Android: cannot resolve method 'findViewById(int)' in AsyncTask

Question

Following the tutorial here I have set up an app that should be able to download an image from a URL and place it into an ImageView. The only issue is that it cannot find findViewById because it is in the Activity class instead of the AsyncTask class. code is here:

package com.example.weather2;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.util.Log;
import android.widget.ImageView;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;


public class ImageDownloader
        extends AsyncTask<String, Integer, Bitmap> {
protected void onPreExecute(){
    //Setup is done here
}
@Override
protected Bitmap doInBackground(String... params) {
    //TODO Auto-generated method stub
    try{
        URL url = new URL(params[0]);
        HttpURLConnection httpCon =
                (HttpURLConnection)url.openConnection();
        if(httpCon.getResponseCode() != 200)
            throw new Exception("Failed to connect");
        InputStream is = httpCon.getInputStream();
        return BitmapFactory.decodeStream(is);
    }catch(Exception e){
        Log.e("Image", "Failed to load image", e);
    }
    return null;
}
protected void onProgressUpdate(Integer... params){
    //Update a progress bar here, or ignore it, I didn't do it
}
protected void onPostExecute(Bitmap img){
    ImageView iv = (ImageView) findViewById(R.id.imageView); //here is where the issue is
    if(iv!=null && img!=null){
        iv.setImageBitmap(img);
    }
}
protected void onCancelled(){
}
}

When I comment out the lines with the issue, I don't get any errors, which makes me think it is working, but I can't tell because there is no image being posted. Any help for solving this issue would be much appreciated.

findViewById is a method of the class Activity – Blackbelt Jan 06 '14 at 14:15 — Blackbelt, Jan 06 '14 at 14:15

score 4 · Accepted Answer · edited May 23 '17 at 12:00

4

findViewById is a method of Activity class.

And you have

 public class ImageDownloader
    extends AsyncTask<String, Integer, Bitmap>  // not a activity class

If you need the data back in activity you could use interface as a callback to the activity.

I would update ui in Activity itself. look @ blackbelts answer

How do I return a boolean from AsyncTask?

Or

Make AsyncTask an inner class of Activity class and update ui in onPostExecute

Also you might find this blog by Alex Loockwood interesting

http://www.androiddesignpatterns.com/2013/04/retaining-objects-across-config-changes.html

edited May 23 '17 at 12:00

Community

1
1

answered Jan 06 '14 at 14:15

Raghunandan

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I know this. How would I go about fixing this? – Sam Borick Jan 06 '14 at 14:16
@SamBorick if you know that why are you trying to do it? – Blackbelt Jan 06 '14 at 14:17
@blackbelt http://stackoverflow.com/questions/16752073/how-do-i-return-a-boolean-from-asynctask this is a usefull reference which i bookmarked – Raghunandan Jan 06 '14 at 14:18
Because I am following the tutorial, and wouldn't know any other way of doing it. I was thinking about passing it back to activity somehow. an example would be much appreciated. – Sam Borick Jan 06 '14 at 14:21
@SamBorick look at my previous comment blackbelt has already answered a similar one – Raghunandan Jan 06 '14 at 14:21
1

@SamBorick also read this http://www.androiddesignpatterns.com/2013/04/retaining-objects-across-config-changes.html. might help you in the future – Raghunandan Jan 06 '14 at 14:28
I made the AsyncTask an inner class and it worked like a charm! Thanks! – Sam Borick Jan 06 '14 at 14:32

score 3 · Answer 2 · answered Jan 06 '14 at 14:20

3

Instead of calling findViewById() in the AsyncTask, pass in the ImageView as a parameter:

public class ImageDownloader extends AsyncTask<String, Integer, Bitmap> {
    final ImageView mImageView;

    public ImageDownloader(ImageView imageView) {
        mImageView = imageView;
    }

    // ...

answered Jan 06 '14 at 14:20

laalto

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would this be a problem during orientation change? – Raghunandan Jan 06 '14 at 14:22
Not really. The ImageView may not be displayed anymore though. In any case, one needs to consider that the views may be recycled (if in an adapter) and the imageview is not the correct one at the time the asynctask completes. For getting something working when working with a tutorial this is ok; for real production use, use a tested imageloader such as Picasso, Volley, Universal-Image-Loader or similar. – laalto Jan 06 '14 at 14:28
agreed. I have mostly used UIL for LazyLoading. – Raghunandan Jan 06 '14 at 14:30

slhddn · Answer 3 · 2014-01-06T14:33:06.353

Use your activity directly when configuring/getting image. This will solve your problem.

public class ImageDownloader extends AsyncTask<String, Integer, Bitmap> {
    private Context context;

    public ImageDownloader(Activity yourActivity) {
        context = activity;
    }

    // ...

    protected void onPostExecute(Void... params){
        ImageView iv = (ImageView) context.findViewById(R.id.imageView); 
        if(iv!=null && img!=null){
            iv.setImageBitmap(img);
        }
    }
}

score 1 · Answer 4 · answered Jul 05 '16 at 10:47

Use cast to AppCompatActivity, such as:

public class ImageDownloader extends AsyncTask<String, Integer, Bitmap> {
private Context context;

public ImageDownloader(Activity yourActivity) {
    context = yourActivity;
}

// ...

protected void onPostExecute(Void... params){
    //Here
    ImageView iv = (ImageView) ((AppCompatActivity) context).findViewById(R.id.imageView); 
    if(iv!=null && img!=null){
        iv.setImageBitmap(img);
    }
}

}

score 0 · Answer 5 · answered Jan 06 '14 at 14:25

You can implement the AsyncTask as a inner class of your activity. And put ImageView iv = (ImageView) findViewById(R.id.imageView); in your activity's onCreate() method. That is what I did in my project.

Or you can pass the activity.this as a parameter to the AsyncTask's constructor. And use ImageView iv = (ImageView)activity. findViewById(R.id.imageView) in your AsyncTask's onPostExecute() method.

Android: cannot resolve method 'findViewById(int)' in AsyncTask

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