Fix problems caused by missing transform data or invalid order of operations when interpolating matrices.

src/Avalonia.Visuals/Matrix.cs

@@ -215,6 +215,28 @@ namespace Avalonia
             return angle * 0.0174532925;
         }
 
+        /// <summary>
+        /// Appends another matrix as post-multiplication operation.
+        /// Equivalent to this * value;
+        /// </summary>
+        /// <param name="value">A matrix.</param>
+        /// <returns>Post-multiplied matrix.</returns>
+        public Matrix Append(Matrix value)
+        {
+            return this * value;
+        }
+
+        /// <summary>
+        /// Prpends another matrix as pre-multiplication operation.
+        /// Equivalent to value * this;
+        /// </summary>
+        /// <param name="value">A matrix.</param>
+        /// <returns>Pre-multiplied matrix.</returns>
+        public Matrix Prepend(Matrix value)
+        {
+            return value * this;
+        }
+
         /// <summary>
         /// Calculates the determinant for this matrix.
         /// </summary>

src/Avalonia.Visuals/Media/Transformation/InterpolationUtilities.cs

@@ -18,11 +18,11 @@ namespace Avalonia.Media.Transformation
         public static Matrix ComposeTransform(Matrix.Decomposed decomposed)
         {
             // According to https://www.w3.org/TR/css-transforms-1/#recomposing-to-a-2d-matrix
-
-            return Matrix.CreateTranslation(decomposed.Translate) *
-                   Matrix.CreateRotation(decomposed.Angle) *
-                   Matrix.CreateSkew(decomposed.Skew.X, decomposed.Skew.Y) *
-                   Matrix.CreateScale(decomposed.Scale);
+            return Matrix.Identity
+                .Prepend(Matrix.CreateTranslation(decomposed.Translate))
+                .Prepend(Matrix.CreateRotation(decomposed.Angle))
+                .Prepend(Matrix.CreateSkew(decomposed.Skew.X, decomposed.Skew.Y))
+                .Prepend(Matrix.CreateScale(decomposed.Scale));
         }
 
         public static Matrix.Decomposed InterpolateDecomposedTransforms(ref Matrix.Decomposed from, ref Matrix.Decomposed to, double progress)

src/Avalonia.Visuals/Media/Transformation/TransformOperation.cs

@@ -86,6 +86,8 @@ namespace Avalonia.Media.Transformation
 
             if (fromIdentity && toIdentity)
             {
+                result.Matrix = Matrix.Identity;
+
                 return true;
             }
 
@@ -179,7 +181,8 @@ namespace Avalonia.Media.Transformation
                 }
                 case OperationType.Identity:
                 {
-                    // Do nothing.
+                    result.Matrix = Matrix.Identity;
+
                     break;
                 }
             }

tests/Avalonia.Visuals.UnitTests/Media/TransformOperationsTests.cs

@@ -129,7 +129,7 @@ namespace Avalonia.Visuals.UnitTests.Media
 
             Assert.Single(operations);
             Assert.Equal(TransformOperation.OperationType.Matrix, operations[0].Type);
-            
+
             var expectedMatrix = new Matrix(1, 2, 3, 4, 5, 6);
 
             Assert.Equal(expectedMatrix, operations[0].Matrix);
@@ -195,7 +195,7 @@ namespace Avalonia.Visuals.UnitTests.Media
         [Theory]
         [InlineData(0d, 10d)]
         [InlineData(0.5d, 15d)]
-        [InlineData(1d,20d)]
+        [InlineData(1d, 20d)]
         public void Can_Interpolate_Rotation(double progress, double angle)
         {
             var from = TransformOperations.Parse("rotate(10deg)");
@@ -225,5 +225,73 @@ namespace Avalonia.Visuals.UnitTests.Media
             Assert.Single(operations);
             Assert.Equal(TransformOperation.OperationType.Matrix, operations[0].Type);
         }
+
+        [Fact]
+        public void Order_Of_Operations_Is_Preserved_No_Prefix()
+        {
+            var from = TransformOperations.Parse("scale(1)");
+            var to = TransformOperations.Parse("translate(50px,50px) scale(0.5,0.5)");
+
+            var interpolated_0 = TransformOperations.Interpolate(from, to, 0);
+
+            Assert.True(interpolated_0.IsIdentity);
+
+            var interpolated_50 = TransformOperations.Interpolate(from, to, 0.5);
+
+            AssertMatrix(interpolated_50.Value, scaleX: 0.75, scaleY: 0.75, translateX: 12.5, translateY: 12.5);
+
+            var interpolated_100 = TransformOperations.Interpolate(from, to, 1);
+
+            AssertMatrix(interpolated_100.Value, scaleX: 0.5, scaleY: 0.5, translateX: 25, translateY: 25);
+        }
+
+        [Fact]
+        public void Order_Of_Operations_Is_Preserved_One_Prefix()
+        {
+            var from = TransformOperations.Parse("scale(1)");
+            var to = TransformOperations.Parse("scale(0.5,0.5) translate(50px,50px)");
+
+            var interpolated_0 = TransformOperations.Interpolate(from, to, 0);
+
+            Assert.True(interpolated_0.IsIdentity);
+
+            var interpolated_50 = TransformOperations.Interpolate(from, to, 0.5);
+
+            AssertMatrix(interpolated_50.Value, scaleX: 0.75, scaleY: 0.75, translateX: 25.0, translateY: 25);
+
+            var interpolated_100 = TransformOperations.Interpolate(from, to, 1);
+
+            AssertMatrix(interpolated_100.Value, scaleX: 0.5, scaleY: 0.5, translateX: 50, translateY: 50);
+        }
+
+        private static void AssertMatrix(Matrix matrix, double? angle = null, double? scaleX = null, double? scaleY = null, double? translateX = null, double? translateY = null)
+        {
+            Assert.True(Matrix.TryDecomposeTransform(matrix, out var composed));
+
+            if (angle.HasValue)
+            {
+                Assert.Equal(angle.Value, composed.Angle);
+            }
+
+            if (scaleX.HasValue)
+            {
+                Assert.Equal(scaleX.Value, composed.Scale.X);
+            }
+
+            if (scaleY.HasValue)
+            {
+                Assert.Equal(scaleY.Value, composed.Scale.Y);
+            }
+
+            if (translateX.HasValue)
+            {
+                Assert.Equal(translateX.Value, composed.Translate.X);
+            }
+
+            if (translateY.HasValue)
+            {
+                Assert.Equal(translateY.Value, composed.Translate.Y);
+            }
+        }
     }
 }