dispose method
override
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).
Implementation
@override
void dispose() {
_pointers!.keys.toList().forEach(_removeState);
assert(_pointers!.isEmpty);
_pointers = null;
super.dispose();
}