angular 5 - loop of observers inside an other observer - efficient way to do it

Question

I have a 'get' call that returns a list of users that contains a user id and a name. After the call succeeded, I need to call another rest for each user to bring his picture.

Currently I am doing this in this way-

component-

users;
userPhotos = {};

constructor(private dashboardService: DashboardService) {
  this.dashboardService.bringUsers().subscribe((data) =>{
    this.users = data;
    this.users.forEach((user) => {
      this.dashboardService.getPicture(user.userID).subscribe((data) => {
        if (data){
          this.userPhotos[user.userID] = window.URL.createObjectURL(data);
        } else {
          this.userPhotos[user.userID] = '';
        }
      });
    });
  });
}

In my html-

<ng-container *ngFor="let user of users">
  <div>
      {{user.displayName}}
  </div>

  <display-picture [image]="userPhotos[user.userID]"></display-picture>
</ng-container>

Where display-picture is a component that just presents the image.

Is there any efficient way using rxjs to do it?

Answer 1

from to iterate each user and mergeScan can be used here to execute the observables. For clarity i break it into two functions. You can also control the number of concurrency with mergeScan

    const getUserPic=users=>from(users).pipe(
        mergeScan((acc, curr:any) =>
            getPicture(curr.userID).pipe(map(photo => {
                acc[curr.userID] = photo
                return acc
            }))
          ,{})
        ) 

bringUsers().pipe(
    tap(users=>this.users=users),
    switchMap(users=> getUserPic(users)),
    last()
).subscribe(console.log)

https://stackblitz.com/edit/rxjs-1bt172

Answer 2

You can use a forkJoin() to merge together all the final emitted values from multiple observables. RxJS 6 supports using an object map of key/value pairs to define which observables, but you'll have to use an array for older versions.

this.dashboardService.bringUsers().pipe(
   switchMap(users => forkJoin(
       users.reduce((acc, user) => (acc[user.userId] = this.dashboardService.getPicture(user.userID), acc), {}
   ))),
).subscribe((users: {[userId: number]: any}) => {
   Object.entries(users).forEach(([userId, data]:[number,any]) => {
      this.userPhotos[user.userID] = data ? window.URL.createObjectURL(data) : '';
   });
});