You generally want to use shareReplay
when you have side-effects or taxing computations that you do not wish to be executed amongst multiple subscribers. It may also be valuable in situations where you know you will have late subscribers to a stream that need access to previously emitted values. This ability to replay values on subscription is what differentiates share
and shareReplay
.
For instance, suppose you have an observable that emits the last visited url. In the first example we are going to use share
:
// simulate url change with subjectconst routeEnd = new Subject<{data: any, url: string}>();// grab url and share with subscribersconst lastUrl = routeEnd.pipe(pluck('url'),share());// initial subscriber requiredconst initialSubscriber = lastUrl.subscribe(console.log);// simulate route changerouteEnd.next({data: {}, url: 'my-path'});// nothing loggedconst lateSubscriber = lastUrl.subscribe(console.log);
In the above example nothing is logged as the lateSubscriber
subscribes to the source. Now suppose instead we wanted to give access to the last emitted value on subscription, we can accomplish this with shareReplay
:
import { Subject } from 'rxjs/Subject';import { ReplaySubject } from 'rxjs/ReplaySubject';import { pluck, share, shareReplay, tap } from 'rxjs/operators';// simulate url change with subjectconst routeEnd = new Subject<{data: any, url: string}>();// grab url and share with subscribersconst lastUrl = routeEnd.pipe(tap(_ => console.log('executed')),pluck('url'),// defaults to all values so we set it to just keep and replay last oneshareReplay(1));// requires initial subscriptionconst initialSubscriber = lastUrl.subscribe(console.log);// simulate route change// logged: 'executed', 'my-path'routeEnd.next({data: {}, url: 'my-path'});// logged: 'my-path'const lateSubscriber = lastUrl.subscribe(console.log);
Note that this is similar behavior to what you would see if you subscribed a ReplaySubject
to the lastUrl
stream, then subscribed to that Subject
:
// simulate url change with subjectconst routeEnd = new Subject<{data: any, url: string}>();// instead of using shareReplay, use ReplaySubjectconst shareWithReplay = new ReplaySubject();// grab url and share with subscribersconst lastUrl = routeEnd.pipe(pluck('url')).subscribe(val => shareWithReplay.next(val));// simulate route changerouteEnd.next({data: {}, url: 'my-path'});// subscribe to ReplaySubject instead// logged: 'my path'shareWithReplay.subscribe(console.log);
In fact, if we dig into the source code we can see a very similar technique is being used. When a subscription is made, shareReplay
will subscribe to the source, sending values through an internal ReplaySubject
:
( source )
return function shareReplayOperation(this: Subscriber<T>, source: Observable<T>) {refCount++;if (!subject || hasError) {hasError = false;subject = new ReplaySubject<T>(bufferSize, windowTime, scheduler);subscription = source.subscribe({next(value) { subject.next(value); },error(err) {hasError = true;subject.error(err);},complete() {isComplete = true;subject.complete();},});}const innerSub = subject.subscribe(this);return () => {refCount--;innerSub.unsubscribe();if (subscription && refCount === 0 && isComplete) {subscription.unsubscribe();}};};}
Example 1: Multiple subscribers sharing source
( Stackblitz )
// RxJS v6+import { Subject, ReplaySubject } from 'rxjs';import { pluck, share, shareReplay, tap } from 'rxjs/operators';// simulate url change with subjectconst routeEnd = new Subject<{data: any, url: string}>();// grab url and share with subscribersconst lastUrl = routeEnd.pipe(tap(_ => console.log('executed')),pluck('url'),// defaults to all values so we set it to just keep and replay last oneshareReplay(1));// requires initial subscriptionconst initialSubscriber = lastUrl.subscribe(console.log)// simulate route change// logged: 'executed', 'my-path'routeEnd.next({data: {}, url: 'my-path'});// logged: 'my-path'const lateSubscriber = lastUrl.subscribe(console.log);
shareReplay 📰 - Official docs
📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/shareReplay.ts