Indicate when a ReplaySubject has reached its "completion" signal

Question

Indicate when a ReplaySubject has reached its "completion" signal

I'm currently looking for an effective way to indicate when a ReplaySubject is empty.

import {ReplaySubject} from 'rxjs/ReplaySubject'; 
const rs = new ReplaySubject<Object>();

 // ...
constructor(){
  this.sub = rs.subscribe(...);
}

Each time the constructor is called, all items from the subject are replayed. But my main question is - is there any event we can monitor to know when the subject is empty?

The only solution I can come up with is triggering a custom event when the subject is done, like so:

rs.next({done:true});

Is sending data to the next() method the most effective way to signal that the ReplaySubject is temporarily empty or out of events?

typescript rxjs rxjs5 angular-observable

Answer 1

Answer №1

When Subject is functioning as an Observer, you have the option to trigger complete() right after the last next() call. This way, you can monitor it using the third parameter of the subscribe(...) method.

const rs = new ReplaySubject<Object>();
rs.next({a: 1})
rs.complete()

rs.subscribe(
    obj => console.log(obj),
    error => {},
    () => console.log('completed')
);
// Result
// {a: 1} completed

Answer 2

When Subject is functioning as an Observer, you have the option to trigger complete() right after the last next() call. This way, you can monitor it using the third parameter of the subscribe(...) method.

const rs = new ReplaySubject<Object>();
rs.next({a: 1})
rs.complete()

rs.subscribe(
    obj => console.log(obj),
    error => {},
    () => console.log('completed')
);
// Result
// {a: 1} completed

Answer 3

Answer №2

One approach could be to create a secondary observable to keep track of the last item being replayed:

const last$ = rs.replay(1);

Then, by using combineLatest and scan, you can monitor when the ReplaySubject has completed replaying:

this.sub = Observable.combineLatest(
  rs,
  last$.take(1)
).scan((acc: { item: Object, isReplay: boolean }, curr: [Object, Object]) => {
    return {item: curr[0], isReplay: acc.isReplay && curr[0] !== curr[1]};
  }, {isReplay: true}
).subscribe(...);

Answer 4

One approach could be to create a secondary observable to keep track of the last item being replayed:

const last$ = rs.replay(1);

Then, by using combineLatest and scan, you can monitor when the ReplaySubject has completed replaying:

this.sub = Observable.combineLatest(
  rs,
  last$.take(1)
).scan((acc: { item: Object, isReplay: boolean }, curr: [Object, Object]) => {
    return {item: curr[0], isReplay: acc.isReplay && curr[0] !== curr[1]};
  }, {isReplay: true}
).subscribe(...);

Indicate when a ReplaySubject has reached its "completion" signal

Answer №1

Answer №2

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