(DART) usage supplement

1. Post operators : ? 、!

String? name;
 // Adding a question mark allows the static check to pass , Represents a nullable type ,String?  Is a nullable type  String.

DateTime? _lastPressedAt; 
DateTime.now().difference(_lastPressedAt!)
// Suffixed !  Will turn the expression on the left into the corresponding   Non empty type 
// Like all other transformations , Use  !  Some static security will be lost . These transformations must be done at runtime 

Reference article ：
Dart Post operator ! flutter Question mark symbol

2.late keyword

late PageController pageController;
// （ effect ） Explicitly declare a non empty variable , But not initialized 

Reference article ：
dart in late The role of keywords