RxJava: how to compose multiple Observables with dependencies and collect all results at the end?

Question

I\'m learning RxJava and, as my first experiment, trying to rewrite the code in the first run() method in this code (cited on Netflix\'s blog as a problem RxJava ca

Answer 1

I think what you are looking for is switchmap. We ran into a similar issue where we have a session service that handles getting a new session from an api, and we need that session before we can get more data. We can add to the session observable that returns the sessionToken for use in our data call.

getSession returns an observable;

public getSession(): Observable{
  if (this.sessionToken)
    return Observable.of(this.sessionToken);
  else if(this.sessionObservable)
    return this.sessionObservable;
  else {
    // simulate http call 
    this.sessionObservable = Observable.of(this.sessonTokenResponse)
    .map(res => {
      this.sessionObservable = null;
      return res.headers["X-Session-Token"];
    })
    .delay(500)
    .share();
    return this.sessionObservable;
  }
}

and getData takes that observable and appends to it.

public getData() {
  if (this.dataObservable)
    return this.dataObservable;
  else {
    this.dataObservable = this.sessionService.getSession()
      .switchMap((sessionToken:string, index:number) =>{
        //simulate data http call that needed sessionToken
          return Observable.of(this.dataResponse)
          .map(res => {
            this.dataObservable = null;
            return res.body;
          })
          .delay(1200)
        })
        .map ( data => {
          return data;
        })
        .catch(err => {
          console.log("err in data service", err);
         // return err;
        })
        .share();
    return this.dataObservable;
  }
}

You will still need a flatmap to combine the not dependent observables.

Plunkr: http://plnkr.co/edit/hiA1jP?p=info

Where I got the idea to use switch map: http://blog.thoughtram.io/angular/2016/01/06/taking-advantage-of-observables-in-angular2.html