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?

Answer 1

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.

Answer 2

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