Flutter状态管理:RxDart

一、前言

  • BLoC《Flutter状态管理:BLoC(Business Logic Component)》
  • 编程概念《编程方式:命令式 vs 函数式》

二、StreamController增加版:Subject

其实无论从订阅或者变换都可以看出, Dart 中的 Stream 已经自带了类似 rx 的效果,但是为了让 rx 的用户们更方便的使用,ReactiveX 就封装了 rxdart 来满足用户的熟悉感,如下图所示为它们的对应关系:

Dart RxDart
StreamController Subject
Stream Observable

rxdart 中, Observable 是一个 Stream,而 Subject 继承了 Observable 也是一个 Stream,并且 Subject 实现了 StreamController 的接口,所以它也具有 Controller 的作用。

如下代码所示是 rxdart 的简单使用,可以看出它屏蔽了外界需要对 StreamSubscriptionStreamSink 等的认知,更符合 rx 历史用户的理解

final subject = PublishSubject();

subject.stream.listen(observerA);
subject.add("AAAA1");
subject.add("AAAA2"));

subject.stream.listen(observeB);
subject.add("BBBB1");
subject.close();

以上方代码为例:

  • PublishSubject 内部实际创建是创建了一个广播 StreamController.broadcast ;
  • 当我们调用 add 或者 addStream 时,最终会调用到的还是我们创建的 StreamController.add;
  • 当我们调用 onListen 时,也是将回调设置到 StreamController 中。
  • rxdart 在做变换时,我们获取到的 Observable 就是 this,也就是 PublishSubject 自身这个 Stream ,而 Observable 一系列的变换,也是基于创建时传入的 stream 对象,比如:
  @override
  Observable asyncMap(FutureOr convert(T value)) =>Observable(_stream.asyncMap(convert));

所以我们可以看出来,rxdart 只是对 Stream 进行了概念变换,变成了我们熟悉的对象和操作符,而这也是为什么 rxdart 可以在 StreamBuilder 中直接使用的原因。

RxDart提供了三种StreamController的变体来应用到不同的场景:

  • PublishSubject
  • BehaviorSubject
  • ReplaySubject

以下来分别讲序这三种场景的使用情况。

2.1、PublishSubject

Flutter状态管理:RxDart_第1张图片

PublishSubject最常见,从图中可看到,listener只能监听到订阅之后的事件:

final subject = PublishSubject();

subject.stream.listen((event) => print("observer1 => $event"));
subject.add(1);
subject.add(2);

subject.stream.listen((event) => print("observer2 => $event"));
subject.add(3);
subject.close();

// 打印输出:
// flutter: observer1 => 1
// flutter: observer2 => 3
// flutter: observer1 => 2
// flutter: observer1 => 3

2.2、BehaviorSubject

Flutter状态管理:RxDart_第2张图片

BehaviorSubject也是广播,与PublishSubject的区别是:它会返回订阅前的最后一次事件:

final subject = BehaviorSubject();

subject.stream.listen((event) => print("observer1 => $event"));
subject.add(1);
subject.add(2);

subject.stream.listen((event) => print("observer2 => $event"));
subject.add(3);
subject.close();

// 打印输出:
// flutter: observer1 => 1
// flutter: observer2 => 2
// flutter: observer2 => 3
// flutter: observer1 => 2
// flutter: observer1 => 3

2.3、ReplaySubject

顾名思义:回放!会将订阅前的事件都发送给新的订阅者:

final subject = ReplaySubject();

subject.stream.listen((event) => print("observer1 => $event"));
subject.add(1);
subject.add(2);

subject.stream.listen((event) => print("observer2 => $event"));
subject.add(3);
subject.close();

// 打印输出:
// flutter: observer1 => 1
// flutter: observer2 => 1
// flutter: observer2 => 2
// flutter: observer2 => 3
// flutter: observer1 => 2
// flutter: observer1 => 3

三、实战演练

3.1、新建Model(CountModel.dart)

import 'package:rxdart/rxdart.dart';

class CountModel {
  BehaviorSubject _subject = BehaviorSubject.seeded(0);

  get stream => _subject.stream;
  get value => _subject.value;

  increment() {
    _subject.add(value + 1);
  }

  decrement() {
    _subject.add(value - 1);
  }
}

3.2、新建页面(RxdartPage.dart)

import 'package:flutter/material.dart';
import 'package:stateresearch/model/CountModel.dart';

class RxdartPage extends StatelessWidget {
  final CountModel _model = CountModel();

