RxJS Subject 详解
iafa6945
8年前
<h2>Observer Pattern</h2> <h3>观察者模式定义</h3> <p>观察者模式又叫发布订阅模式（Publish/Subscribe），它定义了一种一对多的关系，让多个观察者对象同时监听某一个主题对象，这个主题对象的状态发生变化时就会通知所有的观察者对象，使得它们能够自动更新自己。</p> <p>我们可以使用日常生活中，期刊订阅的例子来形象地解释一下上面的概念。期刊订阅包含两个主要的角色：期刊出版方和订阅者，他们之间的关系如下：</p> <ul> <li>期刊出版方 - 负责期刊的出版和发行工作</li> <li>订阅者 - 只需执行订阅操作，新版的期刊发布后，就会主动收到通知，如果取消订阅，以后就不会再收到通知</li> </ul> <p>在观察者模式中也有两个主要角色：Subject (主题) 和 Observer (观察者) 。它们分别对应例子中的期刊出版方和订阅者。接下来我们来看张图，从而加深对上面概念的理解。</p> <p style="text-align:center"><img src="https://simg.open-open.com/show/6746475b66da036126669987ca4edb90.png"></p> <h3>观察者模式结构</h3> <p style="text-align:center"><img src="https://simg.open-open.com/show/2337383f6c5bbf02e5c1b69b844eae0c.png"></p> <h3>观察者模式实战</h3> <p>Subject 类定义</p> <pre> <code class="language-javascript">class Subject { constructor() { this.observerCollection = []; } addObserver(observer) { // 添加观察者 this.observerCollection.push(observer); } deleteObserver(observer) { // 移除观察者 let index = this.observerCollection.indexOf(observer); if(index >= 0) this.observerCollection.splice(index, 1); } notifyObservers() { // 通知观察者 this.observerCollection.forEach((observer)=>observer.notify()); } }</code></pre> <p>Observer 类定义</p> <pre> <code class="language-javascript">class Observer { constructor(name) { this.name = name; } notify() { console.log(`${this.name} has been notified.`); } }</code></pre> <p>使用示例</p> <pre> <code class="language-javascript">let subject = new Subject(); // 创建主题对象 let observer1 = new Observer('semlinker'); // 创建观察者A - 'semlinker' let observer2 = new Observer('lolo'); // 创建观察者B - 'lolo' subject.addObserver(observer1); // 注册观察者A subject.addObserver(observer2); // 注册观察者B subject.notifyObservers(); // 通知观察者 subject.deleteObserver(observer1); // 移除观察者A subject.notifyObservers(); // 验证是否成功移除</code></pre> <p>以上代码成功运行后控制台的输出结果：</p> <pre> <code class="language-javascript">semlinker has been notified. lolo has been notified. lolo has been notified.</code></pre> <h2>Observable subscribe</h2> <p>在介绍RxJS - Subject 之前，我们先来看个示例：</p> <pre> <code class="language-javascript">const interval$ = Rx.Observable.interval(1000).take(3); interval$.subscribe({ next: value => console.log('Observer A get value: ' + value); }); setTimeout(() => { interval$.subscribe({ next: value => console.log('Observer B get value: ' + value); }); }, 1000);</code></pre> <p>以上代码运行后，控制台的输出结果：</p> <pre> <code class="language-javascript">Observer A get value: 0 Observer A get value: 1 Observer B get value: 0 Observer A get value: 2 Observer B get value: 1 Observer B get value: 2</code></pre> <p>通过以上示例，我们可以得出以下结论：</p> <ul> <li>Observable 对象可以被重复订阅</li> <li>Observable 对象每次被订阅后，都会重新执行</li> </ul> <p>上面的示例，我们可以简单地认为两次调用普通的函数，具体参考以下代码：</p> <pre> <code class="language-javascript">function interval() { setInterval(() => console.log('..'), 1000); } interval(); setTimeout(() => { interval(); }, 1000);</code></pre> <p>Observable 对象的默认行为，适用于大部分场景。但有些时候，我们会希望在第二次订阅的时候，不会从头开始接收 Observable 发出的值，而是从第一次订阅当前正在处理的值开始发送，我们把这种处理方式成为组播 (multicast)，那我们要怎么实现呢 ？回想一下我们刚才介绍过观察者模式，你脑海中是不是已经想到方案了。没错，我们可以通过自定义 Subject 来实现上述功能。</p> <h2>自定义 Subject</h2> <p>Subject 类定义</p> <pre> <code class="language-javascript">class Subject { constructor() { this.observers = []; } addObserver(observer) { this.observers.push(observer); } next(value) { this.observers.forEach(o => o.next(value)); } error(error){ this.observers.forEach(o => o.error(error)); } complete() { this.observers.forEach(o => o.complete()); } }</code></pre> <p>使用示例</p> <pre> <code class="language-javascript">const interval$ = Rx.Observable.interval(1000).take(3); let subject = new Subject(); let observerA = { next: value => console.log('Observer A get value: ' + value), error: error => console.log('Observer