Use KeySpline in Animation

src/Avalonia.Animation/Animation.cs

@@ -254,7 +254,7 @@ namespace Avalonia.Animation
                         cue = new Cue(keyframe.KeyTime.TotalSeconds / Duration.TotalSeconds);
                     }
 
-                    var newKF = new AnimatorKeyFrame(handler, cue);
+                    var newKF = new AnimatorKeyFrame(handler, cue, keyframe.KeySpline);
 
                     subscriptions.Add(newKF.BindSetter(setter, control));
 

src/Avalonia.Animation/AnimatorKeyFrame.cs

@@ -24,11 +24,20 @@ namespace Avalonia.Animation
         {
             AnimatorType = animatorType;
             Cue = cue;
+            KeySpline = null;
+        }
+
+        public AnimatorKeyFrame(Type animatorType, Cue cue, KeySpline keySpline)
+        {
+            AnimatorType = animatorType;
+            Cue = cue;
+            KeySpline = keySpline;
         }
 
         internal bool isNeutral;
         public Type AnimatorType { get; }
         public Cue Cue { get; }
+        public KeySpline KeySpline { get; }
         public AvaloniaProperty Property { get; private set; }
 
         private object _value;

src/Avalonia.Animation/Animators/Animator`1.cs

@@ -89,6 +89,9 @@ namespace Avalonia.Animation.Animators
             else
                 newValue = (T)lastKeyframe.Value;
 
+            if (lastKeyframe.KeySpline != null) // TODO: do we use firstKeyFrame or lastKeyframe?!
+                progress = lastKeyframe.KeySpline.GetSplineProgress(progress);
+
             return Interpolate(progress, oldValue, newValue);
         }
 

src/Avalonia.Animation/KeyFrame.cs

@@ -19,6 +19,7 @@ namespace Avalonia.Animation
     {
         private TimeSpan _ktimeSpan;
         private Cue _kCue;
+        private KeySpline _kKeySpline;
 
         public KeyFrame()
         {
@@ -74,6 +75,25 @@ namespace Avalonia.Animation
             }
         }
 
+        /// <summary>
+        /// Gets or sets the KeySpline of this <see cref="KeyFrame"/>.
+        /// </summary>
+        /// <value>The key spline.</value>
+        public KeySpline KeySpline
+        {
+            get
+            {
+                return _kKeySpline;
+            }
+            set
+            {
+                _kKeySpline = value;
+                if (value != null && !value.IsValid())
+                {
+                    throw new ArgumentException($"{nameof(KeySpline)} must have X coordinates >= 0.0 and <= 1.0.");
+                }
+            }
+        }
 
     }
 

src/Avalonia.Animation/KeySpline.cs

@@ -66,7 +66,17 @@ namespace Avalonia.Animation
         public double ControlPointX1
         {
             get => _controlPointX1;
-            set => _controlPointX1 = value;
+            set
+            {
+                if (IsValidXValue(value))
+                {
+                    _controlPointX1 = value;
+                }
+                else
+                {
+                    throw new ArgumentException("Invalid KeySpline X1 value. Must be >= 0.0 and <= 1.0.");
+                }
+            }
         }
 
         public double ControlPointY1
@@ -78,7 +88,17 @@ namespace Avalonia.Animation
         public double ControlPointX2
         {
             get => _controlPointX2;
-            set => _controlPointX2 = value;
+            set
+            {
+                if (IsValidXValue(value))
+                {
+                    _controlPointX2 = value;
+                }
+                else
+                {
+                    throw new ArgumentException("Invalid KeySpline X2 value. Must be >= 0.0 and <= 1.0.");
+                }
+            }
         }
 
         public double ControlPointY2
@@ -94,8 +114,6 @@ namespace Avalonia.Animation
         /// <returns>the spline progress</returns>
         public double GetSplineProgress(double linearProgress)
         {
-            //ReadPreamble();
-
             if (_isDirty)
             {
                 Build();
@@ -113,6 +131,16 @@ namespace Avalonia.Animation
             }
         }
 
+        public bool IsValid()
+        {
+            return IsValidXValue(_controlPointX1) && IsValidXValue(_controlPointX2);
+        }
+
+        private bool IsValidXValue(double value)
+        {
+            return value >= 0.0 && value <= 1.0;
+        }
+
         /// <summary>
         /// Compute cached coefficients.
         /// </summary>