Android: How to update an UI from AsyncTask if AsyncTask is in a separate class?

I hate inner class.

I've a main activity who launches a 'short-life' AsyncTask.

AsyncTask is in a separate file, is not an inner class of main activity

I need async task updates a textView from main Activity.

I know i can update a TextView from onProgressUpdate, if AsyncTask is a inner class

But how from an external, indipendent, async task ?

UPDATE: This looks like working :

In acitivty i call the task

backgroundTask = new BackgroundTask(this);
backgroundTask.execute();

In the constructor i've

public BackgroundTask(Activity myContext)
{
    debug = (TextView) myContext.findViewById(R.id.debugText);
}

where debug was a private field of AsyncTask.

So onProgressUpdate I can

debug.append(text);

Thanks for all of you suggestions

Solution 1:

AsyncTask is always separate class from Activity, but I suspect you mean it is in different file than your activity class file, so you cannot benefit from being activity's inner class. Simply pass Activity context as argument to your Async Task (i.e. to its constructor)

class MyAsyncTask extends AsyncTask<URL, Integer, Long> {

    WeakReference<Activity> mWeakActivity;

    public MyAsyncTask(Activity activity) {
       mWeakActivity = new WeakReference<Activity>(activity);
    }

 ...

and use when you need it (remember to NOT use in during doInBackground()) i.e. so when you would normally call

int id = findViewById(...)

in AsyncTask you call i.e.

Activity activity = mWeakActivity.get();
if (activity != null) {
   int id = activity.findViewById(...);
}

Note that our Activity can be gone while doInBackground() is in progress (so the reference returned can become null), but by using WeakReference we do not prevent GC from collecting it (and leaking memory) and as Activity is gone, it's usually pointless to even try to update it state (still, depending on your logic you may want to do something like changing internal state or update DB, but touching UI must be skipped).

Solution 2:

Using Interface 1) Create one Interface

public interface OnDataSendToActivity {
    public void sendData(String str);
}

2) Implements it in your Activity

public class MainActivity extends Activity implements OnDataSendToActivity{

     @Override
     protected void onCreate(Bundle savedInstanceState) {
          new AsyncTest(this).execute(new String[]{"AnyData"}); // start your task
     }

     @Override
     public void sendData(String str) {
         // TODO Auto-generated method stub

     }

}

3) Create constructor in AsyncTask(Activity activity){} Register your Interface in AsyncTask file and call interface method as below.

public class AsyncTest extends AsyncTask<String, Integer, String> {

    OnDataSendToActivity dataSendToActivity;
    public AsyncTest(Activity activity){
        dataSendToActivity = (OnDataSendToActivity)activity;
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        dataSendToActivity.sendData(result);
    }

}

Here, your OnPostExecute will call after all task done by AsyncTask and will get "result" as a parameter, returned by doInBackground(){ return "";}.

While "dataSendToActivity.sendData(result);" it will call activity's overrided method "public void sendData(String str) {}".

An edge case to remember: Be sure to pass this, i.e. you current activity's context to AsyncTask and not create another instance of your activity, otherwise your Activity will be destroyed and new one is created.

Android: How to update an UI from AsyncTask if AsyncTask is in a separate class?

Solution 1:

Solution 2:

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