How to get combined behaviour of FLAG_ACTIVITY_CLEAR_TOP and FLAG_ACTIVITY_CLEAR_TASK?

Question

There are so many questions on SO related to this, but no such direct questions I could find.

My app has a main activity M, and a sub activity S.

M launchMode is singleTask because I only ever want one instance running and I want S to exist in the same task as M.

S launchMode is standard, because it can have many instances.

In S, there is an Up button which must always take the user to M.

There are two types of task stack (with S on top):

MSSSS...S

If the user presses Up in S, then the stack should become:

M (where M should be in the same state the user left it)

SSSS...S

If the user presses Up in S, then the stack should become:

M (a new instance)

--

So how to construct the Intent for the Up button...

In the first case, FLAG_ACTIVITY_CLEAR_TOP should do the trick, but if we apply this to the second case then the user is dumped back in Home.

In the second case, FLAG_ACTIVITY_CLEAR_TASK will do the trick, but if we apply this to the first case, we discover that M is finished and restarted.

One might also be tempted to try FLAG_ACTIVITY_NEW_TASK, but then we find hitting Back from M will take the user to the screen where he/she hit Up.

Side note: how is this possible when M is singleTask and the activities all have the same (default) task affinity?

So how to correctly construct the Up Intent in this case?

What is the case where you have a lot of `S` but no `M`? Why did `M` finish? This makes everything more difficult. You should examine your applicatiron architecture to see if you can make `M` stay active in the task stack. Then, you probably can get away with standard launch mode for all activities and just use flags to ensure a single instance of `M` at the root — David Wasser, Jan 26 '15 at 12:55
Simply that S is also a potential entry point - e.g. from a widget. It would feel like a major hack to use M as a forwarding activity in that scenario. — Mark, Jan 26 '15 at 13:23

score 1 · Answer 1 · answered Jan 26 '15 at 14:17

1

You cannot construct an UP Intent that will do what you want. Those things are mutually exclusive. What you could do is the following:

Change launch mode of M to singleTop

In M.onCreate(), check if M is at the root of the task by using isTaskRoot(). If it isn't the task root, have M restart itself and clear the task as follows:

Intent relaunch = new Intent(this, M.class);
relauch.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(relaunch);

In your UP Intent, add the following flags: Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP.

This will ensure that if an instance of M already exists in the task, it will clear everything on top of that instance and call onNewIntent() on the existing instance.

If there isn't an existing instance of M in the task, it will launch a new instance on top of the stack. This instance will recognize that it isn't the root activity and it will clear the task and relaunch itself.

Note: This will only work on Android 3.0 and above (API 11 or higher).

answered Jan 26 '15 at 14:17

David Wasser

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Thanks! However, with singleTop, how do you stop multiple instances of M? – Mark Jan 27 '15 at 03:36
In general, you won't create multiple instances. In case you do, the code you have in `onCreate()` will ensure that the task is cleared back to the original `M`. With this code it isn't possible that you end up with more than 1 active instance of `M` in the activity stack. – David Wasser Jan 27 '15 at 09:05
Generally you don't ever need to use special launch modes "singleTask" or "singleInstance". Using these launch modes usually create more problems than it solves, and people who use them generally don't know why they are doing it. Avoid at all costs. – David Wasser Jan 27 '15 at 09:07
Yes, standard launch mode should also work in this case. – David Wasser Jan 27 '15 at 12:00

score 0 · Answer 2 · answered Jan 26 '15 at 12:23

Here is a workaround/hack using a (Lollipop) deprecated API:

ActivityManager am = (ActivityManager)getSystemService(ACTIVITY_SERVICE);
if (M_CLASS_NAME.equals(am.getRunningTasks(1).get(0).baseActivity.getClassName()) {
    // we should be able to just clear top - because M is already running
    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
}
else {
    // okay to assume M is not running (because it is always root of task)
    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
}

How to get combined behaviour of FLAG_ACTIVITY_CLEAR_TOP and FLAG_ACTIVITY_CLEAR_TASK?

2 Answers2