question: public void loop ( double ellapsedMillis ) { for ( SpringSystemListener listener : mListeners ) { listener . onBeforeIntegrate ( this ) ; } advance ( ellapsedMillis ) ; if ( mActiveSprings . isEmpty ( ) ) { mIdle = true ; } for ( SpringSystemListener listener : mListeners ) { listener . onAfterIntegrate ( this ) ; } if ( mIdle ) { mSpringLooper . stop ( ) ; } }
target:  loop the system until idle