StreamBuilder 和 Flutter 中的流问题（接收重复数据）

Question

今天我遇到了流和 StreamBuilder 的问题。问题如下：如果您有多个 StreamBuilder 小部件侦听同一个流，并且您将数据添加到它的接收器中，则该数据将通过流传出您拥有的 StreamBuilder 侦听器的数量，换句话说：如果您有一个 StreamController（或 BehaviorSubject），k 个 StreamBuilder 类型的小部件，并且您尝试执行 StreamController.sink.add(event)，则此事件将通过流发出 k 次，每个 StreamBuilder 一个。这是预期的行为吗（预期行为=输入一个事件并从另一端独立地听取一次事件，而与听众的数量无关）？我能够“修复”这种将几乎所有小部件树封装到一个 StreamBuilder 中的方法，但这不如第一种方法最佳，因为您正在渲染整个树而不是一些小节点小部件。这里我留下了一些代码，如果你想测试它（这段代码是 flutter create project_name project 的修改）。谢谢！（P.D：如果你只是在没有 StreamBuilder 的情况下收听流，这很有效，即：streamController.stream.listen..）

import 'dart:async';

import 'package:flutter/cupertino.dart';
import 'package:rxdart/subjects.dart';

class MyAppBloc with ChangeNotifier {
  int _currentIndex;
  BehaviorSubject<bool> _controller;

  MyAppBloc() {
    _currentIndex = 0;
    _controller = BehaviorSubject<bool>();
  }

  Stream<int> get currentIndex => _controller.stream.map<int>((event) {
        print('[event: $event]');
        _currentIndex++;
        return _currentIndex;
      });

  StreamSink<bool> get increment => _controller.sink;

  void close() {
    _controller.close();
  }
}

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:test_project/bloc/my_app_bloc.dart';

class MyHomePage extends StatelessWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;
  Widget leadingBuilder(MyAppBloc bloc) {
    return StreamBuilder<int>(
      initialData: 0,
      stream: bloc.currentIndex,
      builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
        print('[leadingBuilderSnapshot: $snapshot]');
        return Text(snapshot.data.toString());
      },
    );
  }

  StreamBuilder<int> counterBuilder(MyAppBloc bloc) {
    return StreamBuilder<int>(
      initialData: 0,
      stream: bloc.currentIndex,
      builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
        print('[counterBuilderSnapshot: $snapshot]');
        return Text(
          snapshot.data.toString(),
          style: Theme.of(context).textTheme.headline4,
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    print('[build]');
    final _bloc = Provider.of<MyAppBloc>(context);
    return Scaffold(
      appBar: AppBar(
        leading: Container(
          width: 30,
          height: 30,
          alignment: Alignment.center,
          child: leadingBuilder(_bloc),
        ),
        title: Text(title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            StreamBuilder<int>(
              initialData: 0,
              stream: _bloc.currentIndex,
              builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
                return Text('${snapshot.data}');
              },
            ),
            Text(
              'You have pushed the button this many times:',
            ),
            counterBuilder(_bloc),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () => _bloc.increment.add(true),
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

Answer 1

您必须使用 2 streams/sink 并将增量放在获取流之外。

import 'dart:async';
import 'package:rxdart/subjects.dart';

class Bloc {
  int _counter = 0;

  Bloc() {
    _controller.stream.listen(_incrementStream);
  }

  final _counterStream = BehaviorSubject<int>.seeded(0);

  Stream get presentCounter => _counterStream.stream;
  Sink get _addValue => _counterStream.sink;

  StreamController _controller = BehaviorSubject<bool>();
  StreamSink<bool> get incrementCounter => _controller.sink;

  void _incrementStream(data) {
    _counter += 1;
    _addValue.add(_counter);
  }

  void dispose() {
    _counterStream.close();
    _controller.close();
  }
}



import 'package:flutter/material.dart';
import 'package:increment/bloc.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  Bloc _bloc = Bloc();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            StreamBuilder<int>(
                stream: _bloc.presentCounter,
                builder: (context, snapshot) {
                  return Text(
                    '${snapshot.data}',
                    style: Theme.of(context).textTheme.headline4,
                  );
                }),
            SizedBox(
              height: 60,
            ),
            StreamBuilder<int>(
                stream: _bloc.presentCounter,
                builder: (context, snapshot) {
                  return Text(
                    '${snapshot.data}',
                    style: Theme.of(context).textTheme.headline4,
                  );
                }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _bloc.incrementCounter.add(true);
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Answer 2

因为 currentIndex 是 getter 而您正在使用 map()，所以每次都会创建一个新流你打电话给 bloc.currentIndex，StreamBuilder 会接听。

所以在原来的代码中，实际上有1个StreamControlller和k个Streams。 (k: StreamBuilder的个数)

要解决您的问题，您可以创建一个事件控制器，并在一个块内监听它以执行您的逻辑。（eventStream 只从 bloc 本身监听，它只会被创建一次）

例如：

class MyAppBloc {
  MyAppBloc() {
    _eventController.listen((event) {
      print('[event: $event]');
      _indexController.add(currentIndex.value + 1);
    });
  }

  final _indexController = BehaviorSubject<int>.seeded(0);
  final _eventController = PublishSubject<bool>();

  ValueStream<int> get currentIndex => _indexController.stream;
  StreamSink<bool> get increment => _eventController.sink;

  void close() {
    _indexController?.close();
    _eventController?.close();
  }
}

StreamBuilder 和 Flutter 中的流问题（接收重复数据）

Issue with StreamBuilder and streams in Flutter (receiving duplicated data)

stream

duplicates

sink

flutter

stream-builder