I've been reading doc regarding to launch modes and there is one thing I don't understard. The doc says that singleTask activity is always the root of the stack:
In contrast, "singleTask" and "singleInstance" activities can only begin a task. They are always at the root of the activity stack. Moreover, the device can hold only one instance of the activity at a time — only one such task.
But: if you look at this part of doc at Figure 4, you see that when Activity 2 starts Activity Y (puts that task to foreground), Activity Y was already on the top of the task and will be on the top of the current task, not the root.
I tried this scenario in this simulation app and when I create singleTask activity, it always creates a new task. However, if the only instance already exists, it finishes all activities above this one so the only instance can be the root (and also the only activity in the task).
How could the Activity Y become on the top of the task above the Activity X?
Is there any other reason I'm missing?
PS: I also don't really understand the difference between Task and back stack.
As usual (sigh), the documentation is wrong. In the diagram you referenced, obviously Activity Y can't be defined as singleTask
and yet be the top activity in a background task containing 2 activities.
When testing scenarios with special launch modes singleTask
and singleInstance
, please be aware that taskAffinity
plays an important role in this behaviour, as taskAffinity
takes priority over special launch modes.
Regarding the difference between "task" and "back stack":
A "task" is a stack of activities that can be manipulated as a whole group.
A "back stack" usually refers to a set of activities within a task. Each task has its own stack of activities. This is used to control what happens when the current activity (the one on top of the back stack) finishes. Normally Android returns the user to the activity that is directly underneath (below) the finishing activity in the back stack.
The Android code and documentation often refer to the "root" of a task (this is the activity that was used to start the task) and the "top" or "front" of a task (this is the activity that is currently being shown).
Actually, the documentation lies :-( Here's an example:
In contrast, "singleTask" and "singleInstance" activities can only begin a task.
This statement is usually, but not always correct. For example, let's say I have 2 activities: A
and B
. A
is the launch activity (ie: the one with ACTION=MAIN
and CATEGORY=DEFAULT
) and is defined with standard launch mode. B
is defined with launchMode="singleTask"
. I start the application and Android creates an instance of A
. In A
I then do:
startActivity(new Intent(this, B.class));
This will create a new instance of activity B
and put it on top of A
in the same task. It will not create a new task with activity B
as the root. The reason is that activity A
and activity B
have the same taskAffinity
(by default all activities of an application have the same taskAffinity
), and Android will ignore the launch mode of B
in this case.
The documentation also says:
Moreover, the device can hold only one instance of the activity at a time — only one such task.
Again, taskAffinity
can break this behaviour. Assume again we have A
, B
and C
, all with the same (default) taskAffinity
. A
and C
have standard launch mode, B
has launchMode="singleTask"
. If A
starts B
, the instance of B
ends up not in a new task, but in the same task as A
(see above). Now B
starts C
. Android creates an instance of C
and puts it on top of B
in the same task. Now C
calls:
startActivity(new Intent(this, B.class));
Android creates a new instance of B
and puts this on top of C
in the task. There are now 2 instances of B
and neither of them is the root activity of the task! This behaviour is also due to the fact that taskAffinity
trumps launch mode.
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