Angular 4 教程
Angular 4 教程
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组件通信

2018-06-06 10:46 更新

本文介绍的内容是组件通信的常用方式：@Input、@Output、@ViewChild、模板变量、MessageService、Broadcaster (Angular 1.x $rootScope 中 $on、$broadcast ) 和 Pub - Sub 模式、RxJS Subject 存在的问题。

输入属性 (父组件 -> 子组件)

counter.component.ts

import { Component, Input } from '@angular/core';


@Component({
    selector: 'exe-counter',
    template: `
      <p>当前值: {{ count }}</p>
      <button (click)="increment()"> + </button>
      <button (click)="decrement()"> - </button>
    `
})
export class CounterComponent {
    @Input() count: number = 0;


    increment() {
        this.count++;
    }


    decrement() {
        this.count--;
    }
}

app.component.ts

import { Component } from '@angular/core';


@Component({
  selector: 'exe-app',
  template: `
   <exe-counter [count]="initialCount"></exe-counter>
  `
})
export class AppComponent {
  initialCount: number = 5;
}

输出属性 (子组件 -> 父组件)

counter.component.ts

import { Component, Input, Output, EventEmitter } from '@angular/core';


@Component({
    selector: 'exe-counter',
    template: `
      <p>当前值: {{ count }}</p>
      <button (click)="increment()"> + </button>
      <button (click)="decrement()"> - </button>
    `
})
export class CounterComponent {
    @Input() count: number = 0;


    @Output() change: EventEmitter<number> = new EventEmitter<number>();


    increment() {
        this.count++;
        this.change.emit(this.count);
    }


    decrement() {
        this.count--;
        this.change.emit(this.count);
    }
}

app.component.ts

import { Component } from '@angular/core';


@Component({
  selector: 'exe-app',
  template: `
   <p>{{changeMsg}}</p> 
   <exe-counter [count]="initialCount" 
    (change)="countChange($event)"></exe-counter>
  `
})
export class AppComponent {
  initialCount: number = 5;


  changeMsg: string;


  countChange(event: number) {
    this.changeMsg = `子组件change事件已触发，当前值是: ${event}`;
  }
}

模板变量

child.component.ts

import {Component} from '@angular/core';


@Component({
  selector: 'child-component',
  template: `I'm {{ name }}`
})


export class ChildComponent {
  public name: string;
}

parent.component.ts

import {Component, OnInit} from '@angular/core';
import {ChildComponent} from './child-component.ts';


@Component({
  selector: 'parent-component',
  template: `
    <child-component #child></child-component>
    <button (click)="child.name = childName">设置子组件名称</button>
  `
})


export class ParentComponent implements OnInit {

  
  private childName: string;

  
  constructor() { }


  ngOnInit() { 
    this.childName = 'child-component';
  }
}

@ViewChild 装饰器

child.component.ts

import { Component, OnInit } from '@angular/core';


@Component({
    selector: 'exe-child',
    template: `
      <p>Child Component</p>  
    `
})
export class ChildComponent {
    name: string = '';
}

app.component.ts

import { Component, ViewChild, AfterViewInit } from '@angular/core';
import { ChildComponent } from './child.component';


@Component({
  selector: 'my-app',
  template: `
    <h4>Welcome to Angular World</h4>
    <exe-child></exe-child>
  `,
})
export class AppComponent {


  @ViewChild(ChildComponent)
  childCmp: ChildComponent;


  ngAfterViewInit() {
    this.childCmp.name = 'child-component';
  }
}

使用 MessageService - 基于 RxJS Subject

message.service.ts

import { Injectable } from '@angular/core';
import {Observable} from 'rxjs/Observable';
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();
    }
}

home.component.ts

import { Component } from '@angular/core';
import { MessageService } from './message.service';