A error: ' + error), complete: () => console.log('Observer A complete!') }; var observerB = { next: value => console.log('Observer B get value: ' + value), error: error => console.log('Observer B error: ' + error), complete: () => console.log('Observer B complete!') }; subject.addObserver(observerA); // 添加观察者A interval$.subscribe(subject); // 订阅interval$对象 setTimeout(() => { subject.addObserver(observerB); // 添加观察者B }, 1000);</code></pre> <p>以上代码运行后，控制台的输出结果：</p> <pre> <code class="language-javascript">Observer A get value: 0 Observer A get value: 1 Observer B get value: 1 Observer A get value: 2 Observer B get value: 2 Observer A complete! Observer B complete!</code></pre> <p>通过自定义 Subject，我们实现了前面提到的功能。接下来我们进入正题 - RxJS Subject。</p> <h2>RxJS Subject</h2> <p>首先我们通过 RxJS Subject 来重写一下上面的示例：</p> <pre> <code class="language-javascript">const interval$ = Rx.Observable.interval(1000).take(3); let subject = new Rx.Subject(); let observerA = { next: value => console.log('Observer A get value: ' + value), error: error => console.log('Observer A error: ' + error), complete: () => console.log('Observer A complete!') }; var observerB = { next: value => console.log('Observer B get value: ' + value), error: error => console.log('Observer B error: ' + error), complete: () => console.log('Observer B complete!') }; subject.subscribe(observerA); // 添加观察者A interval$.subscribe(subject); // 订阅interval$对象 setTimeout(() => { subject.subscribe(observerB); // 添加观察者B }, 1000);</code></pre> <h3>RxJS Subject 源码片段</h3> <pre> <code class="language-javascript">/** * Suject继承于Observable */ export class Subject extends Observable { constructor() { super(); this.observers = []; // 观察者列表 this.closed = false; this.isStopped = false; this.hasError = false; this.thrownError = null; } next(value) { if (this.closed) { throw new ObjectUnsubscribedError(); } if (!this.isStopped) { const { observers } = this; const len = observers.length; const copy = observers.slice(); for (let i = 0; i < len; i++) { // 循环调用观察者next方法，通知观察者 copy[i].next(value); } } } error(err) { if (this.closed) { throw new ObjectUnsubscribedError(); } this.hasError = true; this.thrownError = err; this.isStopped = true; const { observers } = this; const len = observers.length; const copy = observers.slice(); for (let i = 0; i < len; i++) { // 循环调用观察者error方法 copy[i].error(err); } this.observers.length = 0; } complete() { if (this.closed) { throw new ObjectUnsubscribedError(); } this.isStopped = true; const { observers } = this; const len = observers.length; const copy = observers.slice(); for (let i = 0; i < len; i++) { // 循环调用观察者complete方法 copy[i].complete(); } this.observers.length = 0; // 清空内部观察者列表 } }</code></pre> <p>通过 RxJS Subject 示例和源码片段，对于 Subject 我们可以得出以下结论：</p> <ul> <li>Subject 既是 Observable 对象，又是 Observer 对象</li> <li>当有新消息时，Subject 会对内部的 observers 列表进行组播 (multicast)</li> </ul> <h2>Angular 2 RxJS Subject 应用</h2> <p>在 Angular 2 中，我们可以利用 RxJS Subject 来实现组件通信，具体示例如下：</p> <p>message.service.ts</p> <pre> <code class="language-javascript">import { Injectable } from '@angular/core'; import { Observable } from 'rxjs'; import { Subject } from 'rxjs/Subject'; @Injectable() export class MessageService { private subject = new Subject<any>(); sendMessage(message: string) { this.subject.next({ text: message }); } clearMessage() { this.subject.next(); } getMessage(): Observable<any> { return this.subject.asObservable(); } }</code></pre> <p>home.component.ts</p> <pre> <code class="language-javascript">import { Component } from '@angular/core'; import { MessageService } from '../_services/index'; @Component({ moduleId: module.id, templateUrl: 'home.component.html' }) export class HomeComponent { constructor(private messageService: MessageService) {} sendMessage(): void { // 发送消息 this.messageService.sendMessage('Message from Home Component to App Component!'); } clearMessage(): void { // 清除消息 this.messageService.clearMessage(); } }</code></pre> <p>app.component.ts</p> <pre> <code class="language-javascript">import { Component, OnDestroy } from '@angular/core'; import { Subscription } from 'rxjs/Subscription'; import { MessageService } from './_services/index'; @Component({ moduleId: module.id, selector: 'app', templateUrl: 'app.component.html' }) export class AppComponent implements OnDestroy { message: any; subscription: Subscription; constructor(private messageService: MessageService) { this.subscription = this.messageService.getMessage() .subscribe(message => { this.message = message; }); } ngOnDestroy() { this.subscription.unsubscribe(); } }</code></pre> <p>以上示例实现的功能是组件之间消息通信，即 HomeComponent 子组件，向 AppComponent 父组件发送消息。代码运行后，浏览器的显示结果如下：</p> <p style="text-align:center"><img src="https://simg.open-open.com/show/e2de1b0566e4d6e746574a76ad0800cf.png"></p> <p><a href="/misc/goto?guid=4959746442729209148" rel="nofollow,noindex">Plunker 示例</a></p> <h2>Subject 存在的问题</h2> <p>因为 Subject 在订阅时，是把 observer 存放到观察者列表中，并在接收到新值的时候，遍历观察者列表并调用观察者上的 next 方法，具体如下：</p> <pre> <code class="language-javascript">next(value) { if (this.closed) { throw new ObjectUnsubscribedError(); } if (!this.isStopped) { const { observers } = this; const len = observers.length; const copy = observers.slice(); for (let i = 0; i < len; i++) { // 循环调用观察者next方法，通知观察者 copy[i].next(value); } } }</code></pre> <p>这样会有一个大问题，如果某个 observer 在执行时出现异常，却没进行异常处理，就会影响到其它的订阅者，具体示例如下：</p> <pre> <code class="language-javascript">const source = Rx.Observable.interval(1000); const subject = new Rx.Subject(); const example = subject.map(x => { if (x === 1) { throw new Error('oops'); } return x; }); subject.subscribe(x => console.log('A', x)); example.subscribe(x => console.log('B', x)); subject.subscribe(x => console.log('C', x)); source.subscribe(subject);</code></pre> <p>以上代码运行后，控制台的输出结果：</p> <pre> <code class="language-javascript">A 0 B 0 C 0 A 1 Rx.min.js:74 Uncaught Error: oops</code></pre> <p><a href="/misc/goto?guid=4959746442821002873" rel="nofollow,noindex">JSBin - Subject Problem Demo</a></p> <p>在代码运行前，大家会认为观察者B 会在接收到 1 值时抛出异常，观察者 A 和 C 仍会正常运行。但实际上，在当前的 RxJS 版本中若观察者 B 报错，观察者 A 和 C 也会停止运行。那么应该如何解决这个问题呢？目前最简单的方式就是为所有的观察者添加异常处理，更新后的代码如下：</p> <pre> <code class="language-javascript">const source = Rx.Observable.interval(1000); const subject = new Rx.Subject(); const example = subject.map(x => { if (x === 1) { throw new Error('oops'); } return x; }); subject.subscribe( x => console.log('A', x), error => console.log('A Error:' + error) ); example.subscribe( x => console.log('B', x), error => console.log('B Error:' + error) ); subject.subscribe( x => console.log('C', x), error => console.log('C Error:' + error) ); source.subscribe(subject);</code></pre> <p><a href="/misc/goto?guid=4959746442903227914" rel="nofollow,noindex">JSBin - RxJS Subject Problem Solved Demo</a></p> <h2>RxJS Subject & Observable</h2> <p>Subject 其实是观察者模式的实现，所以当观察者订阅 Subject 对象时，Subject 对象会把订阅者添加到观察者列表中，每当有 subject 对象接收到新值时，它就会遍历观察者列表，依次调用观察者内部的 next() 方法，把值一一送出。