Implemented BeginResizeDrag

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

@@ -9,6 +9,7 @@ using Perspex.Input.Raw;
 using Perspex.Platform;
 using Perspex.Skia.Android;
 using System;
+using Perspex.Controls;
 
 namespace Perspex.Android.Platform.SkiaPlatform
 {
@@ -131,6 +132,11 @@ namespace Perspex.Android.Platform.SkiaPlatform
             //Not supported
         }
 
+        public void BeginResizeDrag(WindowEdge edge)
+        {
+            //Not supported
+        }
+
         public IDisposable ShowDialog()
         {
             throw new NotImplementedException();

src/Gtk/Perspex.Gtk/WindowImpl.cs

@@ -11,6 +11,7 @@ using Perspex.Platform;
 using Perspex.Input;
 using Perspex.Threading;
 using Action = System.Action;
+using WindowEdge = Perspex.Controls.WindowEdge;
 
 namespace Perspex.Gtk
 {
@@ -170,6 +171,14 @@ namespace Perspex.Gtk
             BeginMoveDrag(1, x, y, 0);
         }
 
+        public void BeginResizeDrag(WindowEdge edge)
+        {
+            int x, y;
+            ModifierType mod;
+            Screen.RootWindow.GetPointer(out x, out y, out mod);
+            BeginResizeDrag((Gdk.WindowEdge) (int) edge, 1, x, y, 0);
+        }
+
         public IDisposable ShowDialog()
         {
             Modal = true;

src/Perspex.Controls/Perspex.Controls.csproj

@@ -171,6 +171,7 @@
     <Compile Include="TreeView.cs" />
     <Compile Include="TreeViewItem.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="WindowEdge.cs" />
     <Compile Include="WrapPanel.cs" />
   </ItemGroup>
   <ItemGroup>

src/Perspex.Controls/Platform/ITopLevelImpl.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
+using Perspex.Controls;
 using Perspex.Input;
 using Perspex.Input.Raw;
 
@@ -90,8 +91,14 @@ namespace Perspex.Platform
         void Show();
 
         /// <summary>
-        /// Starts moving a window with left button being held. Should be called from left mouse button press event handler
+        /// Starts moving a window with left button being held. Should be called from left mouse button press event handler.
         /// </summary>
         void BeginMoveDrag();
+
+        /// <summary>
+        /// Starts resizing a window. This function is used if an application has window resizing controls. 
+        /// Should be called from left mouse button press event handler
+        /// </summary>
+        void BeginResizeDrag(WindowEdge edge);
     }
 }

src/Perspex.Controls/Platform/PlatformManager.cs

@@ -175,6 +175,7 @@ namespace Perspex.Controls.Platform
 
             public void Show() => _tl.Show();
             public void BeginMoveDrag() => _tl.BeginMoveDrag();
+            public void BeginResizeDrag(WindowEdge edge) => _tl.BeginResizeDrag(edge);
 
             public IDisposable ShowDialog() => _window.ShowDialog();
 

src/Perspex.Controls/TopLevel.cs

@@ -369,5 +369,11 @@ namespace Perspex.Controls
         /// Starts moving a window with left button being held. Should be called from left mouse button press event handler
         /// </summary>
         public void BeginMoveDrag() => PlatformImpl.BeginMoveDrag();
+
+        /// <summary>
+        /// Starts resizing a window. This function is used if an application has window resizing controls. 
+        /// Should be called from left mouse button press event handler
+        /// </summary>
+        public void BeginResizeDrag(WindowEdge edge) => PlatformImpl.BeginResizeDrag(edge);
     }
 }

src/Perspex.Controls/WindowEdge.cs

@@ -0,0 +1,17 @@
+namespace Perspex.Controls
+{
+    
+    public enum WindowEdge
+    {
+        //Please don't reorder stuff here, I was lazy to write proper conversion code
+        //so the order of values is matching one from GTK
+        NorthWest = 0,
+        North,
+        NorthEast,
+        West,
+        East,
+        SouthWest,
+        South,
+        SouthEast,
+    }
+}

