[android] new IImageViewHandler API (#3045)

* [android] new IImageViewHandler API

Context:
https://github.com/bumptech/glide
https://github.com/jonathanpeppers/glidex

Currently the way my "proof-of-concept" GlideX library works by
completely bypassing `IImageSourceHandler`. GlideX provides its own
custom `ImageRenderer` and `ImageCellRenderer`. This was required due
to how *opinionated* the Glide library is. Glide's approach is to
never allow a developer access to a `Android.Graphics.Bitmap` or
`Android.Graphics.Drawable` because we would likely do it wrong... and
developers do all the time!

To evolve Xamarin.Forms to where images could be better handled down
the road, I've introduced a new interface:

    public interface IImageViewHandler : IRegisterable
    {
        Task LoadImageAsync(ImageSource imageSource, ImageView imageView, CancellationToken cancellationToken = default(CancellationToken));
    }

The idea is that we can query for `IImageViewHandler` and fall back to
`IImageSourceHandler`. This would allow GlideX to just be an
`IImageViewHandler` and not mess with creating custom renderers.

We can also implement `IImageViewHandler` for `FileImageSource`, to
get some general performance improvements around loading files from
disk:

    string file = ((FileImageSource)imagesource).File;
    if (File.Exists(file))
    {
        //Load with imageView.SetImageURI(Android.Net.Uri)
    }
    else
    {
        //Load with imageView.SetImageResource(int)
    }

I tested this change with new performance tests in `ImageScenarios`:
- Load 100 images using `AndroidResource`
- Load 100 images from disk
- I conveniently prefixed these with `.`, so they appeared first in
  the scenario list

Here are the results from three runs, using `IImageSourceHandler`
versus `IImageViewHandler`, in a HAXM emulator:

`IImageSourceHandler` - 100x `AndroidResource`
- 6059.899
- 3297.885
- 3015.179

`IImageSourceHandler` - 100x from disk
- 12398.71
- 14146.41
- 16060.88

`IImageViewHandler` - 100x `AndroidResource`
- 6748.766
- 2817.975
- 2456.197

`IImageViewHandler` - 100x from disk
- 7326.745
- 4799.001
- 5411.515

There is not going to be as much as an improvement for
`AndroidResource` (maybe not any), since Xamarin.Forms looks for
`Drawable` since: https://github.com/xamarin/Xamarin.Forms/pull/1973

NOTE: that these scenarios are probably too slow to keep, as it seems
these performance tests are geared to fail if they take longer than
250ms. I can remove these before we merge this PR.

Other changes to make this happen:
- `Registrar` was failing with `InvalidCastException` instead of
  returning `null` when querying for `IImageViewHandler`. I switched
  to using `as` instead of a hard cast. I am not sure why it was not
  already setup this way, since
  `Registrar.Registered.GetHandlerForObject<T>` appears to be called
  and checked for `null` everywhere.

* Changes to get this puppy "mergeable"

- Removed code from #1973 that is obsolete, we should not be able to hit that codepath anymore
- Restored: e8660383b0/Xamarin.Forms.Platform.Android/Extensions/ImageViewExtensions.cs (L55)
- Used `SetImageDrawable` and `Drawable` so that `Bugzilla51173_InvalidImage` should pass now
- Made adjustments to the perf scenarios

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/PerformanceGallery/Scenarios/ImageScenarios.cs

@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Linq;
 using Xamarin.Forms.Internals;
 
