1. override
void didStopTrackingLastPointer(int pointer)

Called when the number of pointers this recognizer is tracking changes from one to zero.

The given pointer ID is the ID of the last pointer this recognizer was tracking.

Source 

@override
void didStopTrackingLastPointer(int pointer) {
  switch(_state) {
    case ScaleState.possible:
      resolve(GestureDisposition.rejected);
      break;
    case ScaleState.ready:
      assert(false);  // We should have not seen a pointer yet
      break;
    case ScaleState.accepted:
      break;
    case ScaleState.started:
      assert(false);  // We should be in the accepted state when user is done
      break;
  }
  _state = ScaleState.ready;
}