Guide to creating a series of HTTP requests in Angular 6 with the help of RxJS

Question

Guide to creating a series of HTTP requests in Angular 6 with the help of RxJS

After extensively searching the internet, I have yet to discover a suitable solution for my specific scenario. My current setup involves using the MEAN stack (Angular 6) and implementing a registration form. What I am seeking is a method to make successive HTTP calls to the API where each call relies on the response from the previous one. The structure I envision is as follows:

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 */ }
);

I am inquiring about which RxJS operators I should utilize to accomplish this task. While nesting multiple subscriptions is one approach, I am eager to explore more efficient options with RxJS. Additionally, considerations for error handling are crucial.

angular typescript rxjs angular-httpclient rxjs6

Answer 1

Answer №1

When making consecutive API calls that rely on previous results, it is recommended to utilize the concatMap function.

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 asynchronous method does not rely on the return value of the previous async call, you can use

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

Answer 2

When making consecutive API calls that rely on previous results, it is recommended to utilize the concatMap function.

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 asynchronous method does not rely on the return value of the previous async call, you can use

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

Answer 3

Answer №2

Dealing with a similar issue, I came up with a solution that involves using the pipe and concatMap functions to extract sequential data from an array within a specified time frame.

This method can be applied in situations where multiple requests need to be processed.

Feel free to utilize this approach if it aligns with your requirements.

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

Answer 4

Dealing with a similar issue, I came up with a solution that involves using the pipe and concatMap functions to extract sequential data from an array within a specified time frame.

This method can be applied in situations where multiple requests need to be processed.

Feel free to utilize this approach if it aligns with your requirements.

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

Answer 5

Answer №3

`MergeMap`

is the perfect solution you've been seeking

Calling multiple APIs in sequence can be achieved using mergeMap in RxJS:

firstPOSTCallToAPI('url', data).pipe(
    mergeMap(result1 => secondPOSTCallToAPI('url', result1)),
    mergeMap(result2 => thirdPOSTCallToAPI('url', result2)),
    mergeMap(result3 => fourthPOSTCallToAPI('url', result3)),
    // ...
).subscribe(
    success => { 
      // You will receive the response from the last request (fourthPOSTCallToAPI) here
    },
    errorData => { /* Display error message */ }
);


// Assuming that
// secondPOSTCallToAPI, thirdPOSTCallToAPI and fourthPOSTCallToAPI
// return observables eg. this.http.get<Book>(apiUrl);

Answer 6

`MergeMap`

is the perfect solution you've been seeking

Calling multiple APIs in sequence can be achieved using mergeMap in RxJS:

firstPOSTCallToAPI('url', data).pipe(
    mergeMap(result1 => secondPOSTCallToAPI('url', result1)),
    mergeMap(result2 => thirdPOSTCallToAPI('url', result2)),
    mergeMap(result3 => fourthPOSTCallToAPI('url', result3)),
    // ...
).subscribe(
    success => { 
      // You will receive the response from the last request (fourthPOSTCallToAPI) here
    },
    errorData => { /* Display error message */ }
);


// Assuming that
// secondPOSTCallToAPI, thirdPOSTCallToAPI and fourthPOSTCallToAPI
// return observables eg. this.http.get<Book>(apiUrl);

Answer 7

Answer №4

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/forkJoin';

@Injectable()
export class DataService {

  constructor(private http: HttpClient) { }

  public makeMultipleRequests(): Observable<any[]> {
    let response1 = this.http.get(requestUrl1);
    let response2 = this.http.get(requestUrl2);
    let response3 = this.http.get(requestUrl3);
    return Observable.forkJoin([response1, response2, response3]);
  }
}

The example above demonstrates how to perform multiple HTTP requests, and this can be extended to handle more requests if needed.

import { Component, OnInit } from '@angular/core';
import { DataService } from "../data.service";

@Component({
    selector: 'app-page',
    templateUrl: './page.component.html',
    styleUrls: ['./page.component.css']
})
export class DemoComponent implements OnInit {
    public responseData1: any;
    public responseData2: any;
    public responseData3: any;

    constructor(private dataService: DataService) {}

    ngOnInit() {
        this.dataService.makeMultipleRequests().subscribe(responseList => {
            this.responseData1 = responseList[0];
            this.responseData2 = responseList[1];
            this.responseData3 = responseList[1];
        });
    }
}

Answer 8

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/forkJoin';

@Injectable()
export class DataService {

  constructor(private http: HttpClient) { }

  public makeMultipleRequests(): Observable<any[]> {
    let response1 = this.http.get(requestUrl1);
    let response2 = this.http.get(requestUrl2);
    let response3 = this.http.get(requestUrl3);
    return Observable.forkJoin([response1, response2, response3]);
  }
}

The example above demonstrates how to perform multiple HTTP requests, and this can be extended to handle more requests if needed.

import { Component, OnInit } from '@angular/core';
import { DataService } from "../data.service";

@Component({
    selector: 'app-page',
    templateUrl: './page.component.html',
    styleUrls: ['./page.component.css']
})
export class DemoComponent implements OnInit {
    public responseData1: any;
    public responseData2: any;
    public responseData3: any;

    constructor(private dataService: DataService) {}

    ngOnInit() {
        this.dataService.makeMultipleRequests().subscribe(responseList => {
            this.responseData1 = responseList[0];
            this.responseData2 = responseList[1];
            this.responseData3 = responseList[1];
        });
    }
}

Answer 9

Answer №5

If you have a bunch of emails that need to be sent out sequentially, let me walk you through the process:

sendEmails() {
  this.isLoading = true; 
            const calls = this.emails <<-- assuming this is an array of emails
            .map(email => this.userEmailService.deliver({email: email, userId: 1242}));
            from(calls) <<-- using the from operator.
                .pipe(
                    concatMap(res => res),
                    finalize(() => this.isLoading = false)
                ).subscribe(() => { });
}

I trust you find this explanation helpful.

Answer 10

If you have a bunch of emails that need to be sent out sequentially, let me walk you through the process:

sendEmails() {
  this.isLoading = true; 
            const calls = this.emails <<-- assuming this is an array of emails
            .map(email => this.userEmailService.deliver({email: email, userId: 1242}));
            from(calls) <<-- using the from operator.
                .pipe(
                    concatMap(res => res),
                    finalize(() => this.isLoading = false)
                ).subscribe(() => { });
}

I trust you find this explanation helpful.

Answer 11

Answer №6

If you're looking to call multiple APIs simultaneously, Angular has a useful feature called forkJoin().

forkJoin()

When you use forkJoin(), the data from each API call will be returned in an array in the same order as they were called.

For example:

forkJoin(request1, request2)
    .subscribe(([response1, response2]) => {

If you'd like to learn more about forkJoin, you can visit this link.

I've also provided another helpful answer on Stack Overflow. Feel free to check it out as it may provide additional assistance.

Answer 12

If you're looking to call multiple APIs simultaneously, Angular has a useful feature called forkJoin().

forkJoin()

When you use forkJoin(), the data from each API call will be returned in an array in the same order as they were called.

For example:

forkJoin(request1, request2)
    .subscribe(([response1, response2]) => {

If you'd like to learn more about forkJoin, you can visit this link.

I've also provided another helpful answer on Stack Overflow. Feel free to check it out as it may provide additional assistance.

Guide to creating a series of HTTP requests in Angular 6 with the help of RxJS

Answer №1

Answer №2

Answer №3

`MergeMap`

Answer №4

Answer №5

Answer №6

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