Subject 的其中一个变体就是 BehaviorSubject，它有一个“当前值”的概念。它保存了发送给消费者的最新值。并且当有新的观察者订阅时，会立即从 BehaviorSubject 那接收到“当前值”。

import { Component, OnInit } from '@angular/core';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';@Component({selector: 'app-subject',templateUrl: './subject.component.html',styleUrls: ['./subject.component.css']
})
export class SubjectComponent implements OnInit {constructor() { }ngOnInit() {const subject: BehaviorSubject<string> = new BehaviorSubject<string>('Leo');subject.subscribe( // 观察者A订阅(val: string) => {console.log(`observerA: ${val}`);});subject.next('Raph');subject.next('Mikey');subject.subscribe( // 观察者B订阅(val: string) => {console.log(`observerB: ${val}`);});subject.next('Don');}}