@Override
    public void onBackStackChanged() {
        mShowingBack = (getFragmentManager().getBackStackEntryCount() > 0);

        // When the back stack changes, invalidate the options menu (action bar).
        invalidateOptionsMenu();
    }
Whether or not we're showing the back of the card (otherwise showing the front).
app/src/main/java/home/smart/fly/animations/ui/activity/demos/CardFlipActivity.java