我对 Subject
和 BehaviorSubject
的区别不是很清楚。难道只是因为 BehaviorSubject
有 getValue()
函数吗?
我对 Subject
和 BehaviorSubject
的区别不是很清楚。难道只是因为 BehaviorSubject
有 getValue()
函数吗?
BehaviorSubject保存一个值。当订阅时,它立即发出该值。Subject不保存值。
Subject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示例:
const subject = new Rx.BehaviorSubject(0);
subject.next(1);
subject.subscribe(x => console.log(x));
控制台输出:1
另外:
BehaviorSubject
应该使用初始值创建:new Rx.BehaviorSubject(1)
ReplaySubject
。BehaviorSubject在订阅时会返回初始值或当前值。
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);
输出结果为:
observerA: 0
observerA: 1
observerA: 2
observerB: 2
observerA: 3
observerB: 3
Subject在订阅时不会返回当前值。它仅在调用.next(value)
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);
在控制台上输出以下内容:
observerA: 2
observerB: 2
observerA: 3
observerB: 3
subject.next(3);
时,数字3就在那里。 - Mohammed Safeer我刚刚创建了一个项目,旨在解释所有主题之间的区别:
https://github.com/piecioshka/rxjs-subject-vs-behavior-vs-replay-vs-async
BehaviorSubject
只保存一个值(所以我们需要初始化一个默认值)。当它被订阅时会立即发出该值。另一方面,Subject
不保存值。这意味着在 Subject
中,订阅者只会接收到即将到来的值,而在 BehaviorSubject
中,订阅者将接收到先前的值和即将到来的值。让我们通过一个例子来看看这是如何工作的: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
就像我们之前看到的那样,前两个值是在第二次订阅注册之前从subject输出的,因此第二次订阅没有收到它们,只收到了后续的新值。由于第一次订阅在第一个值输出之前就已经订阅了,所以它获取了所有的值。
现在,让我们将 subject
更改为 BehaviorSubject
并看看区别:
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,请注意第一个订阅者输出0
,因为BehaviorSubject
是使用0
进行初始化的。当第二个订阅者订阅时,它立即发出值'2',因为它是最后一个处理的值,所以它充当了其初始值。
有关BehaviorSubject
和Subject
之间差异的更多信息可以在此处找到。
BehaviorSubject
会在内存中保留被可观察对象最后发出的值,而一个普通的 Subject
不会。
BehaviorSubject
就像具有缓冲区大小为 1 的 ReplaySubject
。
更新:有一些极端情况可以区分这两者。 https://medium.com/javascript-everyday/behaviorsubject-vs-replaysubject-1-beware-of-edge-cases-b361153d9ccf
简而言之: 如果你想在订阅时提供一个初始值,即使到目前为止,Subject 还没有推送任何内容,请使用 BehaviorSubject。如果你想要将最后一个值重放给观察者,即使 Subject 已经关闭,请使用 ReplaySubject(1)。
这可能有助于你理解。
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
这三者都有很大的区别,让我在这里举几个例子:
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
可以看到,当我们在发出信号后订阅主题(即next(...)之后),
next()
相同因此,区别主要在于您的订阅位置,无论是在 next()
之前还是之后。
在实际情况中,我们只在填充主题数据后才会触发事件 [即在next()
之后]。
// 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
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
// 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