@@ -23,4 +24,50 @@ namespace Xamarin.Forms.Controls.GalleryPages.PerformanceGallery.Scenarios
 			View = new Image { Source = "coffee.png" };
 		}
 	}
+
+	[Preserve(AllMembers = true)]
+	internal class ImageScenario3 : PerformanceScenario
+	{
+		const int count = 5;
+
+		public ImageScenario3()
+		: base($"[Image] {count}x AndroidResource")
+		{
+			var source = ImageSource.FromFile("bank.png");
+			var layout = new StackLayout();
+			for (int i = 0; i < count; i++)
+			{
+				layout.Children.Add(new Image { Source = source, HeightRequest = 20 });
+			}
+			View = layout;
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	internal class ImageScenario4 : PerformanceScenario
+	{
+		const int count = 5;
+		static readonly string tempFile;
+
+		static ImageScenario4()
+		{
+			//NOTE: copy image to disk in static ctor, so not to interfere with timing
+			tempFile = Path.Combine(Path.GetTempPath(), $"{nameof(ImageScenario4)}.png");
+			using (var embeddedStream = typeof(ImageScenario4).Assembly.GetManifestResourceStream("Xamarin.Forms.Controls.GalleryPages.crimson.jpg"))
+			using (var fileStream = File.Create(tempFile))
+				embeddedStream.CopyTo(fileStream);
+		}
+
+		public ImageScenario4()
+		: base($"[Image] {count}x from disk")
+		{
+			var source = ImageSource.FromFile(tempFile);
+			var layout = new StackLayout();
+			for (int i = 0; i < count; i++)
+			{
+				layout.Children.Add(new Image { Source = source, HeightRequest = 20 });
+			}
+			View = layout;
+		}
+	}
 }

Xamarin.Forms.Core/Registrar.cs

@@ -53,17 +53,17 @@ namespace Xamarin.Forms.Internals
 			return (TRegistrable)DependencyResolver.ResolveOrCreate(handlerType, args);
 		}
 
-		public TOut GetHandler<TOut>(Type type) where TOut : TRegistrable
+		public TOut GetHandler<TOut>(Type type) where TOut : class, TRegistrable
 		{
-			return (TOut)GetHandler(type);
+			return GetHandler(type) as TOut;
 		}
 
-		public TOut GetHandler<TOut>(Type type, params object[] args) where TOut : TRegistrable
+		public TOut GetHandler<TOut>(Type type, params object[] args) where TOut : class, TRegistrable
 		{
-			return (TOut)GetHandler(type, args);
+			return GetHandler(type, args) as TOut;
 		}
 
-		public TOut GetHandlerForObject<TOut>(object obj) where TOut : TRegistrable
+		public TOut GetHandlerForObject<TOut>(object obj) where TOut : class, TRegistrable
 		{
 			if (obj == null)
 				throw new ArgumentNullException(nameof(obj));
@@ -71,10 +71,10 @@ namespace Xamarin.Forms.Internals
 			var reflectableType = obj as IReflectableType;
 			var type = reflectableType != null ? reflectableType.GetTypeInfo().AsType() : obj.GetType();
 
-			return (TOut)GetHandler(type);
+			return GetHandler(type) as TOut;
 		}
 
-		public TOut GetHandlerForObject<TOut>(object obj, params object[] args) where TOut : TRegistrable
+		public TOut GetHandlerForObject<TOut>(object obj, params object[] args) where TOut : class, TRegistrable
 		{
 			if (obj == null)
 				throw new ArgumentNullException(nameof(obj));
@@ -82,7 +82,7 @@ namespace Xamarin.Forms.Internals
 			var reflectableType = obj as IReflectableType;
 			var type = reflectableType != null ? reflectableType.GetTypeInfo().AsType() : obj.GetType();
 
-			return (TOut)GetHandler(type, args);
+			return GetHandler(type, args) as TOut;
 		}
 
 		public Type GetHandlerType(Type viewType)

Xamarin.Forms.Platform.Android/Extensions/ImageViewExtensions.cs

@@ -31,23 +31,30 @@ namespace Xamarin.Forms.Platform.Android
 
 			imageView.SetImageResource(global::Android.Resource.Color.Transparent);
 
+			bool setByImageViewHandler = false;
 			Bitmap bitmap = null;
-			Drawable drawable = null;
 
