流的颤振重装列表& RxDart

Question

在使用Streams和RxDart调用刷新指示器之后，我有一个关于如何重新加载列表的问题。

下面是我的模型课：

    class HomeState {

      List<Event> result;
      final bool hasError;
      final bool isLoading;

       HomeState({
        this.result,
        this.hasError = false,
        this.isLoading = false,
      });

      factory HomeState.initial() =>
          new HomeState(result: new List<Event>());
      factory HomeState.loading() => new HomeState(isLoading: true);
      factory HomeState.error() => new HomeState(hasError: true);

    }


    class HomeBloc {

      Stream<HomeState> state;
      final EventRepository repository;

      HomeBloc(this.repository) { 
        state = new Observable.just(new HomeState.initial());
      }

      void loadEvents(){
        state = new Observable.fromFuture(repository.getEventList(1)).map<HomeState>((List<Event> list){
          return new HomeState(
              result: list,
              isLoading: false 
          );    
        }).onErrorReturn(new HomeState.error())
        .startWith(new HomeState.loading());
      }

    }

我的小部件：

    class HomePageRx extends StatefulWidget {
      @override
      _HomePageRxState createState() => _HomePageRxState();
    }

    class _HomePageRxState extends State<HomePageRx> {
      HomeBloc bloc;

      _HomePageRxState() {
        bloc = new HomeBloc(new EventRest());
        bloc.loadEvents();
      }

      Future<Null> _onRefresh() async {
        bloc.loadEvents();
        return null;
      }

      @override
      Widget build(BuildContext context) {
        return new StreamBuilder(
            stream: bloc.state,
            builder: (BuildContext context, AsyncSnapshot<HomeState> snapshot) {
              var state = snapshot.data;
              return new Scaffold(
                body: new RefreshIndicator(
                  onRefresh: _onRefresh,
                  child: new LayoutBuilder(builder:
                      (BuildContext context, BoxConstraints boxConstraints) {
                    if (state.isLoading) {
                      return new Center(
                        child: new CircularProgressIndicator(
                          backgroundColor: Colors.deepOrangeAccent,
                          strokeWidth: 5.0,
                        ),
                      );
                    } else {
                      if (state.result.length > 0) {
                        return new ListView.builder(
                            itemCount: snapshot.data.result.length,
                            itemBuilder: (BuildContext context, int index) {
                              return new Text(snapshot.data.result[index].title);
                            });
                      } else {
                        return new Center(
                          child: new Text("Empty data"),
                        );
                      }
                    }
                  }),
                ),
              );
            });
      }
    }

问题是，当我执行拉式刷新列表时，UI不刷新(服务器被调用，刷新指示器的动画也被调用)，我知道这个问题与流有关，但我不知道如何解决。

预期结果:在加载数据之前显示CircularProgressIndicator

有什么帮助吗？谢谢

Answer 1

不应该更改state的实例。

相反，您应该向可观察的对象提交一个新的值。这样，正在监听StreamBuilder的state将被通知一个新的值。

这意味着您不能只在内部拥有一个Observable实例，因为Observable没有任何方法来添加新值。所以你需要一个Subject。

基本上，这会将您的Bloc更改为：

class HomeBloc {
  final Stream<HomeState> state;
  final EventRepository repository;
  final Subject<HomeState> _stateSubject;

  factory HomeBloc(EventRepository respository) {
    final subject = new BehaviorSubject(seedValue: new HomeState.initial());
    return new HomeBloc._(
        repository: respository,
        stateSubject: subject,
        state: subject.asBroadcastStream());
  }

  HomeBloc._({this.state, Subject<HomeState> stateSubject, this.repository})
      : _stateSubject = stateSubject;

  Future<void> loadEvents() async {
    _stateSubject.add(new HomeState.loading());

    try {
      final list = await repository.getEventList(1);
      _stateSubject.add(new HomeState(result: list, isLoading: false));
    } catch (err) {
      _stateSubject.addError(err);
    }
  }
}

另外，请注意loadEvent如何在异常情况下使用addError。而不是用HomeState来推动hasError: true。