Delay release from buffer until promises after it are done - rxjs

Question

Here is my pseudo-code:
const s = new Subject();
s.pipe(
bufferCount(1).pipe(
concatMap(() => new Promise()),
concatMap(() => new Promise()),
concatMap(() => new Promise()),
)
)
s.next('a');
s.next('b');
s.next('c');
I want "b" and "c" held in the buffer UNTIL "a" is done processing.
Is this possible?

Answer 1

I suppose you want a source Observable to trigger some task and then only have the next value from source trigger the next task when the previous task completed. You can achieve this by zipping your source with a second trigger (startNext) that indicates that the previous task is done and the next value from source starting the next task can be emitted.
import { Subject, zip, of, BehaviorSubject } from 'rxjs';
import { map, tap, delay, concatMap } from 'rxjs/operators';
const source = new Subject();
const startNext = new BehaviorSubject(null);
zip(source, startNext)
.pipe(
map(([s, n]) => s), // discard the 'startNext' trigger
concatMap(s => of(s).pipe(delay(1000))),
concatMap(s => of(s).pipe(delay(200))),
concatMap(s => of(s).pipe(delay(3000))),
tap(_ => startNext.next(null))
).subscribe(s => console.log('result for', s));
source.next('a');
source.next('b');
source.next('c');
https://stackblitz.com/edit/rxjs-g5efuc