1. override
void dispose()

Releases any resources used by the object.

This method is called by the owner of this gesture recognizer when the object is no longer needed (e.g. when a gesture recognizer is being unregistered from a GestureDetector, the GestureDetector widget calls this method).

Source 

@override
void dispose() {
  _velocityTrackers.clear();
  super.dispose();
}