Xamarin Android – Using Remote Images in Lists

bitmap-downloader-non-list.cs

/// <summary>
/// An AsyncTask that will asynchronously download an image and assign it to an ImageView.
/// </summary>
public class BitmapDownloaderTask : AsyncTask<String, Java.Lang.Void, Bitmap>
{
	private readonly ImageView _imageView;

	public BitmapDownloaderTask(ImageView imageView)
	{
		if (imageView == null)
			throw new ArgumentNullException("imageView");

		_imageView = imageView;
	}

	public string Url { get; private set; }

	/// <summary>
	/// Called on a background thread when the task is executed.
	/// </summary>
	protected override Bitmap RunInBackground(params string[] @params)
	{
		Url = @params[0];
		return DownloadRemoteImage(Url);
	}

	/// <summary>
	/// Once the image is downloaded, associates it to the imageView
	/// </summary>
	protected override void OnPostExecute(Bitmap bitmap)
	{
		if (IsCancelled)
			bitmap = null;

		imageView.SetImageBitmap(bitmap);
	}

	private Bitmap DownloadRemoteImage(string url)
	{
		if (string.IsNullOrWhiteSpace(url))
			throw new ArgumentException("url must not be null, empty, or whitespace");

		Uri imageUri;
		if (!Uri.TryCreate(url, UriKind.Absolute, out imageUri))
			throw new ArgumentException("Invalid url");

		try
		{
			WebRequest request = HttpWebRequest.Create(imageUri);
			request.Timeout = 10000;

			WebResponse response = request.GetResponse();
			Stream inputStream = response.GetResponseStream();

			return BitmapFactory.DecodeStream(inputStream);
		}
		catch (Exception)
		{
			return null;
		}
	}
}

bitmap-downloader-task-list-compatible.cs

/// <summary>
/// An AsyncTask that will asynchronously download an image and assign it to an ImageView.
/// </summary>
public class BitmapDownloaderTask : AsyncTask<String, Java.Lang.Void, Bitmap>
{
	private readonly WeakReference<ImageView> _imageViewReference;

	public BitmapDownloaderTask(ImageView imageView)
	{
		if (imageView == null)
			throw new ArgumentNullException("imageView");

		_imageViewReference = new WeakReference<ImageView>(imageView);
	}

...

	/// <summary>
	/// Once the image is downloaded, associates it to the imageView
	/// </summary>
	protected override void OnPostExecute(Bitmap bitmap)
	{
		if (IsCancelled)
			bitmap = null;

		if (_imageViewReference != null && bitmap != null)
		{
			ImageView imageView;
			if (!_imageViewReference.TryGetTarget(out imageView))
				return;

			var bitmapDownloaderTask = imageView.GetBitmapDownloaderTask();

			// Change bitmap only if this process is still associated with it.
			// This is necessary as views can be reused by Android, and a newer BitmapDownloader instance may have been attached to it.
			if (this == bitmapDownloaderTask)
				imageView.SetImageBitmap(bitmap);

		}
	}
...
}

bitmap-downloader-usage-2.cs

var task = new BitmapDownloaderTask(imageView);
imageView.SetImageDrawable(new DownloadedDrawable(task, Color.Gray));
imageView.SetMinimumHeight(300);

task.Execute("http://placehold.it/300x300");

bitmap-downloader-usage.cs

var task = new BitmapDownloaderTask(imageView);
imageView.SetImageDrawable(new ColorDrawable(Color.Gray));
imageView.SetMinimumHeight(300);

task.Execute("http://placehold.it/300x300");

bitmap-task-extension.cs

/// <summary>
/// Retrieve the BitmapDownloaderTask from the ImageView's drawable.
/// This will return null if the drawable is not a DownloadedDrawable, or there is no BitmapDownloaderTask attached to it.
/// </summary>
public static BitmapDownloaderTask GetBitmapDownloaderTask(this ImageView imageView)
{
	if (imageView != null)
	{
		var drawable = imageView.Drawable as DownloadedDrawable;
		if (drawable != null)
			return drawable.GetBitmapDownloaderTask();
	}

	return null;
}

downloaded-drawable.cs

/// <summary>
/// A fake Drawable that will be attached to the ImageView while the download is in progress.
/// Contains a reference to the actual download task, so that a download task can be stopped
///  if a new binding is required, and makes sure that only the last started download process can
///  bind its result, independently of the download finish order.
/// </summary>
public class DownloadedDrawable : ColorDrawable
{
	private readonly WeakReference<BitmapDownloaderTask> _bitmapDownloaderTaskReference;

	public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask, Color loadingBackgroundColor)
		: base(loadingBackgroundColor)
	{
		_bitmapDownloaderTaskReference = new WeakReference<BitmapDownloaderTask>(bitmapDownloaderTask);
	}

	public BitmapDownloaderTask GetBitmapDownloaderTask()
	{
		BitmapDownloaderTask task;
		_bitmapDownloaderTaskReference.TryGetTarget(out task);
		return task;
	}
}