主体和行为主体的区别是什么?

我不清楚Subject和BehaviorSubject之间的区别。仅仅是因为BehaviorSubject具有getValue()函数吗?

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最佳答案

一个BehaviorSubject包含一个值。当它被订阅时，它立即发出该值。Subject不包含值。

主题示例(使用RxJS 5 API):

const subject = new Rx.Subject();
subject.next(1);
subject.subscribe(x => console.log(x));

控制台输出将为空

BehaviorSubject例子:

const subject = new Rx.BehaviorSubject(0);
subject.next(1);
subject.subscribe(x => console.log(x));

控制台输出:1

此外:

  • BehaviorSubject应该创建一个初始值:new Rx.BehaviorSubject(1)
  • 如果你想让主题获得之前发布的值，考虑ReplaySubject。
小开

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。它只在__ABC0调用时触发，并返回/输出value

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
小开
我只是创建一个项目，它解释了什么是所有科目之间的差异:
https://github.com/piecioshka/rxjs-subject-vs-behavior-vs-replay-vs-async < / p >

enter image description here

小开

BehaviorSubject将可观察对象发出的最后一个值保存在内存中。常规的Subject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

< p > 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。如果你想让一个观察者重放最后一个值，即使一个Subject已经关闭，使用ReplaySubject(1).

小开

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
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< >强BehaviorSubject < / >强在订阅后产生一个值，< >强主题< / >强 no。

// Subject
const mySubject = new Rx.Subject().subscribe((v) => console.log(v)); // will return nothing


// BehaviorSubject
const myBehaviorSubject = new Rx.BehaviorSubject(666).subscribe((v) => console.log(v)); // will return 666 when subscription occurs
小开 
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
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BehaviorSubject保存一个值(因此我们实际上需要初始化一个默认值)。当它被订阅时，它会发出值立即。另一方面，Subject不包含值。

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

关于BehaviorSubject和Subject的更多区别可以在在这里中找到

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subject改为BehaviorSubject，看看有什么不同:

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是用0初始化的。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。

小开

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

输出截图在这里

正如你所看到的，当我们在发出(i。e(…)),

  1. 主题-这将不会被发射
  2. behaviorSubject -这将使用最后一个值触发一次
  3. ReplaySubject -这将触发3次，与我们拥有的next()一样多

所以区别主要在于你订阅的位置，无论是在next()之前还是在next()之后。

在实际情况下，我们仅在next() [i]之后触发事件。E在我们向受试者填写数据后]。

小开

程序测试所有4个主题类型: Subject, BehaviorSubject, ReplaySubject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


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