Avalonia.Animation
             return Cue.Parse((string)value, culture);
         }
     }
-
-}
+}