gistfile1.java

// LICENSE: USE IT IF YOU NEED IT, BUT PLEASE MENTION AT LEAST MY NAME, THANKS!
// (Juraj Suchán, [email protected])
//
// PREFACE:
// So i've been building client application for one local "closed community" 
// nyx.cz for year and a half where the displayed content is standard html code.
//
// There is Html.fromHtml to the rescue for that! But unfortunatelly, anyone who tried it
// have to deal with custom Html.ImageGetter implementation.
// This interface contains only one method, with only one paramter (url) and it's completelly
// "impossibru" to make it asynchronous anyhow.
// Not to mention, if you want to use it in application targeted to HC/ICS and higher, 
// the NetworkOnMainThreadException will be thrown!
// 
// Here is my workaround (please note that this is snippet rather than full solution):
// The first Html.ImageGetter implementation in the getDrawable method
// fires of AsyncTask to get the image from specified source and stores it in cache.
// Return value for this method is just drawable placeholder (empty image or so).
// The second Html.ImageGetter implementation takes the loaded drawable from cache..
// I have this code inside custom list adapter and it may looks like this:

// TL;DR we need two imageGetter implementations, one for imageDownloading (igcacher) and one for image drawing from cache (igfromcache)
// - when the task from first imageGetter (igcacher) finishes, it sets the resulting drawable to the cache
// - apply the second imageGetter (igfromcache) in task onPostExecute, and you're done.
// - the textView will redraw correctly because of new calling "setText".

//...
static final Map<String, WeakReference<Drawable>> mDrawableCache = Collections.synchronizedMap(new WeakHashMap<String, WeakReference<Drawable>>());
//...
private Drawable mDefaultDrawable;
private Executor mExecutor = Executors.newSingleThreadExecutor();
//...
Html.ImageGetter igfromcache = new Html.ImageGetter() { // this is the second imageGetter implementation we need
	public Drawable getDrawable(String source) {
		if (!mDrawableCache.containsKey(source)) {
			return null;
		}
		return mDrawableCache.get(source).get(); // the image has been already downloaded and it's stored in cache, right?
	}
}
//...
final String message = "some html content with image tasks";
final TextView textView = (TextView)findViewById(R.id.discussion_message_tv);

Html.ImageGetter igcacher = new Html.ImageGetter() { // this is first imageGetter implementation we need
	public Drawable getDrawable(String source) {
		ImageDownloadData imageDownloadData = new ImageDownloadData(source, message, textView);
		ImageDownloadAsyncTask imageDownloadTask = new ImageDownloadAsyncTask(imageDownloadData);
		imageDownloadTask.executeOnExecutor(mExecutor, null); // go for it online!
		return mDefaultDrawable; // this is placeholder image taken from resources.
	}
};

textView.setText(Html.fromHtml(message, igcacher, null)); // the trick starts here, spinoff caching image getter.
//...
class ImageDownloadData {
	private String imageSource;
	private String message;
	private TextView textView;

	public ImageDownloadData(String imageSource, String message, TextView textView) {
		this.imageSource = imageSource;
		this.message = message;
		this.textView = textView;
	}

	public String getImageSource() {
		return imageSource;
	}

	public String getMessage() {
		return message;
	}

	public TextView getTextView() {
		return textView;
	}
}
//...
class ImageDownloadAsyncTask extends AsyncTask<Void, Void, Drawable> {
	private ImageDownloadData imageDownloadData;

	public ImageDownloadAsyncTask(ImageDownloadData imageDownloadData) {
		this.imageDownloadData = imageDownloadData;
	}

	@Override
	protected Drawable doInBackground(Void... params) {
		Drawable drawable = null;
		/* 
		code for downloading bitmap is not required here, do it by yourself, 
		remember, the uri for it is in imageDownloadData.getImageSource() ! 
		*/
		drawable.setBounds(0, 0, drawable.getIntrinsicWidth(), drawable.getIntrinsicHeight());
		return drawable;
	}

	@Override
	protected void onPostExecute(Drawable drawable) {
		// put the downloaded drawable into the cache (so the second image getter can access it)
		if (!mDrawableCache.containsKey(imageDownloadData.getSource())) {
			mDrawableCache.put(imageDownloadData.getSource(), new WeakReference<Drawable>(drawable));
		}				

		TextView textView = imageDownloadData.getTextView();
		String message = imageDownloadData.getMessage();
		// this does the trick, we need to set the message/text again, but with 
		// different imageGetter (the second one) imeplementation !
		textView.setText(MyHtml.fromHtml(content, igfromcache, null)); 
	}
}