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

  Widget _body(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("RxdartPage"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('You have pushed the button this many times:'),
            StreamBuilder(
              stream: _model.stream,
              builder: (BuildContext context, AsyncSnapshot snapshot) {
                return Text("${snapshot.data}");
              },
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _model.increment,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

3.3、修改main文件

import 'package:flutter/material.dart';
import 'package:stateresearch/pages/RxdartPage.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter状态管理',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: RxdartPage(),
    );
  }
}

以上就是简单的局部状态管理的例子,至于全局共享,与BLoC类似,建个BLoC和Provider,再包裹MyApp就行。


Flutter状态管理:BLoC(Business Logic Component)

一、前言

占位

二、什么是BLoC?

BLoC只是一个概念(Reactive Programming,响应式编程),它是基于“dart:async”中的Stream、StreamController来实现的。

Flutter状态管理:RxDart_第3张图片

  • 用StreamBuilder包裹有状态的部件,StreamBuilder将会监听一个流;
  • 这个流来自于BLoC;
  • 有状态小部件中的数据来自于监听的流;
  • 用户交互手势被检测到,产生了事件。例如按了一下按钮;
  • 调用bloc的功能来处理这个事件;
  • 在bloc中处理完毕后将会吧最新的数据add进流的sink中;
  • StreamBuilder监听到新的数据,产生一个新的snapshot,并重新调用build方法;
  • Widget被重新构建;

BLoC能够允许我们完美的分离业务逻辑!再也不用考虑什么时候需要刷新屏幕了,一切交给StreamBuilder和BLoC! BLoC由来自Google的两位工程师 Paolo Soares和Cong Hui设计,并在2018年DartConf期间(2018年1月23日至24日)首次展示!

Flutter状态管理:RxDart_第4张图片

三、BLoC创建方式

创建方式有三种:

  • 局部模式(类似 setState);
  • 全局单例模式(单例模式);
  • Scoped模式;

3.1、全局单例模式

全局单例模式并不推荐,原因在于:持久占用Stream而不会释放(dispose)

3.1.1、新建Model

// CountBloc.dart
import 'dart:async';

class CountBloc {
  int _count = 0;
  StreamController _controller;

  Stream get count => _controller.stream;

  CountBloc() {
    _count = 0;
    _controller = StreamController.broadcast();
  }

  void increment() {
    _controller.sink.add(++_count);
  }

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

CountBloc bloc = CountBloc();

3.1.2、新建两个页面(BlocPage和BlocPageTwo)

// BlocPage.dart
import 'package:flutter/material.dart';
import 'package:stateresearch/bloc/models/CountBloc.dart';
import 'package:stateresearch/pages/BlocPageTwo.dart';

class BlocPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return _body(context);
  }

  Widget _body(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("BlocPage"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('You have pushed the button this many times:'),
            StreamBuilder(
                stream: bloc.count,
                initialData: 0,
                builder: (BuildContext context, AsyncSnapshot snapshot) {
                  return Text("${snapshot.data}");
                }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context).push(MaterialPageRoute(builder: (BuildContext context) {
            return BlocPageTwo();
          }));
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}
// BlocPageTwo.dart
import 'package:flutter/material.dart';
import 'package:stateresearch/bloc/models/CountBloc.dart';

class BlocPageTwo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return _body(context);
  }

  Widget _body(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("BlocPageTwo"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('You have pushed the button this many times:'),
            StreamBuilder(
                stream: bloc.count,
                initialData: 0,
                builder: (BuildContext context, AsyncSnapshot snapshot) {
                  return Text("${snapshot.data}");
                }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          bloc.increment();
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

3.1.3、修改main文件

import 'package:flutter/material.dart';
import 'package:stateresearch/pages/BlocPage.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter状态管理',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: BlocPage(),
    );
  }
}

3.2、Scoped模式

推荐这种模式!该方式类似Provider。

3.2.1、新建Model

import 'dart:async';

class CountBloc {
  int _count = 0;
  StreamController _controller;

  Stream get count => _controller.stream;

