How to return a Bitmap from an Async Task in Android app

Question

Ok, so this code is right off the Android Developer site which sets an ImageView to a Bitmap:

class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
private final WeakReference<ImageView> imageViewReference;
private int data = 0;

public BitmapWorkerTask(ImageView imageView) {
    // Use a WeakReference to ensure the ImageView can be garbage collected
    imageViewReference = new WeakReference<ImageView>(imageView);
}

// Decode image in background.
@Override
protected Bitmap doInBackground(Integer... params) {
    data = params[0];
    return decodeSampledBitmapFromResource(getResources(), data, 100, 100));
}

// Once complete, see if ImageView is still around and set bitmap.
@Override
protected void onPostExecute(Bitmap bitmap) {
    if (imageViewReference != null && bitmap != null) {
        final ImageView imageView = imageViewReference.get();
        if (imageView != null) {
            imageView.setImageBitmap(bitmap);
        }
    }
}
}

And basically, what I want to do is instead of doing

imageView.setImageBitmap(bitmap); I want to return that Bitmap so I can then use it in my Main UI. Is there any way of going about doing this? Sorry about my lack of knowledge on AsyncTask, I just started learning it recently. Thank you!

umerk44 · Accepted Answer · 2014-05-31 06:45:31Z

12

Actually there is no need to return bitmap and than use it in main Ui Thread. You can do this on PostExecute becuase postExecute runs in UI Thread. Your question?? Yes you can return bitmap from AsynTask you can do something like this

private class myTask extends AsyncTask<Void,Void,Bitmap>{


      protected Bitmap doInBackground(Void... params) {

            //do stuff
             return bitmap;
        }

        @Override
        protected void onPostExecute(Bitmap result) {
            //do stuff

            }
        }

Now you can get bitmap by calling

 Bitmap returned_bitmap = new myTask().execute().get()

Again this is not good this will hang your UI. But you can get value from Aysnc like this.

Another method you can do is by implementing callback using interface

public interface MyInterface 
{
   public void onPostExecute();
}

Activity class

 public class MyActivity extends Activity implements MyInterface{

private Bitmap Image;

public void onCreate(Bundle b)
{
    super.onCreate(b);

    new MyTask(this).execute();
}

@Override
public void onPostExecute() {
        //Now you have your bitmap update by do in background 
    //do stuff here 
}


private class MyTask extends AsyncTask<Void, Void, Void>
{
    MyInterface myinterface;

    MyTask(MyInterface mi)
    {
        myinterface = mi;
    }

    @Override
    protected Void doInBackground(Void... params) {
        //firt declare bitmap as class member of MyActivity 
        //update bitmap here and then call

        myinterface.onPostExecute();

        return null;
    }

}

 }

edited May 31, 2014 at 6:45

answered May 30, 2014 at 18:19

umerk44

2,8174 gold badges26 silver badges43 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user1282637 Over a year ago

I know that postExecute runs on the main thread, but I want to feed this bitmap into this adapter.add(new Location(BITMAP, "Place 1", "Description", "Distance", "Hours")); so if this adapter is declared in the main UI, how can I pass this into the adapter?

umerk44 Over a year ago

you can pass bitmap to adapter in postexecute() what's porblem with that?

user1282637 Over a year ago

Thank you, I just misunderstood how AsyncTask works, I think. Thank you

Mika · Accepted Answer · 2014-05-30 17:23:22Z

1

You should save yourself a lot of time and use Picasso. This will save you all of this code and it works with bitmaps. Basically you will write something like this:

Picasso.with(context).load("www.somthing.com/api/blah").placeholder(R.drawable.default_picture).into(imageView);

It's one line of code and you got Async and caching for free...

answered May 30, 2014 at 17:23

Mika

5,8556 gold badges40 silver badges84 bronze badges

1 Comment

ekashking Over a year ago

Picasso and Glide don't work with Map setInfoWindowAdapter when you need to download an image for every pin dynamically.

Rod_Algonquin · Accepted Answer · 2014-05-30 17:19:09Z

0

You could either use a callback method through interface or put the asynctask as a inner class and just call the some method for setting the imageView...

For the callback there is a good example here.

answered May 30, 2014 at 17:19

Rod_Algonquin

26.2k7 gold badges54 silver badges63 bronze badges

Comments

ashishduh · Accepted Answer · 2014-05-30 17:28:54Z

0

First of all, onPostExecute executes on your UI thread, so you don't need to do anything differently from the example. But you can always use anonymous classes if you want:

public class MyActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        AsyncTask task = new BitmapWorkerTask() {
            @Override
            public void onPostExecute(Bitmap bitmap) {
                // Do whatever
            }
        };

        task.execute();
    }
}

answered May 30, 2014 at 17:28

ashishduh

6,6993 gold badges32 silver badges36 bronze badges

Collectives™ on Stack Overflow

How to return a Bitmap from an Async Task in Android app

4 Answers 4

3 Comments

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related