Handle multiple http requests and wait for all to complete using Angular RxJs Observables

Question

I am using Observables, I have a use case where:

1. I have to get "pages" from service which gives me Page[]
2. For each of those "pages" I have to get the "sections" from other service
3. Also on subscribe of all those "sections" I have to do some action based on the data returned.

The concern is once all of those requests are completed then have to do some action.

Answer 1

you need use mergeMap and forkJoin

When using forkJoin you can fire parallel request at the same time and it will wait till all requests are completed.

this.http.get('/api/pages/')
    .map((res: any) => res.json())
    .mergeMap((pages: any[]) => {
      if (pages.length > 0) {
        return Observable.forkJoin(
          pages.map((page: any) => {
            return this.http.get('/api/sections/' + page_id)
              .map((res: any) => {
                let section: any = res.json();
                // do your operation and return
                return section;
              });
          });
        );
      }
      return Observable.of([]);
    });
}

Answer 2

In general when nesting async calls try to stay as flat as possible. Otherwise, you're creating callback hell which make things even more complicated than just with regular iterative code:

this.http.get('/api/pages/')
  .map((res: Response) => res.json())
  .concatMap((pages: Page[]) => {
    const observables = pages.map(p => this.http.get('whatever' + p.id));

    return Observable.forkJoin(observables, (...results) => 
      results.map((result, i) => {
        pages[i].sections = result;
        return pages[i];
      })
    )
  })
  .subscribe(pagesWithSections => ...);

The forkJoin() operator takes as argument a selector function that lets you work with the results of all the source Observables. I'm using it to update the original pages array and return it.