Chaining RxJS Observables from http data in Angular2 with TypeScript

Question

I'm currently trying to teach myself Angular2 and TypeScript after happily working with AngularJS 1.* for the last 4 years! I have to admit I am hating it but I am sure my eureka moment is just around the corner... anyway, I have written a service in my dummy app that will fetch http data from a phoney backend I wrote that serves JSON.

import {Injectable} from 'angular2/core';
import {Http, Headers, Response} from 'angular2/http';
import {Observable} from 'rxjs';

@Injectable()
export class UserData {

    constructor(public http: Http) {
    }

    getUserStatus(): any {
        var headers = new Headers();
        headers.append('Content-Type', 'application/json');
        return this.http.get('/restservice/userstatus', {headers: headers})
            .map((data: any) => data.json())
            .catch(this.handleError);
    }

    getUserInfo(): any {
        var headers = new Headers();
        headers.append('Content-Type', 'application/json');
        return this.http.get('/restservice/profile/info', {headers: headers})
            .map((data: any) => data.json())
            .catch(this.handleError);
    }

    getUserPhotos(myId): any {
        var headers = new Headers();
        headers.append('Content-Type', 'application/json');
        return this.http.get(`restservice/profile/pictures/overview/${ myId }`, {headers: headers})
            .map((data: any) => data.json())
            .catch(this.handleError);
    }

    private handleError(error: Response) {
        // just logging to the console for now...
        console.error(error);
        return Observable.throw(error.json().error || 'Server error');
    }   
}

Now in a Component I wish to run (or chain) both getUserInfo() and getUserPhotos(myId) methods. In AngularJS this was easy as in my controller I would do something like this to avoid the "Pyramid of doom"...

// Good old AngularJS 1.*
UserData.getUserInfo().then(function(resp) {
    return UserData.getUserPhotos(resp.UserId);
}).then(function (resp) {
    // do more stuff...
}); 

Now I have tried doing something similar in my component (replacing .then for .subscribe) however my error console going crazy!

@Component({
    selector: 'profile',
    template: require('app/components/profile/profile.html'),
    providers: [],
    directives: [],
    pipes: []
})
export class Profile implements OnInit {

    userPhotos: any;
    userInfo: any;

    // UserData is my service
    constructor(private userData: UserData) {
    }

    ngOnInit() {

        // I need to pass my own ID here...
        this.userData.getUserPhotos('123456') // ToDo: Get this from parent or UserData Service
            .subscribe(
            (data) => {
                this.userPhotos = data;
            }
        ).getUserInfo().subscribe(
            (data) => {
                this.userInfo = data;
            });
    }

}

I'm obviously doing something wrong... how would I best with Observables and RxJS? Sorry if I am asking stupid questions... but thanks for the help in advance! I have also noticed the repeated code in my functions when declaring my http headers...

Answer 1

For your use case, I think that the flatMap operator is what you need:

this.userData.getUserPhotos('123456').flatMap(data => {
  this.userPhotos = data;
  return this.userData.getUserInfo();
}).subscribe(data => {
  this.userInfo = data;
});

This way, you will execute the second request once the first one is received. The flatMap operator is particularly useful when you want to use the result of the previous request (previous event) to execute another one. Don't forget to import the operator to be able to use it:

import 'rxjs/add/operator/flatMap';

This answer could give you more details:

• Angular HTTP GET with TypeScript error http.get(...).map is not a function in [null]

If you want to only use subscribe method, you use something like that:

this.userData.getUserPhotos('123456')
    .subscribe(
      (data) => {
        this.userPhotos = data;

        this.userData.getUserInfo().subscribe(
          (data) => {
            this.userInfo = data;
          });
      });

To finish, if you would want to execute both requests in parallel and be notified when all results are then, you should consider to use Observable.forkJoin (you need to add import 'rxjs/add/observable/forkJoin'):

Observable.forkJoin([
  this.userData.getUserPhotos(),
  this.userData.getUserInfo()]).subscribe(t=> {
    var firstResult = t[0];
    var secondResult = t[1];
});

Answer 2

What you actually need is the switchMap operator. It takes the initial stream of data (User info) and when completed, replaces it with the images observable.

Here is how I'm understanding your flow:

• Get User info
• Get User Id from that info
• Use the User Id to Get User Photos

Here is a demo. NOTE: I mocked the service but the code will work with the real service.

  ngOnInit() {
    this.getPhotos().subscribe();
  }

  getUserInfo() {
    return this.userData.getUserInfo().pipe(
      tap(data => {
        this.userInfo = data;
      }))
  }
  getPhotos() {
    return this.getUserInfo().pipe(
      switchMap(data => {
        return this.userData.getUserPhotos(data.UserId).pipe(
          tap(data => {
            this.userPhotos = data;
          })
        );
      })
    );
  }