Question

In java I want to download multiple images for my imageview at once. I found this code which does exact that but its only downloading the first image. I think it has something to do with the first line in the doInBackground. How can I tweak this code so it loops imageViews and downloads every image for imageViews.

package com.denederlandsewateren.daos;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.util.Log;
import android.widget.ImageView;

public class DownloadImageTaskDAO extends AsyncTask<ImageView, Void, Bitmap> {

    ImageView imageView = null;

    @Override
    protected Bitmap doInBackground(ImageView... imageViews) {
        this.imageView = imageViews[0];
        return download_Image((String)imageView.getTag());
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        imageView.setImageBitmap(result);
    }

    private Bitmap download_Image(String url) {
        //---------------------------------------------------
        Bitmap bm = null;
        try {
            URL aURL = new URL(url);
            URLConnection conn = aURL.openConnection();
            conn.connect();
            InputStream is = conn.getInputStream();
            BufferedInputStream bis = new BufferedInputStream(is);
            bm = BitmapFactory.decodeStream(bis);
            bis.close();
            is.close();
        } catch (IOException e) {
            Log.e("Hub","Error getting the image from server : " + e.getMessage().toString());
        } 
        return bm;
        //---------------------------------------------------
    }

}

called from here:

public void setImages(String url1, String url2) {
        image1.setTag(url1);
        image2.setTag(url2);
        DownloadImageTaskDAO imageDAO = new DownloadImageTaskDAO();
        imageDAO.execute(image1, image2);
    }

Answer 1

@Override
protected Bitmap doInBackground(ImageView... imageViews) {
    this.imageView = imageViews[0];
    return download_Image((String)imageView.getTag());
}

You are processing only the first argument sent to the AsyncTask. With the parameter ImageView... you can send one, two, N elements or arrays. Loop through the arguments and process each one.

You can use the return from doInBackground, in onPostExecute and personally I try to utilize that as much as I can for the sake of encapsulation.

Overall I suggest you read the AsyncTask reference on Android Developers to get a better understanding of AsyncTasks.

I did rewrite your code and below should theoretically work (it’s not tested).

public class DownloadImageTaskDAO extends AsyncTask<ImageView, Void, ArrayList<DownloadImageTaskDAO.Container>> {

    @Override
    protected ArrayList<Container> doInBackground(ImageView... imageViews) {
         ArrayList<Container> results = new ArrayList<Container>(imageViews.length);

         for (ImageView imageView : imageViews) {
             Container tmp = new Container();
             tmp.holder = imageView;
             tmp.image = download_Image(imageView.getTag().toString());
             results.add(tmp);
         }

         return results;
    }

    @Override
    protected void onPostExecute(ArrayList<Container> result) {
        for (Container container : result) {
            container.holder.setImageBitmap(container.image);
        }
    }

    private Bitmap download_Image(String url) {
        ....
    }

    class Container {
        ImageView holder;
        Bitmap image;
    }
}