android - Prevent activity from loading until task has completed -

i have 3 activities:

1. mainactivity
2. activity a
3. activity b

the user can start process in activity b. show notification helper % completed, , go mainactivity.

but don't want user able app until process had completed, - there button on mainactivity want disabled until process has completed.

anyway check if notificationbar still there when click button, or anyway check if async task still running?

thanks

private class downloadstuff extends asynctask<void, void, void> {      protected void doinbackground(void... params) {        // stuff in background          return null;      }       protected void onpostexecute() {          // disable button      } } 

more info: http://developer.android.com/reference/android/os/asynctask.html