-			IImageSourceHandler handler;
+			if (source != null)
-
-			if (source != null && (handler = Internals.Registrar.Registered.GetHandlerForObject<IImageSourceHandler>(source)) != null)
 			{
-				if (handler is FileImageSourceHandler)
+				var imageViewHandler = Internals.Registrar.Registered.GetHandlerForObject<IImageViewHandler>(source);
-				{
+				if (imageViewHandler != null)
-					drawable = imageView.Context.GetDrawable((FileImageSource)source);
-				}
-
-				if (drawable == null)
 				{
 					try
 					{
-						bitmap = await handler.LoadImageAsync(source, imageView.Context);
+						await imageViewHandler.LoadImageAsync(source, imageView);
+						setByImageViewHandler = true;
+					}
+					catch (TaskCanceledException)
+					{
+						imageController?.SetIsLoading(false);
+					}
+				}
+				else
+				{
+					var imageSourceHandler = Internals.Registrar.Registered.GetHandlerForObject<IImageSourceHandler>(source);
+					try
+					{
+						bitmap = await imageSourceHandler.LoadImageAsync(source, imageView.Context);
 					}
 					catch (TaskCanceledException)
 					{
@@ -63,12 +70,10 @@ namespace Xamarin.Forms.Platform.Android
 				return;
 			}
 
-			if (!imageView.IsDisposed())
+			if (!setByImageViewHandler && !imageView.IsDisposed())
 			{
-				if (bitmap == null && drawable != null)
+				if (bitmap == null && source is FileImageSource)
-				{
+					imageView.SetImageResource(ResourceManager.GetDrawableByName(((FileImageSource)source).File));
-					imageView.SetImageDrawable(drawable);
-				}
 				else
 				{
 					imageView.SetImageBitmap(bitmap);

Xamarin.Forms.Platform.Android/Renderers/FileImageSourceHandler.cs

@@ -3,11 +3,13 @@ using System.Threading;
 using System.Threading.Tasks;
 using Android.Content;
 using Android.Graphics;
+using Android.Widget;
+using Android.Net;
 using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms.Platform.Android
 {
-	public sealed class FileImageSourceHandler : IImageSourceHandler
+	public sealed class FileImageSourceHandler : IImageSourceHandler, IImageViewHandler
 	{
 		// This is set to true when run under designer context
 		internal static bool DecodeSynchronously {
@@ -31,5 +33,28 @@ namespace Xamarin.Forms.Platform.Android
 
 			return bitmap;
 		}
+
+		public Task LoadImageAsync(ImageSource imagesource, ImageView imageView, CancellationToken cancellationToken = default(CancellationToken))
+		{
+			string file = ((FileImageSource)imagesource).File;
+			if (File.Exists(file))
+			{
+				var uri = Uri.Parse(file);
+				if (uri != null)
+					imageView.SetImageURI(uri);
+				else
+					Log.Warning(nameof(FileImageSourceHandler), "Could not find image or image file was invalid: {0}", imagesource);
+			}
+			else
+			{
+				var drawable = ResourceManager.GetDrawable(imageView.Context, file);
+				if (drawable != null)
+					imageView.SetImageDrawable(drawable);
+				else
+					Log.Warning(nameof(FileImageSourceHandler), "Could not find image or image file was invalid: {0}", imagesource);
+			}
+
+			return Task.FromResult(true);
+		}
 	}
 }

Xamarin.Forms.Platform.Android/Renderers/IImageViewHandler.cs

@@ -0,0 +1,14 @@
+using System.Threading;
+using System.Threading.Tasks;
+using Android.Widget;
+
+namespace Xamarin.Forms.Platform.Android
+{
+	/// <summary>
+	/// The successor to IImageSourceHandler, the goal being we can achieve better performance by never creating an Android.Graphics.Bitmap instance
+	/// </summary>
+	public interface IImageViewHandler : IRegisterable
+	{
+		Task LoadImageAsync(ImageSource imagesource, ImageView imageView, CancellationToken cancellationToken = default(CancellationToken));
+	}
+}

Xamarin.Forms.Platform.Android/Xamarin.Forms.Platform.Android.csproj

@@ -165,6 +165,7 @@
     <Compile Include="ContextExtensions.cs" />
     <Compile Include="GetDesiredSizeDelegate.cs" />
     <Compile Include="IDeviceInfoProvider.cs" />
+    <Compile Include="Renderers\IImageViewHandler.cs" />
     <Compile Include="InnerGestureListener.cs" />
     <Compile Include="InnerScaleListener.cs" />
     <Compile Include="IPlatformLayout.cs" />