Confused about singleTop mode

Question

I've read in the android docs that singleTop mode is this:

If an instance of the activity already exists at the top of the current task, the system routes the intent to that instance through a call to its onNewIntent() method, rather than creating a new instance of the activity. The activity can be instantiated multiple times, each instance can belong to different tasks, and one task can have multiple instances (but only if the activity at the top of the back stack is not an existing instance of the activity).

However, my app is behaving differently. My main activity has the singleTop launch mode defined in the manifest file. Here is where it's behaving oddly.

1. Start main activity from launcher.
2. From main activity, start sub activity.
3. When user presses back button (or actionbar home button), it sends intent to main activity with some extras. This means that main activity needs to be updated (depending on the user actions in sub activity.)
4. Main activity is shown with updated display.

-- the odd part is this --

5. From main activity, pressing back button goes back again to main activity.
6. Pressing back button a 2nd time brings up the launcher screen, then my app is put in the background.

On step 5, why does it bring up the main activity again? I thought singleTop will bring to front the main activity which is the current top of the stack in the task. But from that behavior in #5 and #6, it seems like it's creating two instances of main activity instead.

Is my understanding incorrect or something else is going on that I'm not clear of yet. Please help explain/clarify. Thanks.

My sub activity has its onBackPressed method overridden. And likewise, the main activity onNewIntent() handles the extras.

@Override
public void onBackPressed() {
    Intent intent = new Intent(this, MainActivity.class);
    intent.putExtra(MainActivity.UPDATE_ARG, true);
    startActivity(intent);
    super.onBackPressed();
}

Note: If I use singleTask mode, it behaves as I expected of singleTop. But I've read somewhere that singleTask and singleInstance are to be used sparingly.

Answer 1

I've finally made sense out of all this (I think!). I'll explain.

android:launchMode attribute applies to the activity whether it's started from home launcher, from within your app (or from another app).

The intent flag Intent.FLAG_ACTIVITY_CLEAR_TOP is needed to implement the singleTop behavior. Therefore, I passed that flag when creating the intent prior to starting the activity.

For the main activity, here's what I did:

@Override
public void onBackPressed() {
    Intent intent = new Intent(this, MainActivity.class);
    // these flags are important!!
    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    intent.putExtra(MainActivity.UPDATE_ARG, true);
    startActivity(intent);
    super.onBackPressed();
}

And correspondingly, I declared android:launchMode="singleTop" in the manifest file.

Answer 2

From posted doc:

one task can have multiple instances (but only if the activity at the top of the back stack is not an existing instance of the activity).

From your code:

When user presses back button (or actionbar home button), it sends intent to main activity with some extras. This means that main activity needs to be updated (depending on the user actions in sub activity.)

when user presses back button your main activity dose not exist at the top of the current task and your sub activity is at the top because it has not destroyed yet, so it creates another main activity and do not use existing one because that main activity is not top. look at this from the doc, note that the backstack contains current foreground activity:

Answer 3

why does it bring up the main activity again?

Well, because you're starting your activity again when you do:

startActivity(intent);

Any updates in your Activity should be performed in the onStart method (or onResume if applicable).