angular - 主题和行为主题有什么区别？

Question

我不清楚 aSubject和 a之间的区别BehaviorSubject。只是 aBehaviorSubject有这个getValue()功能吗？

Answer 1

一个 BehaviorSubject 拥有一个值。当它被订阅时，它会立即发出值。主题不包含值。

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

行为主体示例：

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

控制台输出：1

此外：

• BehaviorSubject应该使用初始值创建：newRx.BehaviorSubject(1)
• 考虑ReplaySubject您是否希望主题包含多个值

Answer 2

行为主体

BehaviourSubject 将返回订阅的初始值或当前值

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);

带输出：

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

主题

主题不返回订阅的当前值。.next(value)它仅在调用时触发并返回/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);

在控制台上使用以下输出：

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

Answer 3

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

Answer 4

BehaviorSubject将 observable 发出的最后一个值保存在内存中。常客Subject不会。

BehaviorSubject就像ReplaySubject缓冲区大小为 1。

更新：有一些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初始值，即使到目前为止还没有向主题推送任何内容，请使用 BehaviorSubject。如果您希望将最后一个值重播给观察者，即使 Subject 已经关闭，请使用 ReplaySubject(1)。

Answer 5

它可能会帮助你理解。

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 

Answer 6

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
    
    

Answer 7

ABehaviorSubject持有一个值（因此我们实际上需要初始化一个默认值）。当它被订阅时，它会立即发出该值。Subject另一方面，A没有值。

这实际上意味着，Subject订阅者只会收到即将到来的值，BehaviorSubject而订阅者将收到先前的值和即将到来的值。

BehaviorSubject更多关于和之间的区别Subject可以在这里找到

所以，让我们举个例子来看看它会如何表现：

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

就像我们在上面看到的，前两个值是在第二次订阅注册之前从主题输出的，所以它没有得到它们，它只是在订阅后才得到新的值。第一个订阅得到了所有这些，因为它在输出第一个值之前就订阅了。

现在，让我们更改subjecttoBehaviorSubject并查看区别：

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，请注意0自BehaviorSubject初始化以来第一个订阅者如何输出0. 当第二个订阅者订阅时，它会立即发出 '2' 值，因为它是要处理的最后一个值，因此它充当它的初始值。

Answer 8

一个BehaviorSubject在订阅后发出一个值，一个主题号。

// 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