When an Activity goes in pause state, an instance of it remains in the Activity stack that is managed by operating system. And I know after a while, it destroys the instance. Now my question is whether the onDestroy() method is called when operating system destroys the Activity instance after a long while?
Like if I put a Toast inside onDestroy() method, that will be shown when the Activity instance gets destroyed by OS? (I know it will be shown by pressing back button).
The nature of this question makes it hard to test because sometimes it takes a day or more for OS in order to destroy an Activity instance in stack.
There is no guarantee that it will be called. You can see Activity#onDestroy for that.
Note: do not count on this method being called as a place for saving data! For example, if an activity is editing data in a content provider, those edits should be committed in either onPause() or onSaveInstanceState(Bundle), not here. This method is usually implemented to free resources like threads that are associated with an activity, so that a destroyed activity does not leave such things around while the rest of its application is still running. There are situations where the system will simply kill the activity's hosting process without calling this method (or any others) in it, so it should not be used to do things that are intended to remain around after the process goes away.
By simply killing it could be e.g. a System.exit call or something similar where it skips the lifecycle hooks.
Related
I have implemented ActivityLifecycleCallbacks in a private class inside my Application class.
If Home button or Back button is used, lifecycle's onStop() and onPause() methods are getting called. If I kill the activity by swiping out from background, onDestroy() method is not getting called. It gets called when I start my activity next time. I am using moveTaskToBack(true) in case of Back press.
What must be the issue and which lifecycle method should get called when we swipe out from background?
From the documentation of onDestroy():
Note: do not count on this method being called as a place for saving
data! For example, if an activity is editing data in a content
provider, those edits should be committed in either onPause() or
onSaveInstanceState(Bundle), not here. This method is usually
implemented to free resources like threads that are associated with an
activity, so that a destroyed activity does not leave such things
around while the rest of its application is still running. There are
situations where the system will simply kill the activity's hosting
process without calling this method (or any others) in it, so it
should not be used to do things that are intended to remain around
after the process goes away.
Such a situation is swiping the app out of the recent tasks list.
Check out this answer on how to get notified when the app is getting swiped out of Recents.
I remember reading a similar question on StackOverflow. Here was the top answer:
Handle exit application from Task Manager
Handle exit application from Task Manager
Unfortunately there isn't a good answer for handling this situation. If the app is force-killed, onDestroy method isn't necessarily called. According to the documentation.
Note: do not count on this method being called as a place for saving data! For example, if an activity is editing data in a content provider, those edits should be committed in either onPause() or onSaveInstanceState(Bundle), not here.
If you can, clean up in the onPause() method. In order for the user to get to that screen to kill the app, it has to have been backgrounded and thus onPause() would be called. (see documentation)
swiping app from back ground i.e. clearing from recents causes the whole process and task to be killed and that time onDestroy() may not be called , when user presses back from an activity then only that activity is killed not the process.
I have an Android existential question:
Does an App's onDestroy() get called when I kill it by swiping it out of the task manager?
If not... how can I detect a "killing" swiping event on the app, and do some final stuff beore it dies?
onDestroy()
Perform any final cleanup before an activity is destroyed. This can
happen either because the activity is finishing (someone called
finish() on it, or because the system is temporarily destroying this
instance of the activity to save space.
Note: do not count on this method being called as a place for saving data!
There are situations where the system will simply kill the activity's
hosting process without calling this method (or any others) in it, so
it should not be used to do things that are intended to remain around
after the process goes away.
Instead, rely on onPause() and onStop() to do any final work before your app goes out of view.
Is your main concern the "swiping" event or just saving data? :)
In my app, In an special situation I need to start an Activity and put it behind the current showing Activity, I mean if the current Activity, which is being shown by the user was closed, the user now can see the Activity which I just started. how to do that?
You cannot do this.
Activities will always start right away. You cannot manipulate the back stack in the way you want.
You would have to detect the activity closing and then start your new activity then.
You can implement your case by using onDestroy ()
Perform any final cleanup before an activity is destroyed. This can happen either because the activity is finishing (someone called finish() on it, or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the isFinishing() method.
Note: do not count on this method being called as a place for saving data! For example, if an activity is editing data in a content provider, those edits should be committed in either onPause() or onSaveInstanceState(Bundle), not here. This method is usually implemented to free resources like threads that are associated with an activity, so that a destroyed activity does not leave such things around while the rest of its application is still running. There are situations where the system will simply kill the activity's hosting process without calling this method (or any others) in it, so it should not be used to do things that are intended to remain around after the process goes away.
