Reactive-Extensions/RxJS

The Reactive Extensions for JavaScript

Dorus

@Dorus

@ychaikin the subscription is triggered by subscribe(), not by the cold observable.
The cold observable will be called again each time hotObservable emit, until you unsubscribe.

Or you could use take, takeUntil etc. to finish the subscription at one point.

Or even keep it going forever depending on your scenario

You need an unsubscribe call when you want this pipeline to become inactive.

Yaakov Chaikin

@ychaikin

Right... what I am curious about is would it ever make sense to do this with the takeUntil:

hotObservable.pipe(takeUntil(subject), switchMap(value => return coldObservable)).subscribe()`

Note that takeUntil is NOT the last operator.

Dorus

@Dorus

it could be if you want to finish the content of the coldObservable but terminate the connection to the hotObservable. Just remember that there can be lingering activety after subject emitted.

For that last reason you usually put takeUntil at the end. But there could be scenario's where you intentionally do it like this.

Yaakov Chaikin

@ychaikin

Ok, my original thought was the the scenario is only when you still want whatever output from switchMap while not wanting anymore values passed into switchMap... If I understood you correctly, that's what you are confirming as well.

Dorus

@Dorus

yes that's what you'll get

Yaakov Chaikin

@ychaikin

ok. Thanks! Appreciate your time!

freshdev1

@freshdev1

Hello guys. I am new to RxJS and is finding it a little hard to grasp a few things... Hope anyone from you can help me out with it.

Scenario:

Parent Component Contract:
     - Child Component Contract Details
     - Child component Beds details

I am getting a value from an API which I need to pass to both child components. This is how I am doing.

Parent Component (TS):
  customerInfo$: Subject<CustomerInfo> = new Subject<CustomerInfo>();

//then in a method I am doing the following
private getCustomerInfo() {
    var ucid = this.localStorageService.getCompanyUCID();

    this.accountService.GetCustomerInfoObservable(ucid)
    .subscribe(this.customerInfo$);
    //   customerInfoObj => {
    //   this.customerInfo$.next(customerInfoObj);
    // });
  }

Parent Component (HTML):
<app-contract-general-information 
          [generalInformationGroup]="contractForm.controls.BasicInformation"
          [customerInfoObj]="customerInfo$"
          (onNext)="onNext(nav,$event)"
           (selectedWrc)="setWrcObject($event)"
 ></app-contract-general-information>

<app-contract-contrcated-beds
                                    [contractedBedsFormGroup]="contractForm.controls.TenantContractDetails"
                                    [customerInfoObj]="customerInfo$"
                                    (onNext)="onNext(nav,$event)"
                                    [selectedWrc$]="selectedWrc$"
                                    ></app-contract-contrcated-beds>

freshdev1

@freshdev1

In the child this is how I am consuming:

@Input() customerInfoObj: Subject<CustomerInfo>;

this.customerInfoObj
    .subscribe(customerInfo =>{
      console.log(customerInfo);
      this.wrcList = [...customerInfo.WRC];
    })

Same goes for the second component. but nowhere does it works.

Cannot understand why both of the components are not able to get the value.

Daniel Willis

@danww

@freshdev1 Ah, I just commented on your msg in the angular/angular channel. So here, when creating wrcList, I would do something like:

this.wrcList$ = this.customerInfoObj.pipe(
  tap((customerInfo: customerInfoType) => console.log(customerInfo)), // if you need to log this.
  map((customerInfo: customerInfoType) => customerInfo.WRC),
);

freshdev1

@freshdev1

@danww lets continue over there... :smile:

Dorus

@Dorus

@freshdev1 One problem i see is that you are subscribing a subject directly to a stream. This means that when the stream completes, so does the subject. Is this intended?

Also i would not use a Subject in your situation, is there any reason why you cannot use a plain CustomerInfo instead? Or possibly an angular @output EventEmitter ?

Yaakov Chaikin

@ychaikin

If you use a takeUtil with a Subject to manage unsubscribing, do you have to call subject.complete()? I saw somewhere that you don't, but I am wondering why or why not? Can you explain this or point me to an article that speaks specifically about the complete() call on that subject and why it would (or wouldn't) be needed?