</p> <p>Subject 之所以具有 Observable 中的所有方法，是因为 Subject 类继承了 Observable 类，在 Subject 类中有五个重要的方法：</p> <ul> <li>next - 每当 Subject 对象接收到新值的时候，next 方法会被调用</li> <li>error - 运行中出现异常，error 方法会被调用</li> <li>complete - Subject 订阅的 Observable 对象结束后，complete 方法会被调用</li> <li>subscribe - 添加观察者</li> <li>unsubscribe - 取消订阅 (设置终止标识符、清空观察者列表)</li> </ul> <h2>BehaviorSubject</h2> <h3>BehaviorSubject 定义</h3> <p><a href="/misc/goto?guid=4959746442984550855" rel="nofollow,noindex">BehaviorSubject 源码片段</a></p> <pre> <code class="language-javascript">export class BehaviorSubject extends Subject { constructor(_value) { // 设置初始值 super(); this._value = _value; } get value() { // 获取当前值 return this.getValue(); } _subscribe(subscriber) { const subscription = super._subscribe(subscriber); if (subscription && !subscription.closed) { subscriber.next(this._value); // 为新的订阅者发送当前最新的值 } return subscription; } getValue() { if (this.hasError) { throw this.thrownError; } else if (this.closed) { throw new ObjectUnsubscribedError(); } else { return this._value; } } next(value) { // 调用父类Subject的next方法，同时更新当前值 super.next(this._value = value); } }</code></pre> <h3>BehaviorSubject 应用</h3> <p>有些时候我们会希望 Subject 能保存当前的最新状态，而不是单纯的进行事件发送，也就是说每当新增一个观察者的时候，我们希望 Subject 能够立即发出当前最新的值，而不是没有任何响应。具体我们先看一下示例：</p> <pre> <code class="language-javascript">var subject = new Rx.Subject(); var observerA = { next: value => console.log('Observer A get value: ' + value), error: error => console.log('Observer A error: ' + error), complete: () => console.log('Observer A complete!') }; var observerB = { next: value => console.log('Observer B get value: ' + value), error: error => console.log('Observer B error: ' + error), complete: () => console.log('Observer B complete!') }; subject.subscribe(observerA); subject.next(1); subject.next(2); subject.next(3); setTimeout(() => { subject.subscribe(observerB); // 1秒后订阅 }, 1000);</code></pre> <p>以上代码运行后，控制台的输出结果：</p> <pre> <code class="language-javascript">Observer A get value: 1 Observer A get value: 2 Observer A get value: 3</code></pre> <p>通过输出结果，我们发现在 observerB 订阅 Subject 对象后，它再也没有收到任何值了。因为 Subject 对象没有再调用 next() 方法。但很多时候我们会希望 Subject 对象能够保存当前的状态，当新增订阅者的时候，自动把当前最新的值发送给订阅者。要实现这个功能，我们就需要使用 BehaviorSubject。</p> <p>BehaviorSubject 跟 Subject 最大的不同就是 BehaviorSubject 是用来保存当前最新的值，而不是单纯的发送事件。BehaviorSubject 会记住最近一次发送的值，并把该值作为当前值保存在内部的属性中。接下来我们来使用 BehaviorSubject 重新一下上面的示例：</p> <pre> <code class="language-javascript">var subject = new Rx.BehaviorSubject(0); // 设定初始值 var observerA = { next: value => console.log('Observer A get value: ' + value), error: error => console.log('Observer A error: ' + error), complete: () => console.log('Observer A complete!') }; var observerB = { next: value => console.log('Observer B get value: ' + value), error: error => console.log('Observer B error: ' + error), complete: () => console.log('Observer B complete!') }; subject.subscribe(observerA); subject.next(1); subject.next(2); subject.next(3); setTimeout(() => { subject.subscribe(observerB); // 1秒后订阅 }, 1000);</code></pre> <p>以上代码运行后，控制台的输出结果：</p> <pre> <code class="language-javascript">Observer A get value: 0 Observer A get value: 1 Observer A get value: 2 Observer A get value: 3 Observer B get value: 3</code></pre> <p><a href="/misc/goto?guid=4959746443075004696" rel="nofollow,noindex">JSBin - BehaviorSubject</a></p> <h2>ReplaySubject</h2> <h3>ReplaySubject 定义</h3> <p><a href="/misc/goto?guid=4959746443158565883" rel="nofollow,noindex">ReplaySubject 源码片段</a></p> <pre> <code class="language-javascript">export class ReplaySubject extends Subject { constructor(bufferSize = Number.POSITIVE_INFINITY, windowTime = Number.POSITIVE_INFINITY, scheduler) { super(); this.scheduler = scheduler; this._events = []; // ReplayEvent对象列表 this._bufferSize = bufferSize < 1 ? 1 : bufferSize; // 设置缓冲区大小 this._windowTime = windowTime < 1 ? 