  CountBloc() {
    _count = 0;
    _controller = StreamController.broadcast();
  }

  void increment() {
    _controller.sink.add(++_count);
  }

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

3.2.2、新建两个页面(BlocPage和BlocPageTwo)

import 'package:flutter/material.dart';
import 'package:stateresearch/bloc/providers/CountProvider.dart';
import 'package:stateresearch/pages/BlocPageTwo.dart';

class BlocPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return _body(context);
  }

  Widget _body(BuildContext context) {
    final _bloc = CountProvider.of(context);

    return Scaffold(
      appBar: AppBar(
        title: Text("BlocPage"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('You have pushed the button this many times:'),
            StreamBuilder(
                stream: _bloc.count,
                initialData: 0,
                builder: (BuildContext context, AsyncSnapshot snapshot) {
                  return Text("${snapshot.data}");
                }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context).push(MaterialPageRoute(builder: (BuildContext context) {
            return BlocPageTwo();
          }));
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}
import 'package:flutter/material.dart';
import 'package:stateresearch/bloc/providers/CountProvider.dart';

class BlocPageTwo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return _body(context);
  }

  Widget _body(BuildContext context) {
    final _bloc = CountProvider.of(context);

    return Scaffold(
      appBar: AppBar(
        title: Text("BlocPageTwo"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('You have pushed the button this many times:'),
            StreamBuilder(
                stream: _bloc.count,
                initialData: 0,
                builder: (BuildContext context, AsyncSnapshot snapshot) {
                  return Text("${snapshot.data}");
                }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _bloc.increment();
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

3.2.3、修改main文件

import 'package:flutter/material.dart';
import 'package:stateresearch/bloc/providers/CountProvider.dart';
import 'package:stateresearch/pages/BlocPage.dart';

void main() {
  runApp(CountProvider(child: MyApp()));
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter状态管理',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: BlocPage(),
    );
  }
}

四、总结

我们发现,BLoC 和 ScopedModel / Provider 在跨页面间共享数组,

  • 相同点:
    • 都需要在顶层包一层(即包裹 MaterialApp);
    • 包裹的这一层实际是继承于 InheritedWidget ;
  • 不同点在于Model:
    • ScopedModel / Provider 的 Model 是继承于 Listenable,且需要主动调用 notifyListeners;
    • BLoC 的 Model 是通过 StreamController / Stream / Sink / StreamBuilder 的方式来异步刷新;
    • 三者 Model 都可以类似 extends / mixins 方式,将多个 Model / Provider 全并起来做全局共享,
      然而,因为 BLoC 是基于 Stream方式,当观察的 Model 数量太多时,性能可能会急剧下降;

下篇会讲到 Redux ,它是一个很优秀的全局共享解决方案,可以考虑 全局redux + 局部bloc(rxdart) 管理方案!


Flutter状态管理:Redux

二、Redux介绍

有做过H5前端开发的朋友应该很早就接触过这个,Redux在React/VUE中,与在Flutter/Dart中概念一样,没有任何区别;唯一的区别只是使用上的不同。

一句话来介绍Redux:Redux是前端流行的,一种单向(unidirectional)数据流架构!

它主要由三部分组成:

  • Store: 它是整个数据的仓库,存储State对象,管理着整个应用的状态;
  • Reducer:处理与分发事件的方法,通过返回新的State来更新Store;
  • Action: 行为(也可以理解为事件),action将会分发至对应的reducer中;

下图是一个完整的数据触发及更新流程:

  1. View产生Action(传递事件类型及数据);
  2. Redux将Action 派发(dispatch)至对应的 Reducer;
  3. Reducer根据Action的类型,处理完后返回一个全新的 State 至 Store;
  4. Store收到新的State后将来通知相应的监听者(View)更新;

Flutter状态管理:RxDart_第5张图片

我们看到上面整个数据流,都是单向的,由View发起,最后到View的更新;

为啥这样设计?

