Added PositionChanged event in TopLevel

Implements #629

src/Android/Avalonia.Android/Platform/SkiaPlatform/WindowImpl.cs

@@ -87,6 +87,8 @@ namespace Avalonia.Android.Platform.SkiaPlatform
 
         public Action<double> ScalingChanged { get; set; }
 
+        public Action<Point> PositionChanged { get; set; }
+
         public View View => this;
 
         Action ITopLevelImpl.Activated { get; set; }

src/Avalonia.Controls/Avalonia.Controls.csproj

@@ -56,6 +56,7 @@
     <Compile Include="HotkeyManager.cs" />
     <Compile Include="IApplicationLifecycle.cs" />
     <Compile Include="IScrollable.cs" />
+    <Compile Include="PositionChangedEventArgs.cs" />
     <Compile Include="WindowIcon.cs" />
     <Compile Include="IPseudoClasses.cs" />
     <Compile Include="DropDownItem.cs" />

src/Avalonia.Controls/Platform/ITopLevelImpl.cs

@@ -67,6 +67,11 @@ namespace Avalonia.Platform
         /// </summary>
         Action<double> ScalingChanged { get; set; }
 
+        /// <summary>
+        /// Gets or sets a method called when the window's position changes.
+        /// </summary>
+        Action<Point> PositionChanged { get; set; }
+
         /// <summary>
         /// Activates the window.
         /// </summary>

src/Avalonia.Controls/PositionChangedEventArgs.cs

@@ -0,0 +1,27 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+
+namespace Avalonia.Controls
+{
+    /// <summary>
+    /// Provides data for the <see cref="TopLevel.PositionChanged"/> event.
+    /// </summary>
+    public class PositionChangedEventArgs : EventArgs
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PositionChangedEventArgs"/> class.
+        /// </summary>
+        /// <param name="newPos">The new window position.</param>
+        public PositionChangedEventArgs(Point newPos)
+        {
+            NewPosition = newPos;
+        }
+
+        /// <summary>
+        /// Gets the new window position.
+        /// </summary>
+        public Point NewPosition { get; }
+    }
+}

src/Avalonia.Controls/TopLevel.cs

@@ -105,6 +105,7 @@ namespace Avalonia.Controls
             PlatformImpl.Input = HandleInput;
             PlatformImpl.Resized = HandleResized;
             PlatformImpl.ScalingChanged = HandleScalingChanged;
+            PlatformImpl.PositionChanged = HandlePositionChanged;
 
             _keyboardNavigationHandler?.SetOwner(this);
             _accessKeyHandler?.SetOwner(this);
@@ -138,6 +139,11 @@ namespace Avalonia.Controls
         /// </summary>
         public event EventHandler Deactivated;
 
+        /// <summary>
+        /// Fired when the window position is changed.
+        /// </summary>
+        public event EventHandler<PositionChangedEventArgs> PositionChanged;
+
         /// <summary>
         /// Gets or sets the client size of the window.
         /// </summary>
@@ -395,6 +401,16 @@ namespace Avalonia.Controls
             _inputManager.ProcessInput(e);
         }
 
+        /// <summary>
+        /// Handles a window position change notification from 
+        /// <see cref="ITopLevelImpl.PositionChanged"/>.
+        /// </summary>
+        /// <param name="pos">The window position.</param>
+        private void HandlePositionChanged(Point pos)
+        {
+            PositionChanged?.Invoke(this, new PositionChangedEventArgs(pos));
+        }
+
         /// <summary>
         /// Starts moving a window with left button being held. Should be called from left mouse button press event handler
         /// </summary>

src/Gtk/Avalonia.Gtk/WindowImpl.cs

@@ -23,6 +23,8 @@ namespace Avalonia.Gtk
 
         private Size _lastClientSize;
 
+        private Point _lastPosition;
+
         private Gtk.IMContext _imContext;
 
         private uint _lastKeyEventTimestamp;
@@ -52,6 +54,7 @@ namespace Avalonia.Gtk
             DoubleBuffered = false;
             Realize();
             _lastClientSize = ClientSize;
+            _lastPosition = Position;
         }
 
         protected override void OnRealized ()
@@ -175,6 +178,8 @@ namespace Avalonia.Gtk
 
         public Action<double> ScalingChanged { get; set; }
 
+        public Action<Point> PositionChanged { get; set; }
+
         public IPopupImpl CreatePopup()
         {
             return new PopupImpl();
@@ -338,6 +343,14 @@ namespace Avalonia.Gtk
                 _lastClientSize = newSize;
             }
 
+            var newPosition = new Point(evnt.X, evnt.Y);
+
+            if (newPosition != _lastPosition)
+            {
+                PositionChanged(newPosition);
+                _lastPosition = newPosition;
+            }
+
             return true;
         }
 

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -55,6 +55,8 @@ namespace Avalonia.Win32
 
         public Action<double> ScalingChanged { get; set; }
 
+        public Action<Point> PositionChanged { get; set; }
+
         public Thickness BorderThickness
         {
             get
@@ -561,6 +563,10 @@ namespace Avalonia.Win32
                     }
 
                     return IntPtr.Zero;
+
+                case UnmanagedMethods.WindowsMessage.WM_MOVE:
+                    PositionChanged?.Invoke(new Point(ToInt32(lParam) & 0xffff, ToInt32(lParam) >> 16));
+                    return IntPtr.Zero;
             }
 
             if (e != null && Input != null)

src/iOS/Avalonia.iOS/AvaloniaView.cs

@@ -27,6 +27,7 @@ namespace Avalonia.iOS
         private readonly UIViewController _controller;
         private IInputRoot _inputRoot;
         private readonly KeyboardEventsHelper<AvaloniaView> _keyboardHelper;
+        private Point _position;
 
         public AvaloniaView(UIWindow window, UIViewController controller) : base(onFrame => PlatformThreadingInterface.Instance.Render = onFrame)
         {
@@ -72,6 +73,7 @@ namespace Avalonia.iOS
         public Action<Rect> Paint { get; set; }
         public Action<Size> Resized { get; set; }
         public Action<double> ScalingChanged { get; set; }
+        public Action<Point> PositionChanged { get; set; }
 
         public IPlatformHandle Handle => AvaloniaPlatformHandle;
 
@@ -136,7 +138,15 @@ namespace Avalonia.iOS
             //Not supported
         }
 
-        public Point Position { get; set; }
+        public Point Position
+        {
+            get { return _position; }
+            set
+            {
+                _position = value;
+                PositionChanged?.Invoke(_position);
+            }
+        }
 
         public Size MaxClientSize => Bounds.Size.ToAvalonia();
         public void SetTitle(string title)