Asynctask Won't Stop Even When the Activity Has Destroyed

AsyncTask won't stop even when the Activity has destroyed

The answer given by @Romain Guy is correct. Nevertheless, I would like to add a complement of information and give a pointer to a library or 2 that can be used for long running AsyncTask and even more for network oriented asynctasks.

AsyncTasks have been designed for doing stuff in background. And yes, you can stop it using the cancel method. As you download stuff from the Internet, I strongly suggest you take care of your thread when it is the IO blocking state. You should organize your download as follow :

public void download() {
    //get the InputStream from HttpUrlConnection or any other
    //network related stuff
    while( inputStream.read(buffer) != -1 && !Thread.interrupted() ) {
      //copy data to your destination, a file for instance
    }
    //close the stream and other resources
}

Using the Thread.interrupted flag will help your thread to quit properly a blocking io state. Your thread will be more responsive to an invocation of the cancel method.

AsyncTask design flaw

But if your AsyncTask lasts for too long, then you will face 2 different issues :

1. Activities are poorly tied to the activity life cycle and you won't get the result of your AsyncTask if your activity dies. Indeed, yes, you can but it will be the rough way.
2. AsyncTask are not very well documented. A naive, though intuitive, implementation and use of an asynctask can quickly lead to memory leaks.

RoboSpice, the library I would like to introduce, uses a background service to execute this kind of requests. It has been designed for network requests. It provides additional features such as automatic caching of requests' results.

Here is the reason why AsyncTasks are bad for long running tasks. The following reasonning is an adaptation from exerpts of RoboSpice motivations : the app that explains why using RoboSpice is filling a need on the Android platform.

The AsyncTask and Activity life cycle

AsyncTasks don't follow Activity instances' life cycle. If you start an AsyncTask inside an Activity and you rotate the device, the Activity will be destroyed and a new instance will be created. But the AsyncTask will not die. It will go on living until it completes.

And when it completes, the AsyncTask won't update the UI of the new Activity. Indeed it updates the former instance of the activity that
is not displayed anymore. This can lead to an Exception of the type java.lang.IllegalArgumentException: View not attached to window manager if you
use, for instance, findViewById to retrieve a view inside the Activity.

Memory leak issue

It is very convenient to create AsyncTasks as inner classes of your Activities. As the AsyncTask will need to manipulate the views
of the Activity when the task is complete or in progress, using an inner class of the Activity seems convenient : inner classes can
access directly any field of the outer class.

Nevertheless, it means the inner class will hold an invisible reference on its outer class instance : the Activity.

On the long run, this produces a memory leak : if the AsyncTask lasts for long, it keeps the activity "alive"
whereas Android would like to get rid of it as it can no longer be displayed. The activity can't be garbage collected and that's a central
mechanism for Android to preserve resources on the device.

Progress of your task will be lost

You can use some workarounds to create a long running asynctask and manage its life cycle accordingly to the life cycle of the activity. You can either cancel the AsyncTask in the onStop method of your activity or you can let your async task finish, and not loose its progress and relink it to the next instance of your activity.

This is possible and we show how in RobopSpice motivations, but it becomes complicated and the code is not really generic. Moreover, you will still loose the progress of your task if the user leaves the activity and comes back. This same issue appears with Loaders, although it would be a simpler equivalent to the AsyncTask with relinking workaround mentionned above.

Using an Android service

The best option is to use a service to execute your long running background tasks. And that is exactly the solution proposed by RoboSpice. Again, it is designed for networking but could be extended to non-network related stuff. This library has a large number of features.

You can even get an idea of it in less than 30 seconds thanks to an infographics.

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It is really a very very bad idea to use AsyncTasks for long running operations. Nevertheless, they are fine for short living ones such as updating a View after 1 or 2 seconds.

I encourage you to download the RoboSpice Motivations app, it really explains this in-depth and provides samples and demonstrations of the different ways to do some network related stuff.

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If you are looking for an alternative to RoboSpice for non network related tasks (for instance without caching), you could also have a look at Tape.

What happens to running AsyncTasks when the Activity changes?

AsyncTask is an abstract Android class which helps the Android applications to handle the Main UI thread in efficient way. AsyncTask class allows us to perform long lasting tasks/background operations and show the result on the UI thread without affecting the main thread.

1. AsyncTask processes are not automatically killed by the OS. AsyncTask processes run in the background and is responsible for finishing it's own job in any case. You can cancel your AsycnTask by calling cancel(true) method. This will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object) method is called instead of onPostExecute() after doInBackground() returns.

2. After completion of it's operation, the background thread it's working on is stopped. AsyncTask has an onPostExecute() which is called once your work is finished. This method is called after doInBackground() method completes processing. Result from doInBackground() is passed to this method.

How to cancel AsyncTask when Activity finishes?

I don't understand if your "cancel" means rollback but you have a cancel method on the AsyncTask class.

Android AsyncTask returns when Activity that executed it... is gone

A very common hurdle in Android development. When I first stumbled on this issue, I found this Q with a lot of great discussion that illuminated things for me:

Is AsyncTask really conceptually flawed or am I just missing something?

It is imperative on you, the author of the AsyncTask, to ensure that it's orphaned completion is 'safe' or else to cancel/abort. The conventional wisdom is to cancel (or at least logically dissociate via some mechanism of your own rolling, as @iagreen proposes) the Task instance from within your Activity's onPause or onDestroy.

Ideal way to cancel an executing AsyncTask

There is frequent guidance around the web from Android team members that AsyncTask is intended to be for short operations whose lifecycle closely matches the 'intended' lifecycle of its parent activity, so if you find yourself doing longrunning operations and having to resume partial progress, etc you might want to look at explicit thread management or coding up a Service, etc:

http://developer.android.com/reference/android/app/Service.html

android: onPostExecute of Asynchtask after Activity finished

1-I want to know will onPostExecute executes by considering that
activity is not displaying on screen anymore?

yes!

2-In case answer is yest, will it cause exceptions or not? (because of
accessing UI objects which are not longer displayed on screen).

Yes! it may cause Exception because your Instance of your Activity and Views which you use in your AsyncTask are not exist anymore

This Link will help you more : AsyncTask won't stop even when the activity has destroyed

AsyncTask status in Activity's onStop() state

activity stop will not make your async task to stop. Your async task will continue and if you have some reference of view in post execute it will cause exception.

You can also use AsyncTaskLoader instead.

Read about it:

http://mobile.dzone.com/articles/loaders-versus-asynctask

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