Android Process Lifecycle Details

Question

The main activity in my app sometimes calls startActivityForResult, expecting a result that will tell it (the main activity) what information to display next. Looking at the documentation for the process lifecycle, it appears that while the selection activity is active, the main activity is considered a "background" activity and could possibly be killed.

So what happens when the selection activity completes? I see that my activity will be re-created and onCreate is called with the SaveInstance Bundle, but then what? Is onActivityResult then called just as if my main activity had never exited and been re-created?

Also, is there any way to force this behavior in a testing environment, since it should otherwise be a very rare occurrence?

Answer 1

Hint: log statements

The paused state as described in docs is:

If an activity has lost focus but is still visible (that is, a new non-full-sized or transparent activity has focus on top of your activity), it is paused. A paused activity is completely alive (it maintains all state and member information and remains attached to the window manager), but can be killed by the system in extreme low memory situations.

That means, under normal circumstances , your main activity should just transfer the control to onActivityResult() when the selection activity completes.

However, docs also state that:

A background activity (an activity that is not visible to the user and has been paused) is no longer critical, so the system may safely kill its process to reclaim memory for other foreground or visible processes. If its process needs to be killed, when the user navigates back to the activity (making it visible on the screen again), its onCreate(Bundle) method will be called with the savedInstanceState it had previously supplied in onSaveInstanceState(Bundle) so that it can restart itself in the same state as the user last left it.

In such cases, the main activity can be redrawn.

One important point to note is that the docs have never mentioned onActivityResult() as one of their lifecycle methods here

So, it might also be the case where android system treats a sub activity and parent activity (read startActivityforResult() and onActivityResult()) in the same manner as it treats an activity - dialog as stated here:

A visible activity (an activity that is visible to the user but not in the foreground, such as one sitting behind a foreground dialog) is considered extremely important and will not be killed unless that is required to keep the foreground activity running.

Answer 2

Suppose there are two activities A and B and Activity A calls Activity B through startOnActivityResult(intent,200) then your Activity goes to background and Activity B is called (onCreate,onStart,onResume) depends on what you have overridden.

Whenever your B activity calls finish() then your Activity B is destroyed and Activity A comes to foreground. In this case now ie Activity A the call will be onActivityResult -> onStart -> onResume but your onCreate will not be called as it is only called when you call a certain Activity.

Suppose you have not called finish() from activity B and called Activity A through intent then only you onCreate() will be called.

Also onActivityResult() is very useful when you want to retain value of your spinners or you want to notifydatasetchanged() your ListView of your First activity after the event of Third activity. You just have to check your resultCode from the Activities in onActivityResult and perform your actions

Answer 3

The answer is basically "yes": The activity is recreated and the control flows through onCreate(), onActivityResult(), onStart(), and so on. An activity is also destroyed if the user rotates the device, e.g. from portrait to landscape, unless the application did not explicitly prevent this behavior. So simply rotate the device (CTRL-F11 on the emulator) to test.