Observable subscribe called multiple times?

Question

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 completely wrong?

Any advise appreciated.

http://stackoverflow.com/questions/43831736/rxjs-execute-the-handler-only-once — martin, May 08 '17 at 10:12
Hi Martin, thanks for the comment. I do however get, `[ts] Property 'multicast' does not exist on type 'Observable'.`. I may need to investigate a little further. — Richard, May 08 '17 at 10:49
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, May 08 '17 at 11:04
Hi Julia, thank you for your comment. Where do I use `take(1)`? On the `Observable`? — Richard, May 08 '17 at 17:31

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

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.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/take.md

edited Jun 20 '20 at 09:12

Community

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answered May 09 '17 at 01:43

basarat

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Observable subscribe called multiple times?

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