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I'm not clear on the difference between a Subject and a BehaviorSubject. Is it just that a BehaviorSubject has the getValue() function?
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Observable is a Generic, and BehaviorSubject is technically a sub-type of Observable because BehaviorSubject is an observable with specific qualities. An observable can be created from both Subject and BehaviorSubject using subject.
1. A BehaviorSubject behaves like a Subject but it has an extra feature, i.e. it preserves the last emitted value. That means if a value was emitted earlier and if a subscription was added after the value was emitted, then the subscription will give the last value that was emitted.
The ReplaySubject is comparable to the BehaviorSubject in the way that it can send “old” values to new subscribers. It however has the extra characteristic that it can record a part of the observable execution and therefore store multiple old values and “replay” them to new subscribers.
While plain Observables are unicast (each subscribed Observer owns an independent execution of the Observable), Subjects are multicast. A Subject is like an Observable, but can multicast to many Observers. Subjects are like EventEmitters: they maintain a registry of many listeners. Every Subject is an Observable.
A BehaviorSubject holds one value. When it is subscribed it emits the value immediately. A Subject doesn't hold a value.
Subject example (with RxJS 5 API):
const subject = new Rx.Subject(); subject.next(1); subject.subscribe(x => console.log(x)); Console output will be empty
BehaviorSubject example:
const subject = new Rx.BehaviorSubject(0); subject.next(1); subject.subscribe(x => console.log(x)); Console output: 1
In addition:
BehaviorSubject should be created with an initial value: new Rx.BehaviorSubject(1) ReplaySubject if you want the subject to hold more than one value
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BehaviourSubject will return the initial value or the current value on Subscription
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:
observerA: 0 observerA: 1 observerA: 2 observerB: 2 observerA: 3 observerB: 3 Subject does not return the current value on Subscription. It triggers only on .next(value) call and return/output the 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); With the following output on the console:
observerA: 2 observerB: 2 observerA: 3 observerB: 3 If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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