1. override
void handleEvent(PointerEvent event)

Called when a pointer event is routed to this recognizer.

Source 

@override
void handleEvent(PointerEvent event) {
  assert(_state != ScaleState.ready);
  bool configChanged = false;
  if (event is PointerMoveEvent) {
    VelocityTracker tracker = _velocityTrackers[event.pointer];
    assert(tracker != null);
    tracker.addPosition(event.timeStamp, event.position);
    _pointerLocations[event.pointer] = event.position;
  } else if (event is PointerDownEvent) {
    configChanged = true;
    _pointerLocations[event.pointer] = event.position;
  } else if (event is PointerUpEvent) {
    configChanged = true;
    _pointerLocations.remove(event.pointer);
  }

  _update(configChanged, event.pointer);

  stopTrackingIfPointerNoLongerDown(event);
}