Ignore events on stream until condition is satisfied?

Question

I created a stream from button click events. The button corresponds to a create action on the database. Obviously, I want the database action to fire only once (at least until it completes). Is there a way to ignore events on createButtonStream until Api.create returns? i.e., the first event should call #Api.create, subsequent events should be ignored until #Api.create returns.

createButtonStream
   .flatMap(() => Api.create()) //Needs to fire once until doSomething() is called
   .onValue(result => doSomething(result))

The only way that comes to mind is to use global state...and I'd rather not do that.

//i don't wanna do this
let condition = true

createButtonStream
   .filter(() => condition)
   .map(() => condition = false)
   .flatMap(() => Api.create())
   .onValue(result => {condition = true; doSomething(result)})

@iofjuupasli if i'm not mistaken, flatMapFirst will still fire Api.create...it'll just ignore responses until the first one finishes. So in other words, it'll still make calls to the database... which is what I'm trying to prevent. — U Avalos, May 25 '16 at 15:26
It seems that it's not ```var source = Kefir.sequentially(100, [1, 2, 3]); var result = source.flatMapFirst(x => { console.log(x); return Kefir.interval(40, x).take(4); }); result.log(); ``` . Logs to console `1 3` — iofjuupasli, May 25 '16 at 17:43

score 3 · Answer 1 · answered May 25 '16 at 19:56

3

In RxJS you use the flatMapFirst or exhaustMap operator (if using RxJS 5)

createButtonStream
   .flatMapFirst(() => Api.create())       
   .subscribe(result => doSomething(result));

flatMapFirst will silently drop events if they arrive before the first source completes, the method should not get invoked.

answered May 25 '16 at 19:56

paulpdaniels

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Same functionality exists in bacon (https://github.com/baconjs/bacon.js/#observable-flatmapfirst), this answer is better than the one I gave. – OlliM May 26 '16 at 05:36

score 0 · Answer 2 · answered May 25 '16 at 12:38

You can use awaiting. You need a Bus to work around the chicken and egg problem.

const filterBus = new Bacon.Bus()

const createDone = createButtonStream
  .filter(filterBus.toProperty(true))
  .flatMap(() => Api.create())

const inProgress = createButtonStream.awaiting(createDone)
filterBus.plug(inProgress.not())

createDone.onValue(doSomething)

Ignore events on stream until condition is satisfied?

2 Answers2