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I have the following Observable:

dataService.ts

findMessages(chatItem: any): Observable<any[]> {
    return Observable.create((observer) => {
        this.firebaseDataService.findMessages(chatItem).subscribe((firebaseItems: any[]) => {
              // do something
                observer.next(somedata);
        });
    });
}

which calls this function:

firebaseDataService.ts

findMessages(chatItem: any): Observable<any[]> { // populates the firelist
    return this.af.database.list('/message/', {
        query: {
            orderByChild: 'negativtimestamp'
        }
    }).map(items => {
        const filtered = items.filter(
            item => ((item.memberId1 === chatItem.memberId1 && item.memberId2 === chatItem.memberId2)
                || (item.memberId1 === chatItem.memberId2 && item.memberId2 === chatItem.memberId1))
        );
        return filtered;
    });
}

The dataService.findMessages(chatItem) function is only ever called once.

This observes the firebase list. So if any items on the list change, this Observable is fired.

Problem

If the function is accessed, or one item is added to the list, the Observer is fired as expected, but the // do something line is called multiple times. I would only expect it to be called once.

Question

• Is there a way to enforce that it's only called once?
• Or once it's called for the first time, to break out of the
  subscribe?
• Or, is my code pletely wrong?

Any advise appreciated.

I have the following Observable:

dataService.ts

findMessages(chatItem: any): Observable<any[]> {
    return Observable.create((observer) => {
        this.firebaseDataService.findMessages(chatItem).subscribe((firebaseItems: any[]) => {
              // do something
                observer.next(somedata);
        });
    });
}

which calls this function:

firebaseDataService.ts

findMessages(chatItem: any): Observable<any[]> { // populates the firelist
    return this.af.database.list('/message/', {
        query: {
            orderByChild: 'negativtimestamp'
        }
    }).map(items => {
        const filtered = items.filter(
            item => ((item.memberId1 === chatItem.memberId1 && item.memberId2 === chatItem.memberId2)
                || (item.memberId1 === chatItem.memberId2 && item.memberId2 === chatItem.memberId1))
        );
        return filtered;
    });
}

The dataService.findMessages(chatItem) function is only ever called once.

This observes the firebase list. So if any items on the list change, this Observable is fired.

Problem

If the function is accessed, or one item is added to the list, the Observer is fired as expected, but the // do something line is called multiple times. I would only expect it to be called once.

Question

• Is there a way to enforce that it's only called once?
• Or once it's called for the first time, to break out of the
  subscribe?
• Or, is my code pletely wrong?

Any advise appreciated.

• javascript
• typescript
• ionic-framework
• rxjs
• observable

Share Improve this question Follow edited May 8, 2017 at 11:08 Richard asked May 8, 2017 at 9:55 RichardRichard 8,9653434 gold badges123123 silver badges255255 bronze badges 6

• stackoverflow./questions/43831736/… – martin Commented May 8, 2017 at 10:12
• Hi Martin, thanks for the ment. I do however get, [ts] Property 'multicast' does not exist on type 'Observable<any[]>'.. I may need to investigate a little further. – Richard Commented May 8, 2017 at 10:49
• 1 => resolved by: import 'rxjs/add/operator/multicast'; – Richard Commented May 8, 2017 at 10:55
• I am not sure how to construct the parameters for multicast(parameters). It looks like I need a Rx.Subject, but what is this? How do i get this? – Richard Commented May 8, 2017 at 11:04
• use take(1) if you need to have only one emition – Julia Passynkova Commented May 8, 2017 at 15:14

 |  Show 1 more ment

1 Answer 1

Sorted by: Reset to default 3

but the // do something line is called multiple times

It is called in the callback passed to this.firebaseDataService.findMessages(chatItem).subscribe. The subscribe function will fire whenever there is a new item. If you only want to take one item you can use the take operator

More

Docs on take : https://github./Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/take.md

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