Wait for observable in for loop to finish before continuing loop in Angular 5

Question

I'm looping through an array of objects (called projects). The forEach loop contains a service call that returns an observable. I'm trying to wait to process the next project in the array until the observable within the loop completes. What should I use? I tried forkJoin already.

projects .forEach(project => { this.imageService.getProjectImages(project.projectId.toString(), true, true, undefined) .catch(err => observer.error(err)) .finally(() => { // process next project }) .subscribe((image: FileRepresentation) => { data.image = image; this.getSlide(project, data); }); })

forkJoin only works for an array of observables. I have an array of objects. — depiction
– depiction, Commented Feb 3, 2018 at 5:56
use map: projects.map(project => this.imageService.getProjectImages(project.projectId.ToString()...)) will return array of observables. — Harry Ninh
– Harry Ninh, Commented Feb 3, 2018 at 6:27

martin · Accepted Answer · 2018-02-03 08:36:06Z

If you want to run one Observable at the time and only start the next one after the previous one completed then forkJoin is not a good choice because it subscribes to all source Observables right away. A better approach is using so called higher-order Observable and subscribe to one after another with concatAll:

const projects = [ Observable.of(1).delay(1000), Observable.of(2).delay(1000), Observable.of(3).delay(1000), ]; Observable.from(projects) .concatAll() .subscribe(console.log);

This simulates the HTTP call by making an Observable with 1s delay. If you run this example you'll see that it prints each number with 1s delay:

See live demo: http://jsbin.com/zocuma/3/edit?js,console

How would I do this with an existing projects array containing objects?

depiction · Accepted Answer · 2018-02-05 17:32:53Z

I eventually figured out a solution. The key is to use a second function that is called recursively.

Pass all projects and the first project's index to getImagesForProject. Once all images have been received for the first project, check to see if the imageCount is less than maxImages. If yes, call getImagesForProject recursively until the limit is reached.

this.getImagesForProject(projects, 0, 5); getImagesForProject(projects: Project[], index: number, maxImages: number = 5, imageCount?: number) { this.imageService.getProjectImages(projects[index].projectId.toString(), true, true, undefined) .finally(() => { if(imageCount < maxImages) { this.getImagesForProject(projects, data, (index + 1), imageCount); } }) .subscribe(image => { imageCount++; data.image = image; this.getSlide(projects[index], data); }); }

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