异步下载无效，同步下载有效

Question

The issue is that syncronous calls work but the async does not.

Sync call:

using (var bmp = GetImageBitmapFromUrl (item.Image))
{
    imgNewsItem.SetImageBitmap (bmp);
    bmp.Dispose ();
}

Async call:

LoadNewsItemImageAsync (imgNewsItem, item.Image);

Async functionality:

public async void LoadNewsItemImageAsync(ImageView v, string url)
{
    using (Bitmap bmp = await GetImageBitmapFromUrlAsync(url))
    {
        v.SetImageBitmap (bmp);
        bmp.Dispose ();
    }
}

Task<Bitmap> GetImageBitmapFromUrlAsync (string url)
{
    return Task.Run<Bitmap>(() => GetImageBitmapFromUrl (url));
}

The below function works when it is not run via an async task

Bitmap GetImageBitmapFromUrl(string url)
{
    Bitmap imageBitmap = null;
    try {
        using (var webClient = new WebClient())
        {
            var imageBytes = webClient.DownloadData(url);
            if (imageBytes != null && imageBytes.Length > 0)
            {
                imageBitmap = BitmapFactory.DecodeByteArray(imageBytes, 0, 
                    imageBytes.Length);
            }
    } catch (Exception ex) {
        Log.WriteLine (LogPriority.Error, "GetImageFromBitmap Error", ex.Message);
    }

    return imageBitmap;
}

Answer 1

In general, you should not use Task.Run to create asynchronous wrappers for synchronous methods .

In this particular instance, I believe you're running into a problem because Bitmap is a UI-affine type. Bitmap acts just like a UI control; you should only access it from the UI thread.

Consider this implementation of GetImageBitmapFromUrlAsync instead:

async Task<Bitmap> GetImageBitmapFromUrlAsync(string url)
{
  Bitmap imageBitmap = null;
  try {
    using (var webClient = new WebClient())
    {
      var imageBytes = await webClient.DownloadDataTaskAsync(url);
      if (imageBytes != null && imageBytes.Length > 0)
      {
        imageBitmap = BitmapFactory.DecodeByteArray(imageBytes, 0, 
            imageBytes.Length);
      }
  } catch (Exception ex) {
    Log.WriteLine (LogPriority.Error, "GetImageFromBitmap Error", ex.Message);
  }

  return imageBitmap;
}