rxjs: Chaining Async Requests

Question

I'm new to rxjs, and I need to call 3 async requests, which started out as promises, and put the reurned objects into an array in the order in which they were called. Here's what a I have (partly based on this question):

let observeFront = Observable.fromPromise(this.requestAsync(frontSlice, "front"));
let observeMiddle = Observable.fromPromise(this.requestAsync(midSlice, "middle"));
let observeEnd = Observable.fromPromise(this.requestAsync(endSlice, "end"));

let observeCombined = Observable.concat(observeFront, observeMiddle, observeEnd);

observeCombined.subscribe(data => {
  // this logs only one object - need all 3 in proper order
  console.log(data);
  this.resultsEmitter.emit(data)
});

observeFront.subscribe(response => {
  // logs the correct object
  console.log("front result:", response);
});

observeMiddle.subscribe(response => {
  // correct
  console.log("middle result:", response);
});

observeEnd.subscribe(response => {
  // correct
  console.log("end result:", response);
});

How can I emit the result only when I have [frontOb, middleObj, endObj] ?

See http://stackoverflow.com/questions/35247310/angular-2-promise-all-with-rxjs/35247372#35247372 — Eric Martinez, Feb 28 '16 at 13:40

score 1 · Accepted Answer · answered Feb 28 '16 at 14:37

As @Eric suggested you can use forkJoin() instead of concat():

let observeFront = Observable.fromPromise(this.requestAsync(frontSlice, "front"));
let observeMiddle = Observable.fromPromise(this.requestAsync(midSlice, "middle"));
let observeEnd = Observable.fromPromise(this.requestAsync(endSlice, "end"));

let observeJoined = Observable.forkJoin(observeFront, observeMiddle, observeEnd);

observeJoined.subscribe(data => {
  console.log(data); // => [frontOb, middleObj, endObj]
  this.resultsEmitter.emit(data)
});

rxjs: Chaining Async Requests

1 Answers1