一个Observable的例子

import { interval } from "rxjs";
import { take } from "rxjs/operators";

const interval$ = interval(1000).pipe(take(3));

interval$.subscribe(value => console.log("Observer A get value: " + value));

setTimeout(() => {
  interval$.subscribe(value => console.log("Observer B get value: " + value));
}, 1000);

输出

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

可以看到

• Observable 对象可以被重复订阅。
• Observable 对象每次被订阅后，都会重新执行。

一个Subject的例子

import { interval, Subject } from "rxjs";
import { take } from "rxjs/operators";

const interval$ = interval(1000).pipe(take(3));
const subject = new Subject();

const observerA = {
  next: value => console.log("Observer A get value: " + value),
  error: error => console.log("Observer A error: " + error),
  complete: () => console.log("Observer A complete!")
};

const 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);

输出

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!

可以看到

• Subject 是 Observable 对象。
• Subject 是保持内部状态的 Observable 对象。
• Subject 还是 Observe 对象。

除了 Subject 之外，还有BehaviorSubject、ReplaySubject 和 AsyncSubject。

BehaviorSubject

先看一个Subject的例子。

import { Subject } from "rxjs";

const subject = new Subject();

const observerA = {
  next: value => console.log("Observer A get value: " + value),
  error: error => console.log("Observer A error: " + error),
  complete: () => console.log("Observer A complete!")
};

const 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);

输出

Observer A get value: 1
Observer A get value: 2
Observer A get value: 3

这里的observerB没有订阅。
因为 Subject 对象没有再调用 next() 方法。
这里的Subject 不能保存当前的最新状态。
如果希望 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。
使用 BehaviorSubject。

import { BehaviorSubject } from "rxjs";
const subject = new BehaviorSubject(0);

const observerA = {
  next: value => console.log("Observer A get value: " + value),
  error: error => console.log("Observer A error: " + error),
  complete: () => console.log("Observer A complete!")
};

const 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);

输出

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

同时我们看到const subject = new BehaviorSubject(0);有一个初始值为0，它用于表示 Subject 对象当前的状态。
在subject.subscribe(observerA);这条语句执行后，便会输出Observer A get value: 0。

ReplaySubject

如果我们希望新增的订阅者，可以接收到数据源最近发送的几个值。
可以使用ReplaySubject。

import { ReplaySubject } from "rxjs";
const subject = new ReplaySubject(2);

const observerA = {
  next: value => console.log("Observer A get value: " + value),
  error: error => console.log("Observer A error: " + error),
  complete: () => console.log("Observer A complete!")
};

const 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);

输出

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

当你把const subject = new ReplaySubject(2);改为const subject = new ReplaySubject(1);
输出

Observer A get value: 1
Observer A get value: 2
Observer A get value: 3
Observer B get value: 3

AsyncSubject

AsyncSubject 类似于 last 操作符，它会在 Subject 结束后发出最后一个值。

import { AsyncSubject } from "rxjs";
const subject = new AsyncSubject();

const observerA = {
  next: value => console.log("Observer A get value: " + value),
  error: error => console.log("Observer A error: " + error),
  complete: () => console.log("Observer A complete!")
};

const 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);

输出

Observer A get value: 3
Observer A complete!
Observer B get value: 3
Observer B complete!

当你注释掉subject.complete();则什么也不会输出。
因为subject没有结束。

参考：
RxJS Subject