PhiLho PhiLho - 2 months ago 22
Javascript Question

RxJS: concat three promises, distinguish results

I have three promises, Rest requests returning lists of data.

The third one has references (ids) to the first two lists, so I want to map these ids to corresponding names when I have all the data.

The mapping isn't an issue, I just use Lodash for that.

But the issue is to wait for the three promises to resolve before starting to computing this mapping.

I figured out to use

concat()
:

Rx.Observable.concat(p1, p2, p3).subscribe(
function onNext(list)
{
// Assign the list to the corresponding variable in the scope
},
function onError(e)
{
// Notify of error
},
function onCompleted()
{
// Do the mapping
}
);


My problem is that
onNext()
will be called in random order. Ie. I don't know which list I receive at some point, and it is hard to tell from the data it contains.

Is there a way to track which promises produced which list? A kind of zip? concatMap? concatMapObserver? I admit I haven't fully understood the usage of the last two...

Answer

If these are promises we are talking about, I think you can have a look at the forkJoin operator. Cf. https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/forkjoin.md, http://xgrommx.github.io/rx-book/content/observable/observable_methods/forkjoin.html

You could then have something like :

Rx.Observable.forkJoin(p1, p2, p3, function(p1,p2,p3){
/* your combining code here */
})

In short, forkJoin has semantics similar to that of RSVP.all if you know the promises library RSVP.

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