Android destroying activities, killing processes

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First please have a look at this:

img1

onPause() Called when the system is about to start resuming a
previous activity. This is typically used to commit unsaved changes to
persistent data, stop animations and other things that may be
consuming CPU, etc. Implementations of this method must be very quick
because the next activity will not be resumed until this method
returns. Followed by either onResume() if the activity returns back to
the front, or onStop() if it becomes invisible to the user.

onStop() Called when the activity is no longer visible to the user, because another activity has been resumed and is covering this
one. This may happen either because a new activity is being started,
an existing one is being brought in front of this one, or this one is
being destroyed. Followed by either onRestart() if this activity is
coming back to interact with the user, or onDestroy() if this activity
is going away.

So, when you press “HOME” button on your device, your current foreground activity is put onto onPause() then onStop(), the other 4 should remain onStop()

According to Google’s Documents:

  • If an activity in the foreground of the screen (at the top of the stack), it is active or running.
  • 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.
  • If an activity is completely obscured by another activity, it is stopped. It still retains all state and member information, however,
    it is no longer visible to the user so its window is hidden and it
    will often be killed by the system when memory is needed elsewhere.
  • If an activity is paused or stopped, the system can drop the activity from memory by either asking it to finish, or simply killing
    its process. When it is displayed again to the user, it must be
    completely restarted and restored to its previous state.

And, for the process lifecycle:

Process Lifecycle 3. 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.

All the quotes above are come from: Android Developers Reference: Activity

It is confirmed that the system can destroy non-acitve activities and recycle memories when you launched some memory consuming applications. And you can implement like: isFinishing() in your activity and then using “kill” button in DDMS to detect which of your activities is being dropped by system. But I guess the system will destroy the oldest one first. However it is no point to keep other activities when the “Launch Activity” has been recycled.

UPDATE

Here’s some opinions I found from here:

Stopped state

When an activity is not visible, but still in memory, we say it’s in a
stopped state. Stopped activity could be brought back to the front to
become a Running activity again. Or, it could be destroyed and removed
from memory.

The system keeps activities around in a stopped state because it is
likely that the user will still want to get back to those activities
some time soon, and restarting a stopped activity is far cheaper than
starting an activity from scratch. That is because we already have all
the objects loaded in memory and simply have to bring it all up to the
foreground.

Stopped activities can be removed from memory at any point.

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