Added center properties to RotateTransform.

src/Avalonia.Visuals/Media/RotateTransform.cs

@@ -14,6 +14,18 @@ namespace Avalonia.Media
         public static readonly StyledProperty<double> AngleProperty =
             AvaloniaProperty.Register<RotateTransform, double>(nameof(Angle));
 
+        /// <summary>
+        /// Defines the <see cref="CenterX"/> property.
+        /// </summary>
+        public static readonly StyledProperty<double> CenterXProperty =
+            AvaloniaProperty.Register<RotateTransform, double>(nameof(CenterX));
+
+        /// <summary>
+        /// Defines the <see cref="CenterY"/> property.
+        /// </summary>
+        public static readonly StyledProperty<double> CenterYProperty =
+            AvaloniaProperty.Register<RotateTransform, double>(nameof(CenterY));
+
         /// <summary>
         /// Initializes a new instance of the <see cref="RotateTransform"/> class.
         /// </summary>
@@ -32,18 +44,52 @@ namespace Avalonia.Media
             Angle = angle;
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RotateTransform"/> class.
+        /// </summary>
+        /// <param name="angle">The angle, in degrees.</param>
+        /// <param name="centerX">The x-coordinate of the center point for the rotation.</param>
+        /// <param name="centerY">The y-coordinate of the center point for the rotation.</param>
+        public RotateTransform(double angle, double centerX, double centerY)
+            : this()
+        {
+            Angle = angle;
+            CenterX = centerX;
+            CenterY = centerY;
+        }
+
         /// <summary>
         /// Gets or sets the angle of rotation, in degrees.
         /// </summary>
         public double Angle
         {
-            get { return GetValue(AngleProperty); }
-            set { SetValue(AngleProperty, value); }
+            get => GetValue(AngleProperty);
+            set => SetValue(AngleProperty, value);
+        }
+
+        /// <summary>
+        /// Gets or sets the x-coordinate of the rotation center point. The default is 0.
+        /// </summary>
+        public double CenterX
+        {
+            get => GetValue(CenterXProperty);
+            set => SetValue(CenterXProperty, value);
+        }
+
+        /// <summary>
+        /// Gets or sets the y-coordinate of the rotation center point. The default is 0.
+        /// </summary>
+        public double CenterY
+        {
+            get => GetValue(CenterYProperty);
+            set => SetValue(CenterYProperty, value);
         }
 
         /// <summary>
         /// Gets the transform's <see cref="Matrix"/>.
         /// </summary>
-        public override Matrix Value => Matrix.CreateRotation(Matrix.ToRadians(Angle));
+        public override Matrix Value => Matrix.CreateTranslation(-CenterX, -CenterY) *
+            Matrix.CreateRotation(Matrix.ToRadians(Angle)) *
+            Matrix.CreateTranslation(CenterX, CenterY);
     }
 }