PerfectAsyncTask.java

/**
 * This Activity displays the screen's UI, creates a TaskFragment
 * to manage the task, and receives progress updates and results 
 * from the TaskFragment when they occur.
 */
public class MainActivity extends Activity implements TaskFragment.TaskCallbacks {

  private static final String TAG_TASK_FRAGMENT = "task_fragment";

  private TaskFragment mTaskFragment;

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

    FragmentManager fm = getFragmentManager();
    mTaskFragment = (TaskFragment) fm.findFragmentByTag(TAG_TASK_FRAGMENT);

    // If the Fragment is non-null, then it is currently being
    // retained across a configuration change.
    if (mTaskFragment == null) {
      mTaskFragment = new TaskFragment();
      fm.beginTransaction().add(mTaskFragment, TAG_TASK_FRAGMENT).commit();
    }

    // TODO: initialize views, restore saved state, etc.
  }

  // The four methods below are called by the TaskFragment when new
  // progress updates or results are available. The MainActivity 
  // should respond by updating its UI to indicate the change.

  @Override
  public void onPreExecute() { ... }

  @Override
  public void onProgressUpdate(int percent) { ... }

  @Override
  public void onCancelled() { ... }

  @Override
  public void onPostExecute() { ... }
}


// fragment
/**
 * This Fragment manages a single background task and retains 
 * itself across configuration changes.
 */
public class TaskFragment extends Fragment {

  /**
   * Callback interface through which the fragment will report the
   * task's progress and results back to the Activity.
   */
  interface TaskCallbacks {
    void onPreExecute();
    void onProgressUpdate(int percent);
    void onCancelled();
    void onPostExecute();
  }

  private TaskCallbacks mCallbacks;
  private DummyTask mTask;

  /**
   * Hold a reference to the parent Activity so we can report the
   * task's current progress and results. The Android framework 
   * will pass us a reference to the newly created Activity after 
   * each configuration change.
   */
  @Override
  public void onAttach(Activity activity) {
    super.onAttach(activity);
    mCallbacks = (TaskCallbacks) activity;
  }

  /**
   * This method will only be called once when the retained
   * Fragment is first created.
   */
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Retain this fragment across configuration changes.
    setRetainInstance(true);

    // Create and execute the background task.
    mTask = new DummyTask();
    mTask.execute();
  }

  /**
   * Set the callback to null so we don't accidentally leak the 
   * Activity instance.
   */
  @Override
  public void onDetach() {
    super.onDetach();
    mCallbacks = null;
  }

  /**
   * A dummy task that performs some (dumb) background work and
   * proxies progress updates and results back to the Activity.
   *
   * Note that we need to check if the callbacks are null in each
   * method in case they are invoked after the Activity's and
   * Fragment's onDestroy() method have been called.
   */
  private class DummyTask extends AsyncTask<Void, Integer, Void> {

    @Override
    protected void onPreExecute() {
      if (mCallbacks != null) {
        mCallbacks.onPreExecute();
      }
    }

    /**
     * Note that we do NOT call the callback object's methods
     * directly from the background thread, as this could result 
     * in a race condition.
     */
    @Override
    protected Void doInBackground(Void... ignore) {
      for (int i = 0; !isCancelled() && i < 100; i++) {
        SystemClock.sleep(100);
        publishProgress(i);
      }
      return null;
    }

    @Override
    protected void onProgressUpdate(Integer... percent) {
      if (mCallbacks != null) {
        mCallbacks.onProgressUpdate(percent[0]);
      }
    }

    @Override
    protected void onCancelled() {
      if (mCallbacks != null) {
        mCallbacks.onCancelled();
      }
    }

    @Override
    protected void onPostExecute(Void ignore) {
      if (mCallbacks != null) {
        mCallbacks.onPostExecute();
      }
    }
  }
}