Update Activity's UI when it is not visible [Another activity is in foreground] - android-activity

Question

Is it ok to update an Activity when it is not in the foreground. I am not asking if it can be done from a background thread.
Consider this:
I have two activities Activity-A and Activity-B.
I start an AsyncTask from Activity-A and then go to Activity-B. Now after sometime, the AsyncTask finishes and in the onPostExecute() method, I try to update the images that are in Activity-A. All this is happening when Activity-B is in the foreground.
Is the above scenario reasonable or do I have to wait till Activity-A is in the foreground to update its UI?
If I can safely update the UI in the above scenario, what should I do when Activity-A is killed or finished and the AsyncTask still completes and tries to update the UI? [Assuming I have to do a check for isFinishing before updating the UI]
The above is a simplified version of what I am trying to do. I actually have a Custom ImageView that loads images from the server and updates itself when the request is done. So I am wondering what scenarios I have to worry about if the view is updating itself when the activity is not in foreground or has finished/destroyed.
EDIT:
Here is a sample that is working.
public class MainActivity extends Activity implements OnClickListener {
ImageView mImageView;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mImageView = (ImageView) findViewById(R.id.iv_image);
findViewById(R.id.btn_activity_2).setOnClickListener(this);
}
#Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.btn_activity_2:
new BackGroundTask().execute();
startActivity(new Intent(MainActivity.this, Activity2.class));
break;
}
}
private class BackGroundTask extends AsyncTask<Void, Void, Void> {
#Override
protected Void doInBackground(Void... params) {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
#Override
protected void onPostExecute(Void result) {
super.onPostExecute(result);
mImageView.setImageResource(R.drawable.ic_launcher);
}
}
}
The Layout is just a LinearLayout with button and image.

Answer 1

No, it's not. You can't update a UI that's not visible :) Activity A is onPause (or Stopped if needed).
You have to implement a reasonable Activity LIfeCycle so ActivityA can update its UI during onResume(); The AsyncTask should only touch the data that the UI needs to draw itself.
Your "Custom IMage View" has to be able to load the image from a place outside the Activity.
If your CustomImageView is (or can pass as a regular ImageView), you can use something like Picasso to offload the Bitmap handling the correct way.