null safe controller

null_safe_controller.dart

class FooController {
  _State? _state;
    
  bool get hasState => _state != null;
  
  void _attach(_State state) {
    _state = state;
  }
  
  void hello() {
    _state!.hello();
  }
  
  String get value => _state!.value; 
}

class SafeFooController {
  _State? _state;
  
  void _attach(_State state) {
    _state = state;
  }
  
  void Function()? get hello {
    return _state?.hello;
  }
  
  String? get value => _state?.value; 
}

void main() {
  final fooController = FooController();
  final safeFooController = SafeFooController();
  
  if (fooController.hasState) {
    fooController.hello();
    print(fooController.value);
  } else {
    print('empty');
  }
  
  safeFooController.hello?.call();
  print(safeFooController.value ?? 'empty');
}

class _State {
  final String value = 'hello';
  
  void hello() {
    print(value);
  }
}