BlocListener会在监听的Bloc状态发生变化时被触发，并且在新状态被发射之前对任何Bloc变量的更改做出反应。以下是一些带有变量的Bloc的示例代码，展示了当Bloc发出更新状态时，将触发BlocListener的状态更改回调。

class SampleBloc extends Bloc {
  bool isActive = false;
  @override
  SampleState get initialState => SampleInitialState();

  @override
  Stream mapEventToState(SampleEvent event) async* {
    if (event is SampleEventToggle) {
      isActive = !isActive;
      yield SampleStateActive(isActive: isActive);
    }
  }
}

class SampleStateActive extends SampleState {
  final bool isActive;

  SampleStateActive({required this.isActive});

  @override
  List get props => [isActive];
}

class SampleBlocListener extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return BlocListener(
      listener: (BuildContext context, SampleState state) {
        if (state is SampleStateActive) {
          print('isActive value: ${state.isActive}');
        }
      },
      child: Container(),
    );
  }
}