1. override
_ImmediatePointerState createNewPointerState(PointerDownEvent event)

Subclasses should override this method to create per-pointer state objects to track the pointer associated with the given event.

Source 

@override
_ImmediatePointerState createNewPointerState(PointerDownEvent event) {
  return new _ImmediatePointerState(event.position);
}