Question

在Flutter中，当流的值更改时，我怎么称呼Navigator.push？我已经尝试了下面的代码，但出现错误。

StreamBuilder(
        stream: bloc.streamValue,
        builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
          if (snapshot.hasData && snapshot.data == 1) {
            Navigator.push(
              context,
              MaterialPageRoute(builder: (context) => SomeNewScreen()),
            );
          }

          return Text("");
        });

Answer 1

您不应使用StreamBuilder来处理导航。 StreamBuilder用于构建屏幕内容，而无其他内容。

相反，您将必须收听流以手动触发副作用。这是通过使用StatefulWidget并覆盖initState / dispose来完成的：

class Example extends StatefulWidget {
  final Stream<int> stream;

  const Example({Key key, this.stream}) : super(key: key);

  @override
  ExampleState createState() => ExampleState();
}

class ExampleState extends State<Example> {
  StreamSubscription _streamSubscription;

  @override
  void initState() {
    super.initState();
    _listen();
  }

  @override
  void didUpdateWidget(Example oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.stream != widget.stream) {
      _streamSubscription.cancel();
      _listen();
    }
  }

  void _listen() {
    _streamSubscription = widget.stream.listen((value) {
      Navigator.pushNamed(context, '/someRoute/$value');
    });
  }

  @override
  void dispose() {
    _streamSubscription.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

请注意，如果您使用InheritedWidget获取流（通常是BLoC），则将要使用didChangeDependencies而不是initState / didUpdateWidget。 / p>

这导致：

class Example extends StatefulWidget {
  @override
  ExampleState createState() => ExampleState();
}

class ExampleState extends State<Example> {
  StreamSubscription _streamSubscription;
  Stream _previousStream;

  void _listen(Stream<int> stream) {
    _streamSubscription = stream.listen((value) {
      Navigator.pushNamed(context, '/someRoute/$value');
    });
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    final bloc = MyBloc.of(context);
    if (bloc.stream != _previousStream) {
      _streamSubscription?.cancel();
      _previousStream = bloc.stream;
      _listen(bloc.stream);
    }
  }

  @override
  void dispose() {
    _streamSubscription.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

Answer 2

您可以使用以下自定义侦听器扩展StreamBuilder：

typedef StreamListener<T> = void Function(T value);

class StreamListenableBuilder<T> extends StreamBuilder<T> {

  final StreamListener<T> listener;

  const StreamListenableBuilder({
    Key key,
    T initialData,
    Stream<T> stream,
    @required this.listener,
    @required AsyncWidgetBuilder<T> builder,
  }) : super(key: key, initialData: initialData, stream: stream, builder: builder);

  @override
  AsyncSnapshot<T> afterData(AsyncSnapshot<T> current, T data) {
    listener(data);
    return super.afterData(current, data);
  }
}

然后以这种方式连接侦听器：

StreamBuilder(
    stream: bloc.streamValue,
    listener: (value) {
      if (value==1) {
        Navigator.push(
          context,
          MaterialPageRoute(builder: (context) => SomeNewScreen()),
        );
      }
    },
    builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
      return Container();
    });

当BLOC中的流值更改时导航到新屏幕

2 个答案: