Added the GL view to the Forms sample xaml, and hooked it up

samples/SkiaSharpSample.FormsSample/SkiaSharpSample.FormsSample/DetailContentsPage.xaml

@@ -3,9 +3,16 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
              xmlns:views="clr-namespace:SkiaSharp.Views.Forms;assembly=SkiaSharp.Views.Forms"
              x:Class="SkiaSharpSample.FormsSample.DetailContentsPage">
-  <views:SKCanvasView x:Name="canvas" PaintSurface="OnPaintSample">
-    <views:SKCanvasView.GestureRecognizers>
-      <TapGestureRecognizer Tapped="OnTapSample" />
-    </views:SKCanvasView.GestureRecognizers>
-  </views:SKCanvasView>
+  <Grid>
+    <views:SKCanvasView x:Name="canvas" PaintSurface="OnPaintSample">
+      <views:SKCanvasView.GestureRecognizers>
+        <TapGestureRecognizer Tapped="OnTapSample" />
+      </views:SKCanvasView.GestureRecognizers>
+    </views:SKCanvasView>
+    <views:SKGLView x:Name="glview" PaintSurface="OnPaintGLSample" IsVisible="False" HasRenderLoop="False">
+      <views:SKGLView.GestureRecognizers>
+        <TapGestureRecognizer Tapped="OnTapSample" />
+      </views:SKGLView.GestureRecognizers>
+    </views:SKGLView>
+  </Grid>
 </ContentPage>

samples/SkiaSharpSample.FormsSample/SkiaSharpSample.FormsSample/DetailContentsPage.xaml.cs

@@ -24,18 +24,32 @@ namespace SkiaSharpSample.FormsSample
 			{
 				sample = value;
 
-				sample.Init(canvas.InvalidateSurface);
+				sample.Init(RefreshSamples);
 
 				Title = sample.Title;
-				canvas.InvalidateSurface();
+				RefreshSamples();
 			}
 		}
 
 		public void SwitchBackend(SampleBackends backend)
 		{
-			// TODO: implement once the Forms GL is working
-
-			DisplayAlert("Configure Backend", "This functionality is not yet implemented.", "OK");
+			switch (backend)
+			{
+				case SampleBackends.Memory:
+					canvas.IsVisible = true;
+					canvas.InvalidateSurface();
+					glview.IsVisible = false;
+					break;
+				case SampleBackends.OpenGL:
+					glview.IsVisible = true;
+					glview.InvalidateSurface();
+					canvas.IsVisible = false;
+					break;
+				case SampleBackends.Vulkan:
+				default:
+					DisplayAlert("Configure Backend", "This functionality is not yet implemented.", "OK");
+					break;
+			}
 		}
 
 		private void OnTapSample(object sender, EventArgs e)
@@ -47,5 +61,22 @@ namespace SkiaSharpSample.FormsSample
 		{
 			Sample?.DrawSample(e.Surface.Canvas, e.Info.Width, e.Info.Height);
 		}
+
+		private void OnPaintGLSample(object sender, SKPaintGLSurfaceEventArgs e)
+		{
+			Sample?.DrawSample(e.Surface.Canvas, e.RenderTarget.Width, e.RenderTarget.Height);
+		}
+
+		private void RefreshSamples()
+		{
+			if (canvas.IsVisible)
+			{
+				canvas.InvalidateSurface();
+			}
+			if (glview.IsVisible)
+			{
+				glview.InvalidateSurface();
+			}
+		}
 	}
 }