这可能会帮助你理解。

import * as Rx from 'rxjs';

const subject1 = new Rx.Subject();
subject1.next(1);
subject1.subscribe(x => console.log(x)); // will print nothing -> because we subscribed after the emission and it does not hold the value.

const subject2 = new Rx.Subject();
subject2.subscribe(x => console.log(x)); // print 1 -> because the emission happend after the subscription.
subject2.next(1);

const behavSubject1 = new Rx.BehaviorSubject(1);
behavSubject1.next(2);
behavSubject1.subscribe(x => console.log(x)); // print 2 -> because it holds the value.

const behavSubject2 = new Rx.BehaviorSubject(1);
behavSubject2.subscribe(x => console.log('val:', x)); // print 1 -> default value
behavSubject2.next(2) // just because of next emission will print 2 

BehaviorSubject keeps in memory the last value that was emitted by the observable. A regular Subject doesn't. So we can update dynamic titles based on Behaviour Subject.


var bSubject= new Rx.BehaviorSubject(0);  // 0 is the initial value
    
    bSubject.subscribe({
      next: (v) => console.log('observerA: ' + v)  // output initial value, then new values on `next` triggers
    });
    
    bSubject.next(1);  // output new value 1 for 'observer A'
    bSubject.next(2);  // output new value 2 for 'observer A', current value 2 for 'Observer B' on subscription
    
    bSubject.subscribe({
      next: (v) => console.log('observerB: ' + v)  // output current value 2, then new values on `next` triggers
    });
    
    bSubject.next(3);
    
     - With Output
    
    

BehaviorSubject在内存中保存被观察对象发出的最后一个值。一个普通的实验对象没有。

BehaviorSubject类似于缓冲区大小为1的ReplaySubject。

更新:有边缘用例可以区分这两者。https://medium.com/javascript-everyday/behaviorsubject-vs-replaysubject-1-beware-of-edge-cases-b361153d9ccf

TLDR: 如果您想在订阅时提供一个初始值，即使到目前为止还没有将任何内容推送到Subject，也可以使用BehaviorSubject。如果你想让观察者重放最后一个值，即使一个Subject已经关闭，也可以使用ReplaySubject(1)。

我刚刚创建了一个项目来解释所有科目之间的区别: https://github.com/piecioshka/rxjs-subject-vs-behavior-vs-replay-vs-async

BehaviourSubject

behavionsubject将返回订阅上的初始值或当前值

var bSubject= new Rx.BehaviorSubject(0);  // 0 is the initial value

bSubject.subscribe({
  next: (v) => console.log('observerA: ' + v)  // output initial value, then new values on `next` triggers
});

bSubject.next(1);  // output new value 1 for 'observer A'
bSubject.next(2);  // output new value 2 for 'observer A', current value 2 for 'Observer B' on subscription

bSubject.subscribe({
  next: (v) => console.log('observerB: ' + v)  // output current value 2, then new values on `next` triggers
});

bSubject.next(3);

输出:

observerA: 0
observerA: 1
observerA: 2
observerB: 2
observerA: 3
observerB: 3

主题

主题没有返回订阅上的当前值。它只触发.next(value)调用并返回/输出该值

var subject = new Rx.Subject();

subject.next(1); //Subjects will not output this value

subject.subscribe({
  next: (v) => console.log('observerA: ' + v)
});
subject.subscribe({
  next: (v) => console.log('observerB: ' + v)
});

subject.next(2);
subject.next(3);

在控制台上输出如下信息:

observerA: 2
observerB: 2
observerA: 3
observerB: 3

一个BehaviorSubject包含一个值(因此我们实际上需要初始化一个默认值)。当它被订阅时，它立即发出该值。另一方面，Subject不包含值。

这实际上意味着在Subject中，订阅者将只接收到即将到来的值，而在BehaviorSubject中，订阅者将接收到之前的值和即将到来的值。

所以，让我们举个例子来看看它是如何表现的:

let mySubject = new Subject<number>();

mySubject.subscribe(x => console.log("The first Subscription : " + x));

mySubject.next(1);
mySubject.next(2);

mySubject.subscribe(x => console.log("The second Subscription : " + x));

mySubject.next(3);

// The first Subscription : 1
// The first Subscription : 2
// The first Subscription : 3
// The second Subscription : 3

就像我们上面看到的，前两个值是在第二个订阅注册之前从主题输出的，所以它没有得到它们，它只在订阅后得到新的值。第一个订阅获得了所有这些值，因为它是在输出第一个值之前订阅的。

现在，让我们把主题改为BehaviorSubject，看看区别:

let mySubject = new BehaviorSubject<number>(0);

mySubject.subscribe((x) => console.log('The first Subscription : ' + x));

mySubject.next(1);
mySubject.next(2);

mySubject.subscribe((x) => console.log('The second Subscription : ' + x));

mySubject.next(3);

// The first Subscription : 0 (since it's the initial value)
// The first Subscription : 1
// The first Subscription : 2
// The second Subscription : 2 (since it's the initial value for the seconde subscriber)
// The first Subscription : 3
// The second Subscription : 3

现在，注意第一个订阅者是如何输出0的，因为BehaviorSubject初始化为0。当第二个订阅者订阅时，它立即发出'2'值，因为它是要处理的最后一个值，因此它作为它的初始值。

更多关于行为主体和主体之间的区别可以在这里找到