Dorus

@Dorus

@ychaikin Complete will terminate the pipeline from the subject, unsubscribe will do the same from the subscription. So what you do depend on your scenario, there is no real reason to do both however. Once a subscription is finished, all resources will be cleaned up, independent from if it was finished by a complete message or unsubscribe call.
takeUtil in fact will unsubscribe upstream and complete downstream.
Also if you complete the subject, all subscriptions will terminate (even future ones wont be able to run anymore), but if you unsubscribe, other subscription or future subscriptions could still continue.

Yaakov Chaikin

@ychaikin

@Dorus Thanks. All makes sense. I actually found a really good breakdown of when to complete and when it doesn't matter and why. Take a look if interested: https://stackoverflow.com/questions/57007118/do-i-need-to-complete-takeuntil-subject-inside-ngondestroy

Dorus

@Dorus

@ychaikin Oh you're asking about a takeUntil(subject) scenario? If i remember right there was some bug involving this a few versions back, but those should be resolved now. Anyway if you next to the subject, takeUntil will unsubscribe both the source and the inner observable.

Yaakov Chaikin

@ychaikin

@Dorus I think you're referring to shareReplay bug that was fixed.

Dorus

@Dorus

i forgot the details, it was a while back :)

Yaakov Chaikin

@ychaikin

takeUntil(subject) will unsubscribe but I was wondering about completing the subject with complete(), which is a bit different of a topic.

But I think I have it clear now. Thanks!

Dorus

@Dorus

As i said, not necessarily for the takeUntil, but you could do it if there are other subscriptions to the subject that you didn't track.

Althou i highly doubt there are any others.

Yaakov Chaikin

@ychaikin

👍

freshdev1

@freshdev1

@Dorus thank you for taking a time to look at my example. I was squarely wrong about it. @danww already guided me in another channel and I was able to get things to work. The idea was to directly send the observable to the child components and use the ASYN pipe to resolve it.

Dorus

@Dorus

@freshdev1 Ah yes that's also possible. However a bit icky. You should consider to make a service that encapsulates the observable :)

abhip5369

@abhip5369

Hi all, I am using shareReplay() to replay the http response(list of items) in different tabs on same page to fill a dropdown thats there in all the tabs, but if the value is updated from 1 of the tabs the list of items in the dropdown is not updated. Is there a way with Rxjs to achieve that ?

1 reply

Sea * Side Syndication

@sea-side-syndication

Screen Shot 2021-01-14 at 8.20.21 PM.png

what is the correct way to reconnect a websocket? what should I be putting in closeObserver to reconnect to the websocket?

Sea * Side Syndication

@sea-side-syndication

is there a thorough angular/rxjs websocket example you guys could recommend? perhaps one with multiplex?

Sea * Side Syndication

@sea-side-syndication

for those curious, i followed what was written here: https://github.com/lamisChebbi/ng-realtime-dashboard/blob/54ae8b658ad49e1ebbe5fc295a3e991613a4b88d/src/app/services/data.service.ts

freshdev1

@freshdev1

Guys quick question... I think it will be easy for many, I am just new to this so struggling

 this.beds$ = this.wrcService.getWrcBedsByFilter(this.wrcId, workerTypeId, blockId, buildingId, floorId, ['100000002'])
    .pipe(
      tap(bedsResponse => console.log(bedsResponse)),
      filter(bedsResponse => bedsResponse != null),
      flatMap(beds => beds),
      map(bed => {
        bed.numberOfBedsArray = Array.from({length: bed.qty}, (v, i) => i + 1);
        return bed;
      }),
      mergeMap(beds => beds)
    );

here bedsResponse is of type Observable<Bed[]>

I am flatting it to each Bed object

Dorus

@Dorus

You can combine the last 2 calls, if you use mergeMap instead of map, you wont need the mergeMap after.

Or return Observablefrom()

freshdev1

@freshdev1

mapping it. and then want to again return Observable<Bed[]>

Dorus

@Dorus

btw flatmap and mergeMap are identical, it's strange to use both in the same code

mergeMap is the new one you should use, the other is just an allias

freshdev1

@freshdev1

Ok. I didn't know that

how to combine them all to Observable<Bed[]> again after transformation

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