Angular2 - How to chain async service calls (http requests) in a component?

Question

I have a component which first need to call a service that POST something. Then in the same component I want to wait until the POST is done, to call another service which GETs data.

How can I make the GET call wait for the POST call to finish?

In new-version.component.ts:

private createNewVersion(value) {
    ...

    // create new version, then call on all available versions

    // POST call
    this._newVersionService.createNewVersion(vnr);

    // GET call
    this._versionService.getAvailableVersions(); 

    ...
}

In new-version.service.ts:

export class NewVersionService {

response$: Subject<any>;

constructor(private _http: Http) {
    this.response$ = new BehaviorSubject<any>(null);
 }

public createNewVersion(versionNr) {    
    this._http.post('http://localhost:8080/services/' + versionNr, null, {
        method: 'POST',
    })
    .subscribe(response => {
        this.response$.next(response.status);
    },
    error => console.error(error));
}

Thanks!

Answer 1

When a call returns a Promise chain the calls with

someFunction() {
  return returnsPromise()
    .then(result => doSomethingNext())
    .then(result => doSomethingAfterThat());
}

Ensure you have a return that returns the Promise of that chain so the caller of someFunc() also has a chance to time additional work to execute after doSomethingAfterThat() is completed.

When a call returns an Observable then use the complete callback

someFunction() {
  return returnsObservable()
    .subscribe(
      event => doForEachEvent(),
      error => handleError(),
      () => doSomethingNext()
          .then(result => doSomethingAfterThat());
}

doSomethingNext() is executed after the last event and doSomethingAfterThat() is again chained with then() to show how to mix observable and promise. doSomething().

Answer 2

You should be able to concat to achieve sequence, and reduce to collect the emitted values:

var a = this._newVersionService.createNewVersion(vnr);
var b = this._versionService.getAvailableVersions(); 

Rx.Observable.concat(a, b).reduce((acc:Array<any>, x:any) => {
    acc.push(x); return acc;
}, []).subscribe(t=> { 
      var firstEmitted = t[0];
      var secondEmitted = t[1];
});