CODEWITHSUNDEEP
javaandroidmultithreadingandroid-asynctask
fweigl
fweigl

Reputation: 22028

When is onPostExecuted of AsyncTask executed?

Consider the following example Activity: 

public class ExampleActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_info);

        new ExampleTask().execute("");

        // code line 1
        // code line 2
        // code line 3
        // code line 4
    }

    public class ExampleTask extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... address) {

            // some long-running stuff
            return "";
        }

        protected void onPostExecute(String text) {

        }
    }
}

With 

  new ExampleTask().execute("");

we start an AsyncTask which runs off the UI thread. We can not predict when it will be finished. The AsyncTask's onPostExecute method runs on the UI thread again.

Let's say the AsyncTask is done while code line 2 of the onCreate method is being executed. When will the onPostExecute method be executed? Does it wait until onCreate is done or will it be executed immediately?

I think this question could be generalized to how Java (or at least Android) handles Threads that run off the main thread but return to the main thread and how Java/Android schedules two sequences of code that are 'competing' for immediate excution. Thus it would be nice if an answer would provide a little general insight.

Upvotes: 2

Views: 165

Answers (3)

BeingMIAkashs
BeingMIAkashs

Reputation: 1385

onPostExecute will be called, immediately after the task completion in the doInBackground method. see details about AsyncTask

Let's say the AsyncTask is done while code line 2 of the onCreate method is being executed. When will the onPostExecute method be executed? Does it wait until onCreate is done or will it be executed immediately?

Yes it does wait for onCreate completion, onPostExecute will be called after the onCreate method.

Upvotes: 0

Budius
Budius

Reputation: 39836

you can see it all for your self here: https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/os/AsyncTask.java

           // that's on the background thread
line #288: return postResult(doInBackground(mParams)); 

private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        // here it sends the message to the intenral UI handler
        message.sendToTarget(); 
        return result;
    }

that means:

the AsyncTask post a message to its own internal UI handler, that means that onPostExecute will only be executed after everything else that is queued on the UI Looper gets executed

Upvotes: 2

Alexander
Alexander

Reputation: 48272

It will be called after onCreate() finishes because any callbacks on the UI thread are processed sequentially. So, first goes onCreate() then onPostExecute() However, it seems, this is an implementation detail and you should not rely on that.

Upvotes: 1

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