Propagate task cancellation

Cancel TaskCompletionSource when supplied CT is cancelled (ie download request is cancelled by Uno Image). This is because UIL seems to internally cancel previous downloads if a new URI is requested for the same ImageView (without raising IImageListener callbacks), leading to tasks that never return, but are still awaited for subsequent requests to the same URI thanks to the concurrency-management logic.

Additions/ImageLoader.cs

@@ -98,54 +98,61 @@ namespace Com.Nostra13.Universalimageloader.Core
 		{
 			TaskCompletionSource<Android.Graphics.Bitmap> source = new TaskCompletionSource<Android.Graphics.Bitmap>();
 
+
 			options = options ?? new DisplayImageOptions.Builder()
 				.CacheInMemory(true)
 				.CacheOnDisk(true)
 				.Build();
 
-			using (var aware =
-				imageView != null
-					? new ImageViewAwareCancellable(imageView, ct)
-					: null
-				)
+			// Propagate cancellation to the managed TCS, even though we don't propagate it to Universal Image Loader. This is because UIL has 
+			// the air of cancelling downloads itself if another uri is requested for the same ImageView, leading us to wait on a task that 
+			// will never complete (thanks to the concurrency logic reusing existing tasks).
+			using (ct.Register(() => source.TrySetCanceled()))
 			{
-				using (var listener = new ImageListener(source))
+				using (var aware =
+					imageView != null
+						? new ImageViewAwareCancellable(imageView, ct)
+						: null
+					)
 				{
-					if (targetSize != null && imageView != null)
+					using (var listener = new ImageListener(source))
 					{
-						imageView.SetMaxHeight(targetSize.Value.Height);
-						imageView.SetMaxWidth(targetSize.Value.Width);
+						if (targetSize != null && imageView != null)
+						{
+							imageView.SetMaxHeight(targetSize.Value.Height);
+							imageView.SetMaxWidth(targetSize.Value.Width);
 
-						ImageLoader.Instance.DisplayImage(
-							uri,
-							aware,
-							options,
-							listener
-							);
+							ImageLoader.Instance.DisplayImage(
+								uri,
+								aware,
+								options,
+								listener
+								);
+						}
+						else if (targetSize != null && imageView == null)
+						{
+							var targetImageSize = new ImageSize(targetSize.Value.Width, targetSize.Value.Height);
+
+							ImageLoader.Instance.LoadImage(
+								uri,
+								targetImageSize,
+								options,
+								listener
+								);
+						}
+						else
+						{
+							ImageLoader.Instance.LoadImage(
+								uri,
+								options,
+								listener
+								);
+						}
+
+						var target = await source.Task;
+
+						return target;
 					}
-					else if (targetSize != null && imageView == null)
-					{
-						var targetImageSize = new ImageSize(targetSize.Value.Width, targetSize.Value.Height);
-
-						ImageLoader.Instance.LoadImage(
-							uri,
-							targetImageSize,
-							options,
-							listener
-							);
-					}
-					else
-					{
-						ImageLoader.Instance.LoadImage(
-							uri,
-							options,
-							listener
-							);
-					}
-
-					var target = await source.Task;
-
-					return target;
 				}
 			}
 		}
@@ -225,7 +232,7 @@ namespace Com.Nostra13.Universalimageloader.Core
 
 			public void OnLoadingCancelled(string p0, Android.Views.View p1)
 			{
-				_source.SetCanceled();
+				_source.TrySetCanceled();
 			}
 
 			public void OnLoadingComplete(string p0, Android.Views.View p1, Android.Graphics.Bitmap bitmap)