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I have an activity in which I do server sync with a back end server using a subclass of asyctask.

Given it is a network task and might take couple of seconds. I am afraid of the following scenario to take place.

The activity starts, and whenever the asynctask should start to run, it does so. The onPrexecute() is called, executed, and over. Than the doInBackground() is called, and is done so, however, just when the method is being executed, the user presses the home button and swipes the app from the RECENT APPS. (this ofcourse causes the app to terminate and all the onDestroy methods get called of the alive activities..(Correct me if I'm wrong on this one)).

In my onPostExecute() method, I am inserting the data to DB and updating the VIEWs. But since the app is 'terminated' the onPostExecute() method never runs.

my questions are :

  1. When the user presses the home button and gets out of the app and swipes the app, is doInBackground halted at that moment ? that is, it is cut in the middle and does not continue what it does ?
  2. What happens to the data that I was going to get from the server and put inside the DB ? Is it advisable to do put the data in the db inside the doInBackground ?
tony9099
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4 Answers4

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  1. AsyncTask is a background task it will continue to run even if the app is closed and onDestroy() is called. The problem would be when the task enters onPostExecute() and tries to update any views associated with the activity it is bound to, as that activity no longer exists. If you're concerned about this, one thing I've done is to keep a reference to that AsyncTask in the calling activity, and then call the method myAsyncTaskReference.cancel(true) to cancel it in the onDestroy() of the calling activity.

  2. Yes, I would put the DB operations in the doInBackground() method, as that runs in the background on a separate thread, and doesn't require a reference to the app activity.

Have you considered using a service for this type of thing instead? I would strongly recommend an IntentService, which is a subclass of service which is specifically designed to perform a single task in the background (similar to AsyncTask), and then kill itself once completed. It's quite easy to implement and usually only involves overriding one method - onHandleIntent() - which is called when the service starts.

It can handle all your DB operations and it has no reference to an activity and so the problem you're worried about in #1 would never occur. If you need to update a view, you can have your IntentService generate a broadcast once it's completed, and your Activity can register for that broadcast and update it's views accordingly when it receives it. And if your activity is no longer around once the broadcast is sent then it doesn't matter and nothing will fail.

physphil
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  • but wouldnt the IntentService (just like the asynctask), be stopped whenever the app gets killed, so it wont be even able to fire the broadcast.? – tony9099 Oct 28 '13 at 20:37
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    Nope, it won't be stopped. The IntentService is completely separate from the calling activity, so it will continue to run until it finishes its tasks, even if the app that started it is killed. This is why it's perfect for the DB operations, as it will still write all the data it receives to the DB. The broadcast is sent just to notify the calling activity that the service has completed and it should update its views - so if the activity is no longer running it doesn't matter anyways. – physphil Oct 28 '13 at 20:45
  • No. see the link in my answer. I have tested this. – wtsang02 Oct 28 '13 at 20:48
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    Here's a decent tutorial on how to implement an IntentService and a send a broadcast when completed - http://mobile.tutsplus.com/tutorials/android/android-fundamentals-intentservice-basics/ – physphil Oct 28 '13 at 20:58
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  1. When user presses 'Home', your Activtiy will pause but doInBackground will NOT, but may or may not terminate by system when system feels like it. Activity's onPause will be called. Your Asynctask doInBackGround will NOT halt. It will continue to run until the system kills your App process.
  2. Yes, Db operations can take long. Its advisable to put in doInBackground because it runs on another Thread. onpre/onpostexcute runs on the main thread. If you are worried that System may terminate half way of your db operations, you shouse set Transcation, and only when you are done, you called commit.

Check out this post, I have tested it.

Community
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wtsang02
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  1. no, it doesn't stop.

  2. It is relly better to put it to datastorage of some kind and then work with it

It is always better to use service for such goals. AsyncTasks just don't fit here. Ask your service to do the work, and then you may start or quit activities as you wish to.

Eugene
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If swiping app from recent stack, it is equivalent to close the app hence it will kill all tasks related to the process so async task will also get killed by the android system. ( even intent service is also get killed)

It is device dependent also. Manufacturers customised removing app from recents behaviour

He Termis
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