一 Subject主题

Subject是Observable的子类。- Subject是多播的，允许将值多播给多个观察者。普通的 Observable 是单播的。

在 Subject 的内部，subscribe 不会调用发送值的新执行。它只是将给定的观察者注册到观察者列表中，类似于其他库或语言中的 addListener 的工作方式。

要给 Subject 提供新值，只要调用 next(theValue)，它会将值多播给已注册监听该 Subject 的观察者们。

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

@Component({
  selector: 'app-subject',
  templateUrl: './subject.component.html',
  styleUrls: ['./subject.component.css']
})
export class SubjectComponent implements OnInit {

  constructor() { }

  ngOnInit() {
    const subject: Subject<string> = new Subject<string>();
    const subscriptionA: Subscription = subject.subscribe(
      (val: string) => {
        console.log(`observerA: ${val}`);
      }
    );
    const subscriptionB: Subscription = subject.subscribe(
      (val: string) => {
        console.log(`observerB: ${val}`);
      }
    );
    subject.next('Mikey');
    subject.next('Leo');
    subscriptionA.unsubscribe(); // 取消订阅
    subscriptionB.unsubscribe(); // 取消订阅
    subject.next('Raph');
    subject.complete();
  }

}

每个 Subject 都是观察者。 - Subject 是一个有如下方法的对象： next(v)、error(e) 和 complete() ，可以把 Subject 作为参数传给任何 Observable 的 subscribe 方法。

import { Component, OnInit } from '@angular/core';
import { Subject } from 'rxjs/Subject';
import { Subscription } from 'rxjs/Subscription';
import { from } from 'rxjs/observable/from';
import { Observable } from 'rxjs/Observable';

@Component({
  selector: 'app-subject',
  templateUrl: './subject.component.html',
  styleUrls: ['./subject.component.css']
})
export class SubjectComponent implements OnInit {

  constructor() { }

  ngOnInit() {
    const subject: Subject<string> = new Subject<string>();
    const subscriptionA: Subscription = subject.subscribe(
      (val: string) => {
        console.log(`observerA: ${val}`);
      }
    );
    const subscriptionB: Subscription = subject.subscribe(
      (val: string) => {
        console.log(`observerB: ${val}`);
      }
    );

    const observable: Observable<string> = from(['Raph', 'Don']);
    observable.subscribe(subject);

  }

}