Derived classes must call through to the super class's implementation of this method. If they do not, an exception will be thrown.
Source : http://developer.android.com
I've read several posts that describe the difference between onStart() and onResume(): onStart() is called when the activity becomes visible, onResume() is called when the activity is ready for interaction from the user. fine.
I've always just added code to onPause() and onResume(), and never bothered with onStart() and onStop().
Can anyone give some concrete examples of what you might do in onStart(), vs. onResume()? Same goes for onStop() and onPause(), how is onStop() useful? I must be missing something fundamental here.
onStop() will (for example) be called when you leave the activity for some other activity (edit: almost. see commonswares comment about dialog themed activities).
For example if you use startActivity() in activity A to start activity B. When you press back in activity B you will return to activity A and onStart will be called.
This differs from some of the reasons onPause might be called without onStop being called. If for example the screen times out or you press the standy button onPause will be called, but probably not onStop (depending on memory available and whatnot), so it is a "lighter pause". onStop will be probably be called eventually even in this case, but not immediately.
Ok, but what's the use
Often there is no specific use, but there might be. Since your activities will keep its memory state on the stack even after you start some other activity, that stack will increase with the number of activities started (height of the stack).
This can lead to large memory usage in some applications. After a while the framework will kick in and kill some activities on the stack, but this is rather blunt and will probably mean a lot of states to be retained when returning.
So an example use for onStart/onStop is if you want to release some state when leaving an activity for another and recreate it when you get back.
I have used it to set listadapters to null, empty image caches and similar (in very specific applications). If you want to free the memory used by visible views in a listadapter you can recreate it in onstart and let the views be picked up by the gc. This will increase the likelyhood that the rest of the memory state of the activity will live on.
Some resources can be deemed good enough to save while the activity instance is alive and some only when it is on the front of the stack. It is up to you to decide what is best in your application and the granularity of create/start/resume gives you that.
onStart() works after onCreate() ended its task.
It's a good place to put a broadcastReceiver or initialize some state about the UI that should display consistently anytime the user comes back to this activity.
onResume() works when you come back to your Intent or Activity by pressing the back button. So onPause will be called every time a different activity comes to the foreground.
i think that your question is pretty explained here on the doc : read about the Activity Life Cycle
Looking at the Activity Life Cycle diagram, I notice that onPause() and onStop() can both lead to the "process" being killed. This would require onCreate() to be called when the user wants to resume their application. The point being that onStop() is not necessarily called, nor is onDestroy(), but that onPause() may be the only event that the Activity may see. This being the case, onPause() must handle saving the application status so that the user can return back to it later, regardless of whether onStop() is called or not.
I can see onDestroy() being used to clean up Activity specific resources that would naturally be eliminated in a process kill action. Is there anything else that onDestroy() would be good for?
And what would onStop() be good for? Why would I want to override it for?
If I got your question right: It depends what you want to do with your application. Let's say you are programming application that uses GPS. In the onStop() which is called when the activity is no longer visible to the user, you can remove these requests. Or you can stop the some service if your application is running any. Or you can save preferences (not recommended, do it in onPause() instead), or you can close permanent connection to a server.....If I think of anything else, I'll add more...
If you have read the doc further, you'll see following:
Saving activity state
The introduction to Managing the Activity Lifecycle briefly mentions
that when an activity is paused or stopped, the state of the activity
is retained. This is true because the Activity object is still held in
memory when it is paused or stopped—all information about its members
and current state is still alive. Thus, any changes the user made
within the activity are retained in memory, so that when the activity
returns to the foreground (when it "resumes"), those changes are still
there.
However, when the system destroys an activity in order to recover
memory, the Activity object is destroyed, so the system cannot simply
resume it with its state intact. Instead, the system must recreate the
Activity object if the user navigates back to it. Yet, the user is
unaware that the system destroyed the activity and recreated it and,
thus, probably expects the activity to be exactly as it was. In this
situation, you can ensure that important information about the
activity state is preserved by implementing an additional callback
method that allows you to save information about the state of your
activity and then restore it when the the system recreates the
activity.
Summary: After completing onStop() Activity object is still alive in memory. And this will help the system to restore the activity.
Very basic example: consider you are showing your activity to user, and suddenly your friend calls you! Rest you can understand..
So now it it up to you that, which are the resources/objects/connections should be released on which event.
Another example would be to register and unregister a broadcast receiver.
Note that usually these things are placed in onResume and onPause, the difference is subtle though, onResume/onPause are called when the activity gets placed behind another activity, onStart/onStop are called when the activity is no longer visible in the screen.