CODEWITHSUNDEEP
javascripttypescriptrxjsrxjs5
llCorvinuSll
llCorvinuSll

Reputation: 342

RxJS function that emit last value from one observable then other emit true

Hellow, I'm trying to create function of observable (OBS) and subject(SUB) that stores last item from OBS, while SUB has F value, and emit it (and only it) when SUN becomes T

   OBS ---a----b----c----d----e----f----g----h-----
   SUB ------F----------T------------F-------T-----
   OUT -----------------c--------------------h-----

I tried to solve this with

OBS.window(SUB)
        .withLatestFrom(SUB)
        .switchMap(([window, status]) => {

            if(status === F) {
                return window.combineLatest(SUB, (cmd, status) => {
                    if(status === T) {
                        return null;
                    };

                    return cmd;
                }).last((e) => {
                    return !!e;
                })
            }

            return Observable.empty<Command>();
        }).filter((cmd) => {
            return !!cmd;
        })

but it doesn't work

Upvotes: 4

Views: 764

Answers (2)

llCorvinuSll
llCorvinuSll

Reputation: 342

I found own solution: 

OBS
    .buffer(SUB)
    .withLatestFrom(SUB)
    .map(([buffer, t]) => {
        if(!buffer || !buffer.length) {
            return null;
        }

        if(t === T) {
            return buffer[buffer.length - 1];
        }

        return null;
    })
    .filter((e) => !!e);

this variant has next behavior 

 ---a----b----c----d----e----f----g----h-----
 ------F----------T------------F-------T-----
 -----------------c--------------------h-----

and don't generate output if window between F and T is empty

 ---a--------------d----e----f----g----h-----
 ------F----------T------------F-------T-----
 --------------------------------------h-----

Upvotes: 2

paulpdaniels
paulpdaniels

Reputation: 18663

So it seems like you want something like:

SUB
  // Only emit changes in the status
  .distinctUntilChanged()
  // Only forward true values down stream
  .filter(t => t === T)
  // Only emit the latest from OBS when you get a T from SUB
  // Remap it so only cmd is forwarded
  .withLatestFrom(OBS, (_, cmd) => cmd)

Upvotes: 2

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