Do not destroy android activity and should not be in the history

Question

I have a problem with Android and the transitions with activities. What I want to have is the following:

1. MainActivity calls Activity B.
2. Activity B calls MainActivity (for example, via Back Button, same instance)
3. MainActivity calls the same instance of Activity B. (Same instance is important because Activity B takes a long time to start)
4. Activity B calls MainActivity (for example, via Back Button, same instance)
5. MainActivity Back Button is pressed and app is terminated.

What I tried:

Main Activity:

android: launch mode = "single task"

Start Activity B:

Intent intent = new Intent (this, B.class);
this.startActivity (intent);

Activity B:

android: launch mode = "SingleInstance"

Start MainActivity:

Intent callerIntent = new Intent (this, MainActivity.class);
CallerIntent.addFlags (Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP);
start activity (caller intent);

Unfortunately point 5 does not work.

I have searched for a long time, but unfortunately found nothing or searched for the bottle words. I hope somebody can help me.

I also have a small demo app on which you can test the behavior: AndroidStudioZipFile (30 days available)

Answer 1

What is the behavior in Step 5? I think, it will be showing Activity B after step-5 which is still in the backstack since is it never destroyed or finished! if you do not want to save anything on back press of MainActivity, can you call

@Override
public void onBackPressed() {
    this.finish();
    System.exit(0);
}

or you may also try to clear the back stack when back is pressed on MainActivity! Hope this helps.

Answer 2

Do not use special launch modes for this!

MainActivity should call ActivityB like this:

Intent intent = new Intent(this. ActivityB.class);
intent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
startActivity(intent);

ActivityB should call MainActivity like this:

Intent intent = new Intent(this. MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
startActivity(intent);

This will allow you to flip back and forth between the 2 activities, without creating any new instances. You will need to override onBackPressed() in one or both activities.

If you want the BACK button in MainActivity to exit the app, you will need to be a bit tricky. An easy way to do this is as follows:

Declare a static variable in MainActivity like this:

public static boolean exit = false;

in MainActivity.onBackPressed(), do the following:

exit = true;
super.onBackPressed();

This will cause MainActivity to finish. If MainActivity is the only active Activity, then you are done. However, it is possible that ActivityB is underneath MainActivity. To force ActivityB to finish in this situation, add this to ActivityB.onResume():

super.onResume();
if (MainActivity.exit) {
    finish();
    return;
}
// Rest of onResume() code goes here...