handling observables when one is dependent on data from another

Question

When one observable runs it is depedent on data which comes from another obseravble and I can't work out how to hanlde this dependency correctly.

One observable gets data from Firebase and by subscribing it creates a simple number array called novelsRead: Array

The other observable gets a response from an api and by subscribing it is meant to filter out all the records who's ids are present in novelsRead[].

The issue is, when a response comes from the api, novelsRead[] is still empty because Firebase hasn't responded yet so nothing will be filtered from the api response.

Code below:

export class HomePage {

currentnovels: any;
novels: any;
unreadnovels: any;
nextnovels: any;
novel: any;
resultsPageNumber: number = 1;
novelFullPosterPath: any;
novelsread: Array<number> = [];

private basePosterUrlMedium = 'http://image.novels.org/t/p/w500';
private basePosterUrlSmall = 'http://image.novels.org/t/p/w185';

constructor(private http: Http,
    private novelsApi: NovelsApiService,
    private dataService: DataService,
) {
    //this takes data from Firebase and pushes it to simple array of ids (numbers)
    this.dataService.list('novels-read')
        .subscribe(data => {
            data.map(results => {
                this.novelsread.push(results.novelsId);
            })
        })
}


ngAfterViewInit() {
    this.novelsApi.getnovelsByPage(this.resultsPageNumber)
    .subscribe(data => {
        this.novels = data.results;
        this.novels.map(data => {

            data.full_poster_path = this.basePosterUrlMedium + data.poster_path;
            return data;
        })
            .filter(data => {
                let found = this.novelsread.indexOf(data.id);
                //It seems when the api responds, this.novelsRead is still empty [] because Firebase has not responded yet
                console.log("this novelsread[0] is ", this.novelsread[0]);
                console.log("data.id found is ", found);
                return data;
            })
    })
}

I am looking for the cleanest solution whether that's using events, or observable chains or any other suggestions like moving functions from the constructor to ngAfterViewInit and vice versa.

I did look at code examples for combining observables using combineLatest but found the syntax very convoluted and wondered if there is a cleaner way of achiveing what I need, even if that involves waiting for events.

Answer 1

Using forkJoin should help you. It allows to handle result only when both requests complete: https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/forkjoin.md

OR if your 2nd request depends on 1st response - use switchMap

const request1$ = Rx.Observable.of('response1').delay(2000);
const request2$ = Rx.Observable.of('response2').delay(100);

Rx.Observable.forkJoin(request1$, request2$)
  .subscribe(res => console.log(`forkJoin: ${res}`));

request1$.switchMap(res1 => {
  console.log(`switchMap: ${res1}`);
  return request2$;
}).subscribe(res2 => console.log(`switchMap: ${res2}`));

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