Add spline easing class

src/Avalonia.Animation/Easing/SplineEasing.cs

@@ -0,0 +1,85 @@
+namespace Avalonia.Animation.Easings
+{
+    /// <summary>
+    /// Eases a <see cref="double"/> value
+    /// using a user-defined cubic bezier curve.
+    /// Good for custom easing functions that doesn't quite
+    /// fit with the built-in ones. 
+    /// </summary>
+    public class SplineEasing : Easing
+    {
+        /// <summary>
+        /// X coordinate of the first control point
+        /// </summary>
+        private double _x1;
+        public double X1
+        {
+            get => _x1;
+            set
+            {
+                _x1 = value; _internalKeySpline.ControlPointX1 = _x1;
+            }
+        }
+
+        /// <summary>
+        /// Y coordinate of the first control point
+        /// </summary>
+        private double _y1;
+        public double Y1
+        {
+            get => _y1;
+            set
+            {
+                _y1 = value; _internalKeySpline.ControlPointY1 = _y1;
+            }
+        }
+
+        /// <summary>
+        /// X coordinate of the second control point
+        /// </summary>
+        private double _x2 = 1.0d;
+        public double X2
+        {
+            get => _x2;
+            set
+            {
+                _x2 = value;
+                _internalKeySpline.ControlPointX2 = _x2;
+            }
+        }
+
+        /// <summary>
+        /// Y coordinate of the second control point
+        /// </summary>
+        private double _y2 = 1.0d;
+        public double Y2
+        {
+            get => _y2;
+            set
+            {
+                _y2 = value;
+                _internalKeySpline.ControlPointY2 = _y2;
+            }
+        }
+
+        private KeySpline _internalKeySpline;
+
+        public SplineEasing(double x1 = 0d, double y1 = 0d, double x2 = 1d, double y2 = 1d) : base()
+        {
+            this._internalKeySpline = new KeySpline();
+            this.X1 = x1;
+            this.Y1 = y1;
+            this.X2 = x2;
+            this.Y1 = y2;
+        }
+
+        public SplineEasing()
+        {
+            this._internalKeySpline = new KeySpline();
+        }
+
+        /// <inheritdoc/>
+        public override double Ease(double progress) =>
+            _internalKeySpline.GetSplineProgress(progress);
+    }
+}