How to make a sequence of http requests in Angular 6 using RxJS

Question

I've been looking for a solution all over the web, but couldn't find anything that fits my user case. I'm using the MEAN stack (Angular 6) and I have a registration form. I'm looking for a way to execute multiple HTTP calls to the API and each one is dependent on the returned result from the previous one. I need something that looks like this:

firstPOSTCallToAPI('url', data).pipe(
    result1 => secondPOSTCallToAPI('url', result1)
    result2 => thirdPOSTCallToAPI('url', result2)
    result3 => fourthPOSTCallToAPI('url', result3)
    ....
).subscribe(
    success => { /* display success msg */ },
    errorData => { /* display error msg */ }
);

What combination of RxJS operators do I need to use to achieve this? One possible solution would be to nest multiple subscriptions, but I want to avoid that and do it better with RxJS. Also need to think about error handling.

Solution 1

For calls that depend on previous result you should use concatMap

firstPOSTCallToAPI('url', data).pipe(
    concatMap(result1 => secondPOSTCallToAPI('url', result1))
      concatMap( result2 => thirdPOSTCallToAPI('url', result2))
       concatMap(result3 => fourthPOSTCallToAPI('url', result3))
    ....
).subscribe(
    success => { /* display success msg */ },
    errorData => { /* display error msg */ }
);

if your async method does not depend on return value of the precedent async call you can use

   concat(method(),method2(),method3()).subscribe(console.log)

Solution 2

I have faced same problem, this is my solution use pipe and concatMap for array for get sequence data for time period between start and end time.

This is general solution when we have array request.

I share for whom concern.

 let currentReplayData = [];
 let timerange = [[t1, t2], [t3, t4]]; // array of start and end time
 from(timerange).pipe(
      concatMap(time => <Observable<any>>this.dataService.getData(time[0],time[1]))
      ).subscribe(val => {
        //console.log(val)
        this.currentReplayData = this.currentReplayData.concat(val);
      });

Solution 3

MergeMap

is exact what you are looking for

firstPOSTCallToAPI('url', data).pipe(
    mergeMap(result1 => secondPOSTCallToAPI('url', result1)),
    mergeMap(result2 => thirdPOSTCallToAPI('url', result2)),
    mergeMap(result3 => fourthPOSTCallToAPI('url', result3)),
    // ...
).subscribe(
    success => { 
      // here you will get response of LAST request (fourthPOSTCallToAPI)
    },
    errorData => { /* display error msg */ }
);


// I assume that
// secondPOSTCallToAPI, thirdPOSTCallToAPI and fourthPOSTCallToAPI
// returns obserwable eg. return this.http.get<Book>(apiUrl);

Comments

[mchl18]

forkJoin sends all at once AFAIK, using concat ensures the previous one completes

[Andi Aleksandrov]

Thanks for the reply. I've looked into forkJoin but it doesn't look like a working solution since i need request2 to use response data returned from request1.

[Sachin Shah]

In that case , forkJoin will not use. I suggest to check second link. It will work in you case.

[Andi Aleksandrov]

Exactly what I was looking for!!! Thank you very much, you have ended my suffering!

[Drenai]

In the case above concatMap is not doing what you think it is. You could just have easily used a mergeMap here

[Drenai]

@FanCheung It works fine, but if you replaced the concatMap in your example with a mergeMap or switchMap it will still work. The wait until complete aspect of concatMap is only apparent if you are piping an observable that is streaming multiple data, whereas the firstPOSTCallToAPI only streams a single event, as does the second, third and fourth call

[Alan Smith]

You beautiful human!! Got me out of a bind here. Thanks!

[macawm]

I found this very helpful. blog.angular-university.io/rxjs-higher-order-mapping