Stop renderer on Window minimize.

Adds a new `WindowStateChanged` event to `IWindowImpl` and when a window is minimized, stops the associated renderer (only implemented on win32 currently).

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -16,6 +16,11 @@ namespace Avalonia.Platform
         /// </summary>
         WindowState WindowState { get; set; }
 
+        /// <summary>
+        /// Gets or sets a method called when the <see cref="WindowState"/> changes.
+        /// </summary>
+        Action<WindowState> WindowStateChanged { get; set; }
+
         /// <summary>
         /// Sets the title of the window.
         /// </summary>

src/Avalonia.Controls/Window.cs

@@ -136,6 +136,7 @@ namespace Avalonia.Controls
             : base(impl)
         {
             impl.Closing = HandleClosing;
+            impl.WindowStateChanged = HandleWindowStateChanged;
             _maxPlatformClientSize = PlatformImpl?.MaxClientSize ?? default(Size);
             Screens = new Screens(PlatformImpl?.Screen);
         }
@@ -308,6 +309,18 @@ namespace Avalonia.Controls
             return args.Cancel;
         }
 
+        protected virtual void HandleWindowStateChanged(WindowState state)
+        {
+            if (state == WindowState.Minimized)
+            {
+                Renderer.Stop();
+            }
+            else if (state == WindowState.Normal)
+            {
+                Renderer.Start();
+            }
+        }
+
         /// <summary>
         /// Hides the window but does not close it.
         /// </summary>

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -42,6 +42,7 @@ namespace Avalonia.DesignerSupport.Remote
         public Func<bool> Closing { get; set; }
         public IPlatformHandle Handle { get; }
         public WindowState WindowState { get; set; }
+        public Action<WindowState> WindowStateChanged { get; set; }
         public Size MaxClientSize { get; } = new Size(4096, 4096);
         public event Action LostFocus;
 

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -32,6 +32,8 @@ namespace Avalonia.DesignerSupport.Remote
         public Point Position { get; set; }
         public Action<Point> PositionChanged { get; set; }
         public WindowState WindowState { get; set; }
+        public Action<WindowState> WindowStateChanged { get; set; }
+
         public IRenderer CreateRenderer(IRenderRoot root) => new ImmediateRenderer(root);
         public void Dispose()
         {

src/Gtk/Avalonia.Gtk3/WindowImpl.cs

@@ -43,7 +43,9 @@ namespace Avalonia.Gtk3
                 }
             }
         }
-        
+
+        public Action<WindowState> WindowStateChanged { get; set; }
+
         public IDisposable ShowDialog()
         {
             Native.GtkWindowSetModal(GtkWidget, true);

src/OSX/Avalonia.MonoMac/WindowImpl.cs

@@ -49,6 +49,8 @@ namespace Avalonia.MonoMac
             }
         }
 
+        public Action<WindowState> WindowStateChanged { get; set; }
+
         bool IsZoomed => IsDecorated ? Window.IsZoomed : UndecoratedIsMaximized;
 
         public bool UndecoratedIsMaximized => Window.Frame == Window.Screen.VisibleFrame;

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -104,6 +104,30 @@ namespace Avalonia.Win32.Interop
             ForceMinimize = 11
         }
 
+        public enum SysCommands
+        {
+            SC_SIZE = 0xF000,
+            SC_MOVE = 0xF010,
+            SC_MINIMIZE = 0xF020,
+            SC_MAXIMIZE = 0xF030,
+            SC_NEXTWINDOW = 0xF040,
+            SC_PREVWINDOW = 0xF050,
+            SC_CLOSE = 0xF060,
+            SC_VSCROLL = 0xF070,
+            SC_HSCROLL = 0xF080,
+            SC_MOUSEMENU = 0xF090,
+            SC_KEYMENU = 0xF100,
+            SC_ARRANGE = 0xF110,
+            SC_RESTORE = 0xF120,
+            SC_TASKLIST = 0xF130,
+            SC_SCREENSAVE = 0xF140,
+            SC_HOTKEY = 0xF150,
+            SC_DEFAULT = 0xF160,
+            SC_MONITORPOWER = 0xF170,
+            SC_CONTEXTHELP = 0xF180,
+            SC_SEPARATOR = 0xF00F,
+        }
+
         public enum SystemMetric
         {
             SM_CXSCREEN = 0,  // 0x00

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -77,6 +77,8 @@ namespace Avalonia.Win32
 
         public Action<Point> PositionChanged { get; set; }
 
+        public Action<WindowState> WindowStateChanged { get; set; }
+
         public Thickness BorderThickness
         {
             get
@@ -627,6 +629,25 @@ namespace Avalonia.Win32
 
                     return IntPtr.Zero;
 
+                case UnmanagedMethods.WindowsMessage.WM_SYSCOMMAND:
+                    if (WindowStateChanged != null)
+                    {
+                        switch ((SysCommands)wParam)
+                        {
+                            case SysCommands.SC_MINIMIZE:
+                                WindowStateChanged(WindowState.Minimized);
+                                break;
+                            case SysCommands.SC_MAXIMIZE:
+                                WindowStateChanged(WindowState.Maximized);
+                                break;
+                            case SysCommands.SC_RESTORE:
+                                WindowStateChanged(WindowState.Normal);
+                                break;
+                        }
+                    }
+
+                    break;
+
                 case UnmanagedMethods.WindowsMessage.WM_MOVE:
                     PositionChanged?.Invoke(new Point((short)(ToInt32(lParam) & 0xffff), (short)(ToInt32(lParam) >> 16)));
                     return IntPtr.Zero;