@Override
  public void onPageSelected(int position) {
    if (attachedState) {
      // this is the main event we're interested in!
      setSelectedPage(position);
    } else {
      // when not attached, don't animate the move, just store immediately
      setCurrentPageImmediate();
    }

    // forward onward to any registered listener
    if (pageChangeListener != null) {
      pageChangeListener.onPageSelected(position);
    }
  }
As this class <b>must</b> act as the {@link ViewPager.OnPageChangeListener} for the ViewPager
(as set by {@link #setViewPager(android.support.v4.view.ViewPager)}).  Applications may set a
listener here to be notified of the ViewPager events.

@param onPageChangeListener
java/com/google/android/apps/authenticator/howitworks/PagingIndicator.java