如何从Android上的asynctask返回图像

Question

I am writing an application where I am building a list view in which each item has an image. The content of the list view is generated by an XML file that is stored locally, the pictures are fetched from Amazon AWS S3.

I want to write a class AWSImageFetcher that will be responsible for the authentication first (by using another dedicated class) and then for fetching the images.

I understood that it is best practice on Android to subclass AsyncTask in cases like this to perform the network requests. I am now wondering how I should return the images from the AWSImageFetcher class to the list view.

I am coming from iOS where I would just write a delegate for the AWSImageFetcher which would be invoked after the images have been fetched, but this doesn't feel right on Android. Should I use a listener class instead? How would you solve a situation like this on Android in an elegant way?

Answer 1

Try this form

onPreExecute executes first in UIThread, later doInbakground function execute in parellel thread then after that postExecute run in UIThread

private class AWSImageFetcher extends AsyncTask<String, Void, Bitmap> 
{
    boolean authenticated;

    @Override
    protected void onPreExecute() 
    {
        super.onPreExecute();
        authenticated=authenticate();
    }

    @Override
    protected Bitmap doInBackground(String... urls) 
    {
        Bitmap b=null;
        if(authenticated)
        {
                URL imageUrl = new URL(urls[0]);
            HttpURLConnection conn = (HttpURLConnection) imageUrl
                    .openConnection();
            conn.setConnectTimeout(TIME_OUT_IN_MILLI_SECONDS);
            conn.setReadTimeout(TIME_OUT_IN_MILLI_SECONDS);
            conn.setInstanceFollowRedirects(true);
            InputStream is = conn.getInputStream();
            OutputStream os = new FileOutputStream(f);
            Utils.CopyStream(is, os);
            os.close();
            b = decodeFile(f);
        }
        return b;
    }

    @Override
    protected void onPostExecute(Bitmap result) 
    {
        super.onPostExecute(result);
        if(result!=null)
        {               
            //use bitmap image in result    
        }
       else
       {   
          //Image is not available
       }    

    }

 }


//decodes image and scales it to reduce memory consumption
private Bitmap decodeFile(File f) {
    try {
        // decode image size
        BitmapFactory.Options o = new BitmapFactory.Options();
        o.inJustDecodeBounds = true;
        FileInputStream stream1 = new FileInputStream(f);
        BitmapFactory.decodeStream(stream1, null, o);
        stream1.close();

        // Find the correct scale value. It should be the power of 2.
        final int REQUIRED_SIZE = 70;
        int width_tmp = o.outWidth, height_tmp = o.outHeight;
        int scale = 1;
        while (true) {
            if (width_tmp / 2 < REQUIRED_SIZE
                    || height_tmp / 2 < REQUIRED_SIZE)
                break;
            width_tmp /= 2;
            height_tmp /= 2;
            scale *= 2;
        }

        // decode with inSampleSize
        BitmapFactory.Options o2 = new BitmapFactory.Options();
        o2.inSampleSize = scale;
        FileInputStream stream2 = new FileInputStream(f);
        Bitmap bitmap = BitmapFactory.decodeStream(stream2, null, o2);
        stream2.close();
        return bitmap;
    } catch (FileNotFoundException e) {
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;
}