Thank you for such a detailed reply!
On 6/6/2010 9:34 PM, Mark Murphy wrote:
Leigh McRae wrote:
Thing is
that the docs says onSaveInstanceState() will be called before
onPause()
No, it doesn't. In fact, it says just the opposite:
"If called, this method will occur before onStop(). There are no
guarantees about whether it will occur before or after onPause()."
http://developer.android.com/reference/android/app/Activity.html#onSaveInstanceState(android.os.Bundle)
Looks like it says both :) Still whether it's before onPause() or
onStop() doesn't matter much.
"To capture that state before the activity is killed, you can implement
an ||onSaveInstanceState()
<http://developer.android.com/reference/android/app/Activity.html#onSaveInstanceState%28android.os.Bundle%29>||
method for the activity. Android calls this method before making the
activity vulnerable to being destroyed — that is, before |onPause()| is
called. "
http://developer.android.com/guide/topics/fundamentals.html#lcycles
but how does the system know at this point whether the
activity will be killed by the system or the user?
First, it doesn't matter in most cases.
Second, onSaveInstanceState() may be called seconds or days before the
process is killed, depending on what the user is doing.
Third, the process is not typically killed. Usually, activities will be
destroyed gracefully, going through onStop() and onDestroy(). A process
termination is required only when there's an emergency need for RAM.
Fourth, "the system" knows whether the user pressed the BACK button.
For the BACK button that is trivial and very clear. What happens I
launch the browser from my app and it covers my app as in my example. I
would go through onPause() and onStop(). Both docs say that
onSaveInstanceState() would have been called by now but the system
doesn't know if it will run out of memory at some point with the
browser. Similar would be when the user hits the HOME key and starts
other apps.
Here is a use case:
1) My activity starts up and is running.
2) At some point I want to show a web page so I use an Intent to start
an activity.
3) The web browser covers my app/activity so I would expect onPause()
to be called followed by onStop().
Correct.
At this point it's my understanding that all bets are off and I can be
killed at any point without be called.
Correct. Unlikely, but correct.
If it can happen it needs to be handled.
4) Since the system has enough memory onSaveInstance() doesn't get
called.
I have no idea where you came up with this.
I would expect onSaveInstanceState() to have been called in this
scenario, before onStop(), and around the time of onPause().
It's been inferred since the system has no way of knowing for SURE if it
will have to kill the activity, at which time I have already been put in
the background and received onStop(). Now for the trivial case where the
system starts another activity and needs more memory it makes sense.
5) The user presses home and decides to open some app which requires a
lot of memory.
6) System wants more memory and decides to kill my app but I am
already stopped (onPause() has been called). What happens here?
Your app sees a bright light at the end of a tunnel, with the sound of
harps in the background. Unless it's malware, in which case there's the
strong odor of fire and brimstone.
:-)
LOL.
I am finding this a really serious problem since onPause() pretty much
needs to be treated as onDestory() as far as I can tell. If my app has
to be able to survive being killed without onStop() and onDestory()
being called, why bother having them?
Because the process is generally not killed. That's like asking why we
bother returning values from methods, because a method might raise a
RuntimeException at any point, so why bother returning values? The
reason we return values is because it's useful and random
RuntimeExceptions are, er, the exception.
Also, bear in mind that if your process is killed, a lot of cleanup you
might ordinarily do in onStop() or onDestroy() (e.g., unregister
listeners, stop threads) is no longer relevant.
--
Leigh McRae
www.lonedwarfgames.com
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