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
    
    

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

这三个都很不同，让我再举几个例子，

const subject = new Rx.Subject();
const behaviorSubject = new Rx.BehaviorSubject([]);
const relaySubject = new Rx.ReplaySubject();


subject.next(1)

behaviorSubject.next(1); 
behaviorSubject.next(2); 
behaviorSubject.next(3);

relaySubject.next(1); 
relaySubject.next(2); 
relaySubject.next(3);

subject.subscribe(val => console.log('From Subject', val)); // this will not emits
behaviorSubject.subscribe(val => console.log('From BehaviorSubject', val)); // this will emits only last value
relaySubject.subscribe(val => console.log('From ReplaySubject', val)); // this will emit all values

输出截图

正如你所看到的，当我们在发出(i。e(…)),

主题-这将不会被发射 behaviorSubject -这将使用最后一个值触发一次 ReplaySubject -这将发射3次，因为我们有很多next()

所以区别主要在于订阅的位置，是在next()之前还是在next()之后。

在实际情况下，我们只在next() [i。E在我们向受试者填写数据后]。

程序测试所有4个主题类型: 主题，行为主题，重玩主题和AsyncSubject

// 1. Subject - only value after subscribed
var subject = new Subject();
subject.next(1);
subject.next(2);
subject.complete();
subject.subscribe(
  (data) => this.log("Subject="+data),
  (error) => this.log(error),
  () => this.log('Complete Subject')
);
subject.next(3);
subject.next(4);

// 2. BehaviorSubject - only last value before subscribed and all after subscription
// calls on initalization, mandatory to specify a value
var subjectb = new BehaviorSubject<any>(5);
subjectb.next(1);
subjectb.next(2);
subjectb.complete();
subjectb.subscribe(
  (data) => this.log("Behavior="+data),
  (error) => this.log(error),
  () => this.log('Complete Behavior')
);

// 3. ReplaySubject - all specified last values before subscribed and all after subscription
// Does not call on initalization, no default value
var subjectr = new ReplaySubject(5);
subjectr.next(1);
subjectr.next(2);
subjectr.complete();
subjectr.subscribe(
  (data) => this.log("Replay="+data),
  (error) => this.log(error),
  () => this.log('Complete Replay')
);

// 4. AsyncSubject - only last values before calling complete
var subjecta = new AsyncSubject();
subjecta.next(1);
subjecta.next(2);
subjecta.complete();
subjecta.subscribe(
  (data) => this.log("Async="+data),
  (error) => this.log(error),
  () => this.log('Complete Async')
);

https://stackblitz.com/edit/example-rxjs-subject-e8vj9y?embed=1&file=app/app.component.ts

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