public void addDrawerListener(@NonNull DrawerListener listener){
  if (listener == null) {
    return;
  }
  if (mListeners == null) {
    mListeners=new ArrayList<DrawerListener>();
  }
  mListeners.add(listener);
}

public void addDrawerListener ( @ NonNull DrawerListener listener ) { if ( listener == null ) { return ; } if ( mListeners == null ) { mListeners = new ArrayList < DrawerListener > ( ) ; } mListeners . add ( listener ) ; }
adds the specified listener to the list of listeners that will be notified of drawer events .
