onEnter property Null safety
Triggered when a mouse pointer, with or without buttons pressed, has entered the region and validForMouseTracker is true.
This callback is triggered when the pointer has started to be contained by the region, either due to a pointer event, or due to the movement or disappearance of the region. This method is always matched by a later onExit.
@override final PointerEnterEventListener? onEnter;