[Xwt] Allow Gradient instances to be shared among toolkits using InitForToolkit pattern

Xwt.Gtk/Xwt.CairoBackend/CairoContextBackendHandler.cs

@@ -265,8 +265,10 @@ namespace Xwt.CairoBackend
 		public override void SetPattern (object backend, object p)
 		{
 			var cb = (CairoContextBackend)backend;
+			var toolkit = ApplicationContext.Toolkit;
 
 			Cairo.Context ctx = cb.Context;
+			p = toolkit.GetSafeBackend (p);
 			if (p is ImagePatternBackend) {
 				cb.PatternAlpha = ((ImagePatternBackend)p).Image.Alpha;
 				p = ((ImagePatternBackend)p).GetPattern (ApplicationContext, ((CairoContextBackend)backend).ScaleFactor);

Xwt.WPF/Xwt.WPFBackend/ContextBackendHandler.cs

@@ -245,6 +245,8 @@ namespace Xwt.WPFBackend
 		public override void SetPattern (object backend, object p)
 		{
 			var c = (DrawingContext) backend;
+			var toolkit = ApplicationContext.Toolkit;
+			p = toolkit.GetSafeBackend (p);
 			if (p is ImagePattern)
 				p = ((ImagePattern)p).GetBrush (c.ScaleFactor);
 			c.SetPattern ((System.Windows.Media.Brush)p);

Xwt.XamMac/Xwt.Mac/ContextBackendHandler.cs

@@ -241,7 +241,8 @@ namespace Xwt.Mac
 		public override void SetPattern (object backend, object p)
 		{
 			CGContextBackend gc = (CGContextBackend)backend;
-			gc.CurrentStatus.Pattern = p;
+			var toolkit = ApplicationContext.Toolkit;
+			gc.CurrentStatus.Pattern = toolkit.GetSafeBackend (p);
 		}
 
 		void SetupPattern (CGContextBackend gc)

Xwt/Xwt.Drawing/Context.cs

@@ -335,7 +335,7 @@ namespace Xwt.Drawing
 			get { return pattern; }
 			set {
 				pattern = value;
-				handler.SetPattern (Backend, GetBackend (value));
+				handler.SetPattern (Backend, value);
 			}
 		}
 		

Xwt/Xwt.Drawing/Gradient.cs

@@ -25,6 +25,7 @@
 // THE SOFTWARE.
 
 using System;
+using System.Collections.Generic;
 using Xwt.Backends;
 
 
@@ -32,11 +33,36 @@ namespace Xwt.Drawing
 {
 	public abstract class Gradient: Pattern
 	{
+		List<KeyValuePair<double, Color>> stops;
+
 		public Gradient AddColorStop (double pos, Color color)
 		{
 			ToolkitEngine.GradientBackendHandler.AddColorStop (Backend, pos, color);
+			(stops ?? (stops = new List<KeyValuePair<double, Color>> ())).Add (new KeyValuePair<double, Color> (pos, color));
 			return this;
 		}
+
+		public void InitForToolkit (Toolkit t)
+		{
+			if (ToolkitEngine != t) {
+				var handler = t.GradientBackendHandler;
+				var backend = CreateGradientBackend (handler);
+				SetBackend (handler, backend);
+				if (stops != null)
+					foreach (var stop in stops)
+						handler.AddColorStop (backend, stop.Key, stop.Value);
+			}
+		}
+
+		protected override object OnCreateBackend ()
+		{
+			var handler = ToolkitEngine.GradientBackendHandler;
+			var backend = CreateGradientBackend (handler);
+			SetBackend (handler, backend);
+			return backend;
+		}
+
+		protected abstract object CreateGradientBackend (GradientBackendHandler handler);
 	}
 }
 

Xwt/Xwt.Drawing/LinearGradient.cs

@@ -31,9 +31,19 @@ namespace Xwt.Drawing
 {
 	public sealed class LinearGradient: Gradient
 	{
+		double xStart, yStart, xEnd, yEnd;
+
 		public LinearGradient (double xStart, double yStart, double xEnd, double yEnd)
 		{
-			SetBackend (ToolkitEngine.GradientBackendHandler, ToolkitEngine.GradientBackendHandler.CreateLinear (xStart, yStart, xEnd, yEnd));
+			this.xStart = xStart;
+			this.xEnd = xEnd;
+			this.yStart = yStart;
+			this.yEnd = yEnd;
+		}
+
+		protected override object CreateGradientBackend (GradientBackendHandler handler)
+		{
+			return handler.CreateLinear (xStart, yStart, xEnd, yEnd);
 		}
 	}
 }

Xwt/Xwt.Drawing/Pattern.cs

@@ -39,6 +39,10 @@ namespace Xwt.Drawing
 
 		internal void SetBackend (DisposableResourceBackendHandler handler, object backend)
 		{
+			if (this.handler != null) {
+				Dispose ();
+				GC.ReRegisterForFinalize (this);
+			}
 			Backend = backend;
 			this.handler = handler;
 			if (handler.DisposeHandleOnUiThread)

Xwt/Xwt.Drawing/RadialGradient.cs

@@ -31,9 +31,21 @@ namespace Xwt.Drawing
 {
 	public sealed class RadialGradient: Gradient
 	{
+		double cx0, cy0, radius0, cx1, cy1, radius1;
+
 		public RadialGradient (double cx0, double cy0, double radius0, double cx1, double cy1, double radius1)
 		{
-			SetBackend (ToolkitEngine.GradientBackendHandler, ToolkitEngine.GradientBackendHandler.CreateRadial (cx0, cy0, radius0, cx1, cy1, radius1));
+			this.cx0 = cx0;
+			this.cy0 = cy0;
+			this.radius0 = radius0;
+			this.cx1 = cx1;
+			this.cy1 = cy1;
+			this.radius1 = radius1;
+		}
+
+		protected override object CreateGradientBackend (GradientBackendHandler handler)
+		{
+			return handler.CreateRadial (cx0, cy0, radius0, cx1, cy1, radius1);
 		}
 	}
 }

Xwt/Xwt/Toolkit.cs

@@ -525,6 +525,8 @@ namespace Xwt
 				}
 
 				font.InitForToolkit (this);
+			} else if (obj is Gradient) {
+				((Gradient)obj).InitForToolkit (this);
 			} else if (obj is IFrontend) {
 				if (((IFrontend)obj).ToolkitEngine != this)
 					throw new InvalidOperationException ("Object belongs to a different toolkit");