Reactive-Extensions/RxJS

The Reactive Extensions for JavaScript

Dorus

@Dorus

I wonder if i missed a way to simplify this...

Renke Grunwald

@renke

yes, that makes sense, I guess, that would prevent it from completing before src1 completes.

Dorus

@Dorus

yup

toArray react to completion and you dont want src2 completing to trigger that before src1 does.

Renke Grunwald

@renke

I still don't understand why delayWhen is not the right choice here:

      const foo = first$.pipe(
        publish(firstSubject$ => {
          return merge(
            firstSubject$,
            second$.pipe(
              delayWhen(() =>
                concat(firstSubject$.pipe(ignoreElements()), of())
              )
            )
          );
        })
      );

I would imagine that when firstSubject$ completes there is no leakage from delayWhen whatsoever

Dorus

@Dorus

You should test it. But i dont think this will allow second$ to emit once first is done, not after the initial burst.

Idk, does firstSubject$ remember it completed?

Renke Grunwald

@renke

oh, i see the factory is called on each emit of second$?

Dorus

@Dorus

@renke Quick test does show it works. I said nothing :D https://rxviz.com/v/B8ZABXaJ

Actually, thinking back, i think the subject stays in completion status and will emit complete() right away once you subscribe again on new commits from second$

So that's a very neat solution

Renke Grunwald

@renke

it feels a bit more natural so to say

that aside, thanks for your help, I think I learned a lot here, especially about publish

Dorus

@Dorus

Glad to help, i was half asleep yesterday so it's a miracle i gave at least somewhat working code xD

Renke Grunwald

@renke

it certainly pushed me into the right direction, although your shareReplay snippet confused me a bit :-D

Dorus

@Dorus

it's like publish but with a replaySubject backing the inner stream.

Just a ranom brainwave of mine i guess :P

I do like this new found way to using delayWhen. gotta memorize it.

Renke Grunwald

@renke

I am still puzzled why there is no publishReplay that takes a selector function though. Does that not make sense or why does only publish support that.

Dorus

@Dorus

https://rxjs.dev/api/operators/publishReplay does show selectorOrScheduler

@renke

Renke Grunwald

@renke

Oh, I see, maybe was looking at shareReplay which seem to lack it indeed

Dorus

@Dorus

@renke Oops, my mistake. I named shareReplay and i also forgot to set the first 2 params. Idk i always confuse share and publish, even thou i know very well what they do.

Will Mruzek

@mruzekw

Hi there, say I have a high-order observable. It basically contains a set of set of numbers.

I'd like the first inner observable to be listened to until the next inner observable is available. Then the next one should be listened to. I know there's switchMap, but I can't seem to get it to work.

const setOfSetOfNums$ = merge(
  of([4,5,6,7]).pipe(delay(50)),
  of([0,1,2,3]),
).pipe(
  distinctUntilChanged(),
  switchMap(context => of(...context)),
)

zip(interval(1000), setOfSetOfNums$).pipe(
  map(([_, hand]) => hand),
)

I would expect this to output something like:

---0---1---4---5---6---7--->

But it spits out:

---0---1---2---3---4---5---6---7--->

Derek

@derekkite

@mruzekw not quite clear what you need. 0-1-? or 0 - 4 - 5 - 6 - 7 - 1 then what? maybe describe what you are trying to accomplish

Will Mruzek

@mruzekw

I would like the delayed observable to interrupt the non-delayed one

Thus the desired output of ---0---1---4---5---6---7--->

In actuality, it's a user event that generates a new set of nums to listen to, but this should replicate the idea...I think?

If there's a new observable in setOfSetOfNums$, we should immediately switch to that

Dorus

@Dorus

@mruzekw the problem is you zip after switchMap

thus you have them first emit all events at the same time, and then queue up at zip after the switchMap

Observables have no so called backpressure

Derek

@derekkite

of(4,5,6,7).pipe(
    delay(50), 
    switchMap((of(1,2,3,4))
)

Dorus

@Dorus

That's not even valid code

Will Mruzek

@mruzekw

@Dorus There's no way to add any backpressure?

Would it be any different if a click was the trigger to switch observables?

Derek

@derekkite

@mruzekw there are examples of a search bar where the event triggers an http call that is interrupted via switchmap if the user types something more.

Will Mruzek

@mruzekw

To make this even simpler, is there a way to receive a stream of numbers one at a time? Say when I click a button?

Karsten Pruehss

@kpruehss

@mruzekw if you want a stream of individual numbers rather then all the numbers at once, you should use from() instead of of() i think

Will Mruzek

@mruzekw

I mean, I'm sure I can create a stream of numbers with either one

The question is getting each num one at a time

Err, per click in this case

Dorus

@Dorus

@mruzekw You could put zip before the switchMap, or inside.

const setOfSetOfNums$ = merge(
  of([4,5,6,7]).pipe(delay(50)),
  of([0,1,2,3]),
).pipe(
  switchMap(context => zip(interval(1000), from(context))),
  map(([_, hand]) => hand)
)

Will Mruzek

@mruzekw

Thanks, @Dorus! That worked

Now is there a way to do a head-tail type operation on an observable?

Dorus

@Dorus

There is finalize or concat to add actions on the tail. Or just use the complete callback.

I'm not exactly sure what you mean

Will Mruzek

@mruzekw

Well, I'd like the first value of an observable to pass through, but the rest be released on click

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