How to get last value from a Subject?

Question

I have two Angular2 components which need to share data via a service:

@Injectable()
export class SearchService {
  private searchResultSource = new Subject<string>()
  searchResult$ = this.searchResultSource.asObservable()

  setSearchResults(_searchResult: string): void {
    this.searchResultSource.next(_searchResult)
  }
}

Suppose ComponentA is rendered and it emits an event via SearchService.setSearchResults. Then the user navigates to ComponentB, which also subscribes to searchResult$. However, ComponentB will never observe the event emitted by ComponentA because it was not subscribed to searchResult$ at the time ComponentA emitted an event, as it did not exist.

How can I create an Observable which emits the last event to every new subscriber?

A comprehensible and up-to-date article explaining the `AsyncSubject`, `BehaviorSubject` and `ReplaySubject` can be found here: https://medium.com/@luukgruijs/understanding-rxjs-behaviorsubject-replaysubject-and-asyncsubject-8cc061f1cfc0 — conceptdeluxe, Sep 13 '18 at 00:18
I found this to be a clear simple (no angular/TS) explanation about the three Subjects with live code to play with https://coryrylan.com/blog/rxjs-observables-versus-subjects — DKebler, Jan 03 '20 at 17:38
Also as of rxjs 6 there is shareReply operator see comments here. https://stackoverflow.com/questions/41730542/how-to-convert-an-observable-to-a-replaysubject-in-rxjs, docs here https://www.learnrxjs.io/operators/multicasting/sharereplay.html — DKebler, Jan 03 '20 at 17:56

score 66 · Accepted Answer · edited Sep 13 '18 at 00:20

66

BehaviorSubject immediately emits the last value to new subscribers:

@Injectable()
export class SearchService {

  private searchResultSource = new BehaviorSubject<string>('');

  setSearchResults(_searchResult: string): void {
      this.searchResultSource.next(_searchResult);
  }
}

ReplaySubject emits all previous events to new subscribers.

edited Sep 13 '18 at 00:20

conceptdeluxe

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answered May 12 '16 at 04:42

Günter Zöchbauer

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So the BehaviorSubject has to have an initial value always? Or maybe the initial value is not passed across subscriptions – Marc Guillem Jul 06 '22 at 10:36
1

Yes, it has to have an initial value - which could be null though. – Günter Zöchbauer Jul 07 '22 at 13:35

score 21 · Answer 2 · edited Aug 01 '17 at 22:33

21

You can use the ReplaySubject to always get the last value of the Observer, something like this :

@Injectable()
export class SearchService {

  private searchResultSource = new ReplaySubject<string>(1);

  setSearchResults(_searchResult: string): void {
      this.searchResultSource.next(_searchResult);
  }
}

And just subscribe as normal.
A more advanced example can be found here : caching results with angular2 http service

edited Aug 01 '17 at 22:33

royhowie

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answered May 11 '16 at 23:15

Tiberiu Popescu

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Can you explain "Subscribe as normal"? Is there any case where subscription doesnt work? – sabithpocker Jun 21 '17 at 16:07
I meant that I don't need to show how you subscribe to this ReplaySubject, as you don't need to do anything special on the subscription side, just subscribe and you'll get the last value. – Tiberiu Popescu Jun 21 '17 at 18:40
Okay cool. I had a situation where `BehaviorSubject` subsription didnt work but `ReplaySubject` did. Was trying to understand why. – sabithpocker Jun 22 '17 at 07:16
4

I'm still very new to this Rx stuff and observables (aren't we all at this point?!) but this answer helped me quite a bit. For anyone in my position... I originally chose a `ReplaySubject` (opposed to `BehaviorSubject`) because you don't need to initialize a `ReplaySubject` with a value. But subscribing to a `ReplaySubject` observable by default gives you all values that were broadcast during its lifetime. initialiazing the `ReplaySubject` with a buffer size of 1 like in @tibbus answer worked for me! – default_noob_network Jun 30 '17 at 18:06

How to get last value from a Subject?

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