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

@@ -217,6 +217,34 @@ namespace Perspex.Win32.Interop
             WA_CLICKACTIVE,
         }
 
+        public enum HitTestValues
+        {
+            HTERROR = -2,
+            HTTRANSPARENT = -1,
+            HTNOWHERE = 0,
+            HTCLIENT = 1,
+            HTCAPTION = 2,
+            HTSYSMENU = 3,
+            HTGROWBOX = 4,
+            HTMENU = 5,
+            HTHSCROLL = 6,
+            HTVSCROLL = 7,
+            HTMINBUTTON = 8,
+            HTMAXBUTTON = 9,
+            HTLEFT = 10,
+            HTRIGHT = 11,
+            HTTOP = 12,
+            HTTOPLEFT = 13,
+            HTTOPRIGHT = 14,
+            HTBOTTOM = 15,
+            HTBOTTOMLEFT = 16,
+            HTBOTTOMRIGHT = 17,
+            HTBORDER = 18,
+            HTOBJECT = 19,
+            HTCLOSE = 20,
+            HTHELP = 21
+        }
+
         [Flags]
         public enum WindowStyles : uint
         {

src/Windows/Perspex.Win32/WindowImpl.cs

@@ -226,7 +226,25 @@ namespace Perspex.Win32
         public void BeginMoveDrag()
         {
             UnmanagedMethods.DefWindowProc(_hwnd, (int) UnmanagedMethods.WindowsMessage.WM_NCLBUTTONDOWN,
-                new IntPtr(2), IntPtr.Zero);
+                new IntPtr((int)UnmanagedMethods.HitTestValues.HTCAPTION), IntPtr.Zero);
+        }
+
+        static readonly Dictionary<WindowEdge, UnmanagedMethods.HitTestValues> EdgeDic = new Dictionary<WindowEdge, UnmanagedMethods.HitTestValues>
+        {
+            {WindowEdge.East, UnmanagedMethods.HitTestValues.HTRIGHT},
+            {WindowEdge.North, UnmanagedMethods.HitTestValues.HTTOP },
+            {WindowEdge.NorthEast, UnmanagedMethods.HitTestValues.HTTOPRIGHT },
+            {WindowEdge.NorthWest, UnmanagedMethods.HitTestValues.HTTOPLEFT },
+            {WindowEdge.South, UnmanagedMethods.HitTestValues.HTBOTTOM },
+            {WindowEdge.SouthEast, UnmanagedMethods.HitTestValues.HTBOTTOMRIGHT },
+            {WindowEdge.SouthWest, UnmanagedMethods.HitTestValues.HTBOTTOMLEFT },
+            {WindowEdge.West, UnmanagedMethods.HitTestValues.HTLEFT}
+        };
+
+        public void BeginResizeDrag(WindowEdge edge)
+        {
+            UnmanagedMethods.DefWindowProc(_hwnd, (int) UnmanagedMethods.WindowsMessage.WM_NCLBUTTONDOWN,
+                new IntPtr((int) EdgeDic[edge]), IntPtr.Zero);
         }
 
         public virtual IDisposable ShowDialog()

src/iOS/Perspex.iOS/PerspexView.cs

@@ -16,6 +16,7 @@ using Perspex.Skia.iOS;
 using UIKit;
 using Perspex.iOS.Specific;
 using ObjCRuntime;
+using Perspex.Controls;
 
 namespace Perspex.iOS
 {
@@ -112,6 +113,11 @@ namespace Perspex.iOS
             //Not supported
         }
 
+        public void BeginResizeDrag(WindowEdge edge)
+        {
+            //Not supported
+        }
+
         public Size MaxClientSize => Bounds.Size.ToPerspex();
         public void SetTitle(string title)
         {