@Component({
    selector: 'exe-home',
    template: `
    <div>
        <h1>Home</h1>
        <button (click)="sendMessage()">Send Message</button>
        <button (click)="clearMessage()">Clear Message</button>
    </div>`
})


export class HomeComponent {
    constructor(private messageService: MessageService) {}

    
    sendMessage(): void {
        this.messageService.sendMessage('Message from Home Component to App Component!');
    }


    clearMessage(): void {
        this.messageService.clearMessage();
    }
}

app.component.ts

import { Component, OnDestroy } from '@angular/core';
import { Subscription } from 'rxjs/Subscription';
import { MessageService } from './message.service';


@Component({
    selector: 'my-app',
    template: `
    <div>
       <div *ngIf="message">{{message.text}}</div>
       <exe-home></exe-home>
    </div>
    `
})


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();
    }
}

使用 Broadcaster - 基于 RxJS Subject

实现 Angular 1.x 中的 $rootScope 对象中 $on 和 $broadcast 的功能。

broadcaster.ts

import { Injectable } from '@angular/core';
import {Subject} from 'rxjs/Subject';
import {Observable} from 'rxjs/Observable';
import 'rxjs/add/operator/filter';
import 'rxjs/add/operator/map';


interface BroadcastEvent {
  key: any;
  data?: any;
}


@Injectable()
export class Broadcaster {
  private _eventBus: Subject<BroadcastEvent>;


  constructor() {
    this._eventBus = new Subject<BroadcastEvent>();
  }


  broadcast(key: any, data?: any) {
    this._eventBus.next({key, data});
  }


  on<T>(key: any): Observable<T> {
    return this._eventBus.asObservable()
      .filter(event => event.key === key)
      .map(event => <T>event.data);
  }
}

child.component.ts

import { Component } from '@angular/core';


@Component({
    selector: 'child'
})
export class ChildComponent {
  constructor(private broadcaster: Broadcaster) {}

  
  registerStringBroadcast() {
    this.broadcaster.on<string>('MyEvent')
      .subscribe(message => {
        ...
      });
  }


  emitStringBroadcast() {
    this.broadcaster.broadcast('MyEvent', 'some message');
  }
}

本文主要是介绍组件通讯的思路，提供的都是相对简单的示例。如果想深入了解，请参考 - angular.cn - component-communication。

我有话说

1.在实际开发中，我们也经常使用 Pub (发布) - Sub (订阅模式) 来实现模块之间的消息通讯。接下来我们看一下 Pub - Sub 的核心点：

  • 至少包含 subscribe() 和 publish() 两个方法，subscribe() 用于实现消息订阅，publish() 方法用于发布消息。
  • 支持订阅不同的消息类型，且对于任何一种消息类型都可以添加多个观察者。内部实现一般使用 key-value 结构进行消息类型和观察者列表的存储。
  • 订阅观察者后，最好返回一个对象或函数对象，用于取消订阅。

具体示例如下(详细信息，请参考 - [Pub/Sub JavaScript Object](Pub/Sub JavaScript Object))：

var events = (function(){
  var topics = {};
  var hOP = topics.hasOwnProperty;


  return {
    subscribe: function(topic, listener) {
      // 如果topic类型不存在，则创建
      if(!hOP.call(topics, topic)) topics[topic] = [];

      
      // 添加listener
      var index = topics[topic].push(listener) -1;


      // 返回对象用于移除listener
      return {
        remove: function() {
          delete topics[topic][index];
        }
      };
    },
    publish: function(topic, info) {
      if(!hOP.call(topics, topic)) return;


      topics[topic].forEach(function(item) {
            item(info != undefined ? info : {});
      });
    }
  };
})();

使用示例：

var subscription = events.subscribe('/page/load', function(obj) {
    // 事件处理
});


events.publish('/page/load', {
    url: '/some/url/path' 
});

2.RxJS Subject 在使用中存在一个问题，就是如果某个 observer (观察者) 在执行的时候出现异常，却没有进行异常处理，那么就会影响到其它的观察者。解决该问题，最简单的方式就是为所有的观察者添加异常处理。具体问题如下：

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);

以上代码运行后，控制台的输出结果：

A 0
B 0
C 0
A 1
Rx.min.js:74 Uncaught Error: oops

解决方案：

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);

关于 RxJS Subject 的详细信息，请查看 - RxJS Subject。

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