java - WeakReference/AsyncTask pattern in android

Question

I have a question regarding this simple frequently occurring situation in android .

We have a main activity , we invoke an AsyncTask alongwith the reference of the mainactivity , so that that the AsyncTask can update the views on the MainActivity.

I will break down the event into steps

• MainActivity creates an AyncTask , passes its reference to it .
• AysncTask , starts it's work , downloading ten files for example
• The user changed the orientation of the device. This results in an orphan pointer in the AsyncTask
• When the AsyncTask completes , and tries to access the activity to update the status , it crashes , because of the null pointer .

The solution for the above is to keep a WeakReference in the AsyncTask as recommended by the book "Pro Android 4"

WeakReference<Activity> weakActivity;

in method onPostExecute

Activity activity = weakActivity.get();
if (activity != null) {
   // do your stuff with activity here
}

How does this resolve the situation ?

My question it , if my asynctask is downloading ten files , and upon completion of 5 the activity is restarted (because of an orientation change) then would my FileDownloadingTask be invoked once again ?.

What would happen to the previous AsyncTask that was initially invoked ?

Thank you , and I apologize for the length of the question .

Answer 1

How does this resolve the situation ?

The WeakReference allows the Activity to be garbage collected, so you don't have a memory leak.

A null reference means that the AsyncTask cannot blindly try to update a user-interface that is no longer attached, which would throw exceptions (e.g. view not attached to window manager). Of course you have to check for null to avoid NPE.

if my asynctask is downloading ten files , and upon completion of 5 the activity is restarted (because of an orientation change) then would my FileDownloadingTask be invoked once again ?.

Depends on your implementation, but probably yes - if you don't deliberately do something to make a repeat download unnecessary, such as caching the results somewhere.

What would happen to the previous AsyncTask that was initially invoked ?

In earlier versions of Android it would run to completion, downloading all of the files only to throw them away (or perhaps cache them, depending on your implementation).

In newer Android's I am suspicious that AsyncTask's are being killed along with the Activity that started them, but my basis for suspicion is only that the memory-leak demo's for RoboSpice (see below) do not actually leak on my JellyBean devices.

If I may offer some advice: AsyncTask is not suitable for performing potentially long running tasks such as networking.

IntentService is a better (and still relatively simple) approach, if a single worker thread is acceptable to you. Use a (local) Service if you want control over the thread-pool - and be careful not to do work on the main thread!

RoboSpice seems good if you are looking for a way to reliably perform networking in the background (disclaimer: I have not tried it; I am not affiliated). There is a RoboSpice Motivations demo app in the play store which explains why you should use it by demo-ing all the things that can go wrong with AsyncTask - including the WeakReference workaround.

See also this thread: Is AsyncTask really conceptually flawed or am I just missing something?

Update:

I created a github project with an example of downloading using IntentService for another SO question (How to fix android.os.NetworkOnMainThreadException?), but it is also relevant here, I think. It has the added advantage that, by returning the result via onActivityResult, a download that is in flight when you rotate the device will deliver to the restarted Activity.