Better error handling for image loading errors on iOS/Android (#849)

* First run at removing async void image update methods

Consistent error logging and IsLoading on Android,iOS,UWP

Move error logging into image handlers for better messages

Add demo of custom ImageRenderer error handling

Update docs

Make the test smaller so the results don't get pushed offscreen

Fix namespace error

* Update error handling for fast image renderer

* Update 37625 test to use image we control

* Add java disposed check to avoid ObjectDisposedException in async operations

* Add disposed checks to legacy renderer; null check element before SetIsLoading

* Check disposed on GetDesiredSize for fast renderer
Use local disposed member where possible for disposed check

* Check for disposal after async handlers in iOS

* Add disposal checks after async methods in Windows

* Reset linker settings on project; reduce redundant casts in ImageViewExtensions

1 files changed, 38 insertions, 0 deletions

diff --git a/Xamarin.Forms.ControlGallery.Android/BrokenImageSourceHandler.cs b/Xamarin.Forms.ControlGallery.Android/BrokenImageSourceHandler.cs
new file mode 100644
index 00000000..390d77c9
--- /dev/null
+++ b/Xamarin.Forms.ControlGallery.Android/BrokenImageSourceHandler.cs
@@ -0,0 +1,38 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Android.Content;
+using Android.Graphics;
+using Xamarin.Forms;
+using Xamarin.Forms.ControlGallery.Android;
+using Xamarin.Forms.Controls.Issues;
+using Xamarin.Forms.Platform.Android;
+
+[assembly: ExportRenderer(typeof(_51173Image), typeof(_51173CustomImageRenderer))]
+namespace Xamarin.Forms.ControlGallery.Android
+{
+	public sealed class BrokenImageSourceHandler : IImageSourceHandler
+	{
+		public Task<Bitmap> LoadImageAsync(ImageSource imagesource, Context context, CancellationToken cancelationToken = default(CancellationToken))
+		{
+			throw new Exception("Fail");
+		}
+	}
+
+	public class _51173CustomImageRenderer : ImageRenderer
+	{
+		protected override async Task TryUpdateBitmap(Image previous = null)
+		{
+			try
+			{
+				await UpdateBitmap(previous).ConfigureAwait(false);
+			}
+			catch (Exception ex)
+			{
+				await Application.Current.MainPage.DisplayAlert("Image Error 51173", $"The image failed to load, here's why: {ex.Message}", "OK");
+			}
+		}
+	}
+}
+
+