How to make sure only one ImageSource load is in flight at a time

threadsafe_imagesource

// https://bugzilla.xamarin.com/show_bug.cgi?id=21981#c5

using Xamarin.Forms;

[assembly: ExportImageSourceHandler(typeof(UriImageSource), typeof(iOS.Renderers.ImageLoaderSourceHandler))]

namespace iOS.Renderers
{
    using System.Collections.Generic;
    using System.Threading;
    using System.Threading.Tasks;

    using MonoTouch.UIKit;

    using Xamarin.Forms;
    using Xamarin.Forms.Platform.iOS;

    public sealed class ImageLoaderSourceHandler : IImageSourceHandler
    {
        private static readonly Dictionary<string,
          TaskCompletionSource<object>> CurrentCalls = new Dictionary<string,
          TaskCompletionSource<object>>();

        private static readonly object LockObj = new object();

        public async Task<UIImage> LoadImageAsync(ImageSource imagesource,
          CancellationToken cancelationToken = new CancellationToken(), float scale = 1)
        {
            var uriImageSource = imagesource as UriImageSource;

            if (uriImageSource != null && uriImageSource.Uri != null)
            {
                var key = uriImageSource.Uri.ToString();
                TaskCompletionSource<object> existingTask = null;

                lock (LockObj)
                {
                    if (CurrentCalls.ContainsKey(key))
                        existingTask = CurrentCalls[key];
                }

                if (existingTask != null)
                    await existingTask.Task;

                var task = new TaskCompletionSource<object>();

                lock (LockObj)
                    CurrentCalls.Add(key, task);

                var originalImageLoaderSourceHandler = new
                  Xamarin.Forms.Platform.iOS.ImageLoaderSourceHandler();
                var uiImage = await originalImageLoaderSourceHandler
                  .LoadImageAsync(imagesource, cancelationToken, scale);

                lock (LockObj)
                    CurrentCalls.Remove(key);

                task.SetResult(null);

                return uiImage;
            }

            return null;
        }
    }
}