The ID of the primary pointer this recognizer is tracking.
If this recognizer is no longer tracking any pointers, this field holds the ID of the primary pointer this recognizer was most recently tracking. This enables the recognizer to know which pointer it was most recently tracking when acceptGesture or rejectGesture is called (which may be called after the recognizer is no longer tracking a pointer if, e.g. GestureArenaManager.hold has been called, or if there are other recognizers keeping the arena open).
int? get primaryPointer => _primaryPointer;