1 : windowTime; } next(value) { const now = this._getNow(); this._events.push(new ReplayEvent(now, value)); this._trimBufferThenGetEvents(); super.next(value); } _subscribe(subscriber) { const _events = this._trimBufferThenGetEvents(); // 过滤ReplayEvent对象列表 let subscription; if (this.closed) { throw new ObjectUnsubscribedError(); } ... else { this.observers.push(subscriber); subscription = new SubjectSubscription(this, subscriber); } ... const len = _events.length; // 重新发送设定的最后bufferSize个值 for (let i = 0; i < len && !subscriber.closed; i++) { subscriber.next(_events[i].value); } ... return subscription; } } class ReplayEvent { constructor(time, value) { this.time = time; this.value = value; } }</code></pre> <h3>ReplaySubject 应用</h3> <p>有些时候我们希望在 Subject 新增订阅者后，能向新增的订阅者重新发送最后几个值，这时我们就可以使用 ReplaySubject ，具体示例如下：</p> <pre> <code class="language-javascript">var subject = new Rx.ReplaySubject(2); // 重新发送最后2个值 var observerA = { next: value => console.log('Observer A get value: ' + value), error: error => console.log('Observer A error: ' + error), complete: () => console.log('Observer A complete!') }; var observerB = { next: value => console.log('Observer B get value: ' + value), error: error => console.log('Observer B error: ' + error), complete: () => console.log('Observer B complete!') }; subject.subscribe(observerA); subject.next(1); subject.next(2); subject.next(3); setTimeout(() => { subject.subscribe(observerB); // 1秒后订阅 }, 1000);</code></pre> <p>以上代码运行后，控制台的输出结果：</p> <pre> <code class="language-javascript">Observer A get value: 1 Observer A get value: 2 Observer A get value: 3 Observer B get value: 2 Observer B get value: 3</code></pre> <p>可能会有人认为 ReplaySubject(1) 是不是等同于 BehaviorSubject，其实它们是不一样的。在创建BehaviorSubject 对象时，是设置初始值，它用于表示 Subject 对象当前的状态，而 ReplaySubject 只是事件的重放。</p> <p><a href="/misc/goto?guid=4959746443234889007" rel="nofollow,noindex">JSBin - ReplaySubject</a></p> <h2>AsyncSubject</h2> <h3>AsyncSubject 定义</h3> <p><a href="/misc/goto?guid=4959746443324437220" rel="nofollow,noindex">AsyncSubject 源码片段</a></p> <pre> <code class="language-javascript">export class AsyncSubject extends Subject { constructor() { super(...arguments); this.value = null; this.hasNext = false; this.hasCompleted = false; // 标识是否已完成 } _subscribe(subscriber) { if (this.hasError) { subscriber.error(this.thrownError); return Subscription.EMPTY; } else if (this.hasCompleted && this.hasNext) { // 等到完成后，才发出最后的值 subscriber.next(this.value); subscriber.complete(); return Subscription.EMPTY; } return super._subscribe(subscriber); } next(value) { if (!this.hasCompleted) { // 若未完成，保存当前的值 this.value = value; this.hasNext = true; } } }</code></pre> <h3>AsyncSubject 应用</h3> <p>AsyncSubject 类似于 last 操作符，它会在 Subject 结束后发出最后一个值，具体示例如下：</p> <pre> <code class="language-javascript">var subject = new Rx.AsyncSubject(); var observerA = { next: value => console.log('Observer A get value: ' + value), error: error => console.log('Observer A error: ' + error), complete: () => console.log('Observer A complete!') }; var observerB = { next: value => console.log('Observer B get value: ' + value), error: error => console.log('Observer B error: ' + error), complete: () => console.log('Observer B complete!') }; subject.subscribe(observerA); subject.next(1); subject.next(2); subject.next(3); subject.complete(); setTimeout(() => { subject.subscribe(observerB); // 1秒后订阅 }, 1000);</code></pre> <p>以上代码运行后，控制台的输出结果：</p> <pre> <code class="language-javascript">Observer A get value: 3 Observer A complete! Observer B get value: 3 Observer B complete!</code></pre> <p><a href="/misc/goto?guid=4959746443404100362" rel="nofollow,noindex">JSBin - AsyncSubject</a></p> <h2>参考资源</h2> <ul> <li><a href="/misc/goto?guid=4959746443490382158" rel="nofollow,noindex">Understanding Subjects in RxJS</a></li> <li><a href="/misc/goto?guid=4959746443576918746" rel="nofollow,noindex">30 天精通 RxJS (22) - 什么是 Subject</a></li> <li><a href="/misc/goto?guid=4959746443655383975" rel="nofollow,noindex">Communicating Between Components with Observable & Subject</a></li> </ul> <p> </p> <p>来自：https://juejin.im/post/58dca39861ff4b006b03b80c</p> <p> </p>