  • 单一数据源:整个应用的状态都在一个Store中;
  • 状态只读: 无法直接修改Store中的数据,只能通过action -> reducer来完成;
  • 纯函数改变:reducer就是一个纯函数,只处理逻辑并返回全新的State;

三、Redux之Middleware

小节二介绍了Redux最基本的原理,但是,如何用Redux来做一些异步操作,比如:加载数据、请求API等?这里就引出来了Redux的中间件(Middleware),中间件能够让我们使得action在到达reducer之前,做些其它“动作”!有了中间件,我们不但可以请求API,还可以改变action,使得分发到其它reducer中去;

Flutter状态管理:RxDart_第6张图片

上图是有Middleware的流程图。

四、引入Redux相关的第三方库

// pubspec.yaml
dependencies:
  flutter:
    sdk: flutter

  redux: ^4.0.0+3
  flutter_redux: ^0.6.0
  • redux:基础库,包含了Store、Reducer和Middleware;
  • flutter_redux:用于Flutter的封装好的库(类似React中的react-redux):
    • StoreProvider:用于整个APP顶层,提供给所有的Widgets所需的Store;
    • StoreBuilder: 接收Store的变化通知;
    • StoreConnector:可替代StoreBuilder,能够将Store转成ViewModel;

五、Redux使用方式

Redux在Flutter中的使用与在JavaScript中的使用方式稍微有点不同,为啥?
因为JavaScript是弱类型语言,而Dart是强类型语言,这就使得在JS中每个reducer可以独立管理,而在Flutter中需要由一个大对象来管理!

5.1、目录结构

无论在JS中还是在Flutter中,通常都将action、reducer、store各自建一目录,放在redux目录下,目录结构如下:

.src
 |-- pages/
 |-- redux/
       |-- actions/
             |-- XxxAction.dart
             |-- YyyAction.dart
       |-- reducers/
             |-- XxxReducer.dart
             |-- YyyReducer.dart
            |-- index.dart      // App整个的reducers
       |-- states/              //(Flutter中需要,JS不需要)
            |-- XxxState.dart
            |-- YyyState.dart
            |-- index.dart      // App整个的状态对象
       |-- store/
            |-- index.dart      // App整个的store
 |-- main.dart

5.2、创建Action

// CountAction.dart
class SetCountAction {
  final int value;
  SetCountAction(this.value);
}
class IncrementCountAction {}
class DecrementCountAction {}
// FirstAction.dart
class SetFirstAction {
  final String value;
  SetFirstAction(this.value);
}

5.3、创建State

// CountState.dart
class CountState {
  final int count;
  CountState(this.count);
}
// FirstState.dart
class FirstState {
  String title;
  FirstState(this.title);
}
// index.dart (App整个的State大对象)
import 'package:stateresearch/redux/states/CountState.dart';
import 'package:stateresearch/redux/states/FirstState.dart';

class AppState {
  final FirstState firstState;
  final CountState countState;
  AppState(this.firstState, this.countState);
}

5.4、创建Reducer

// CountReducer.dart
import 'package:redux/redux.dart';
import 'package:stateresearch/redux/actions/CountAction.dart';
import 'package:stateresearch/redux/states/CountState.dart';

// 以下用两种方式创建 reducer
// 1. 基本的 switch-case
// 2. 使用 combineReducers + TypedReducer

//CountState countReducer(state, action) {
//  switch (action.runtimeType) {
//    case IncrementCountAction:
//      return CountState(state.count + 1);
//
//    case DecrementCountAction:
//      return CountState(state.count - 1);
//
//    case SetCountAction:
//      return CountState((action as SetCountAction).value);
//
//    default:
//      return state;
//  }
//}

// 使用 combineReducers 避免写 switch-case
// TypedReducer 的作用就是将 Function 与 Action 关联
final countReducer = combineReducers([
  TypedReducer(_increment),
  TypedReducer(_decrement),
  TypedReducer(_set),
]);

CountState _increment(state, action) => CountState(state.count + 1);
CountState _decrement(state, action) => CountState(state.count - 1);
CountState _set(state, SetCountAction action) => CountState(action.value);
// FirstReducer
import 'package:stateresearch/redux/actions/FirstAction.dart';
import 'package:stateresearch/redux/states/FirstState.dart';

FirstState firstReducer(state, action) {
  switch (action.runtimeType) {
    case SetFirstAction:
      return FirstState((action as SetFirstAction).value);
    default:
      return state;
  }
}
// index.dart
import 'package:stateresearch/redux/reducers/CountReducer.dart';
import 'package:stateresearch/redux/reducers/FirstReducer.dart';
import 'package:stateresearch/redux/states/index.dart';

