ListItem lazyload AsyncTask image display problem.

Question

I am using listview to display data, inside listview i am using image in every listeitem.

I am following this tutorial http://developer.android.com/guide/samples/ApiDemos/src/com/example/android/apis/view/List4.html

new Handler().post(new Runnable(){

       @Override 
       public void run() {
        //Need to subclass to use Asynctask
        class DownloadImage extends AsyncTask<Void, Void, Void>{
         @Override
         protected Void doInBackground(Void... params) {
           Drawable dImage = Util.getImageFromURL(imageURL); 
            getImageIcon().setImageDrawable(dImage);
          return null;
         }

        }
        new DownloadImage().execute();
       }

      });

this looks good for lazzy loading image, but there is one problem it won't display any image until user touch screen or try to scroll results or any interaction by the user to handset device keys.

If i try to scroll results it dispaly all list item images.... what may go wrong ???

Answer 1

Hi Faisal,

It's hard to say without more context. In particular, where is this code being called from, and what does getImageIcon() do??

One problem I see is that you're setting the image icon in a background thread rather than in the UI thread. This could be the reason why it doesn't seem to change until a ui event triggers a refresh. Try changing DownloadImage to something like the following:

class DownloadImage extends AsyncTask<Void, Void, Drawable>{
        @Override
        protected Drawable doInBackground(Void... params) {
            return Util.getImageFromURL(imageURL); 
        }

        @Override
        protected void onPostExecute( Drawable d ) {
            getImageIcon().setImageDrawable(d);
        }

}
new DownloadImage().execute();

If I'm right, that should fix the problem.

If that does work, great, although I think your code could use a little cleanup. Provided that this is all taking place in the UI thread (which is probably true given that you're instantiating a new Handler in this thread), you shouldn't need to use a Handler at all, nor should you need to create a new Runnable. Just strip those out and create a new DownloadImage instance directly and call execute().