AppState reducers(AppState state, action) {
  return AppState(
    firstReducer(state.firstState, action),
    countReducer(state.countState, action),
  );
}

5.5、创建Store

// index.dart
import 'package:redux/redux.dart';
import 'package:stateresearch/redux/reducers/index.dart';
import 'package:stateresearch/redux/states/CountState.dart';
import 'package:stateresearch/redux/states/FirstState.dart';
import 'package:stateresearch/redux/states/index.dart';

final store = Store(
  reducers,
  initialState: AppState(
    FirstState(null),
    CountState(0),
  ),
);

5.6、新建两个页面(ReduxPage和ReduxPage2)

ReduxPage在build中,也可以直接用StoreBuilder(参考ReduxPage2中写法),因为StoreBuilder也是InheritedWidget。

// ReduxPage.dart
import 'dart:math';

import 'package:flutter/material.dart';
import 'package:flutter_redux/flutter_redux.dart';
import 'package:stateresearch/pages/ReduxPage2.dart';
import 'package:stateresearch/redux/actions/CountAction.dart';
import 'package:stateresearch/redux/actions/FirstAction.dart';
import 'package:stateresearch/redux/states/CountState.dart';
import 'package:stateresearch/redux/states/index.dart';

class ReduxPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return _body(context);
  }

  Widget _body(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("ReduxPage")),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('You have pushed the button this many times:'),
            StoreConnector(
              converter: (store) {
                return store.state.countState;
              },
              builder: (context, state) {
                return Text("${state.count}");
              },
            )
          ],
        ),
      ),
      floatingActionButton: StoreBuilder(
        builder: (context, store) {
          return Row(
            children: [
              FloatingActionButton(
                onPressed: () => store.dispatch(IncrementCountAction()),
                tooltip: 'Increment',
                child: Icon(Icons.add),
                heroTag: 'Increment',
              ),
              FloatingActionButton(
                onPressed: () => store.dispatch(DecrementCountAction()),
                tooltip: 'Decrement',
                child: Icon(Icons.close),
                heroTag: 'Decrement',
              ),
              FloatingActionButton(
                onPressed: () => store.dispatch(SetCountAction(100)),
                tooltip: 'Set',
                child: Icon(Icons.settings),
                heroTag: 'Set',
              ),
              FloatingActionButton(
                onPressed: () => store.dispatch(SetFirstAction("chris's age = ${Random().nextInt(100)}")),
                tooltip: 'First',
                child: Icon(Icons.event),
                heroTag: 'First',
              ),
              FloatingActionButton(
                onPressed: () {
                  Navigator.of(context).push(MaterialPageRoute(builder: (context) {
                    return ReduxPage2();
                  }));
                },
                tooltip: 'NextPage',
                child: Icon(Icons.message),
                heroTag: 'NextPage',
              )
            ],
          );
        },
      ),
    );
  }
}
// ReduxPage2.dart
import 'package:flutter/material.dart';
import 'package:flutter_redux/flutter_redux.dart';
import 'package:stateresearch/redux/states/index.dart';

class ReduxPage2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return _body(context);
  }

  Widget _body(BuildContext context) {
    return StoreBuilder(
      builder: (context, store) => Scaffold(
        appBar: AppBar(
          title: Text("${store.state.firstState.title}"),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text('You have pushed the button this many times:'),
              Text("${store.state.countState.count}")
            ],
          ),
        ),
      ),
    );
  }
}

5.7、修改main文件

// main.dart
import 'package:flutter/material.dart';
import 'package:flutter_redux/flutter_redux.dart';
import 'package:stateresearch/pages/ReduxPage.dart';
import 'package:stateresearch/redux/states/index.dart';
import 'package:stateresearch/redux/store/index.dart';

void main() {
  runApp(StoreProvider(store: store, child: MyApp()));
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter状态管理',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: ReduxPage(),
    );
  }
}

六、总结

正因为Redux在Flutter中与在JS中不同,因此,在Flutter中,建议:

  • 如果只是局部使用,不涉及到跨 widget / page 共享,那么最好不要使用 Redux;
  • 相反,Redux 只放全局共享数据 (毕竟是大对象);

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