What is the process for unsubscribing through an HTTP post request within an Angular application

Question

What is the process for unsubscribing through an HTTP post request within an Angular application

There is a POST API set up through API Gateway on AWS, but it seems to be returning an infinite loop of arrays, as shown in the image below. How can I make it return just one array?

Multiple requests

Below is the code snippet:

Angular

import { Component, OnInit, ViewChild, OnChanges, OnDestroy } from '@angular/core';
import { HttpClient } from '@common/http'; 
import { GetService } from '../get.service';
import { Observable, Subject } from 'rxjs';
import { IonSlides, IonSlide } from '@ionic/angular';

@Component({
  selector: 'app-home',
  templateUrl: 'home.page.html',
  styleUrls: ['home.page.scss'],
})
export class HomePage implements OnInit   {
  protected ngUnsubscribe: Subject<> = new Subject<>();

constructor(public http:HttpClient) {}
  @ViewChild(IonSlides,{'static':false}) slides: IonSlides;

  slideOpts = {
    initialSlide: 1,
    speed: 400
 };
public result:any
public res:any
data:Observable<>;
ata:Observable<>;

ngOnInit(){
}

  getdata(){
    this.ata=this.http.post("XXXXXXXXXXXXXXXXXXXXXXXXX",{"freq":1});
    this.ata.subscribe(data=>{console.log(data)})

    return this.res
}

INDEX:any

slideChanged() {
    this.INDEX =this.slides.getActiveIndex().then(data=>console.log(data))
    return this.INDEX
}

HTML

<ion-header>
  <ion-toolbar>
    <ion-title>
      Ionic Blank
    </ion-title>
  </ion-toolbar>
</ion-header>
<p *ngFor="let a of getdata()">{{a}}</p>
<ion-card>
  <ion-card-content>
  <ion-slides  [options]="slideOpts" (ionSlideWillChange)="slideChanged()">
    <ion-slide>
        <h1></h1>
    </ion-slide>
  </ion-slides>
</ion-card-content>
</ion-card>

angular typescript api subscription unsubscribe

Answer 1

Answer №1

It is clear that the issue does not involve the API. This can be confirmed as you are utilizing Angular's HttpClient, which requires additional setup to handle multiple requests. The problem arises from your page being rendered multiple times while Angular's renderer is managing the layout.

The root of the problem can be traced back to this line of code:

<p *ngFor="let a of getdata()">{{a}}</p >

Each time this line is rendered, Angular invokes the getdata() function, triggering another request.

To resolve this issue, it's crucial to separate the result list from the function that requests it. Based on your code, it appears that data is an Observable. To address this, consider implementing the following:

<p *ngFor="let a of data | async">{{a}}</p>

Make sure to utilize the async pipe as shown above.

Refer to Angular's documentation

The async pipe subscribes to an Observable or Promise, returning the most recent value emitted. Upon receiving a new value, the async pipe triggers a check for changes in the component. When the component is destroyed, the async pipe unsubscribes automatically to prevent memory leaks.

Remember to call getdata() in an appropriate location, typically within the ngOnInit() method.

Additional tip: Managing responses from Observable subscriptions

If there is a need to receive only one response from an Observable subscription, you can achieve this by using the take operator.

import { take } from 'rxjs/operators';

[...]

someService.getObservable(...)
           .pipe( take(1) ) // <-- take first response
           .subscribe( (r) => { ... } );

Answer 2

It is clear that the issue does not involve the API. This can be confirmed as you are utilizing Angular's HttpClient, which requires additional setup to handle multiple requests. The problem arises from your page being rendered multiple times while Angular's renderer is managing the layout.

The root of the problem can be traced back to this line of code:

<p *ngFor="let a of getdata()">{{a}}</p >

Each time this line is rendered, Angular invokes the getdata() function, triggering another request.

To resolve this issue, it's crucial to separate the result list from the function that requests it. Based on your code, it appears that data is an Observable. To address this, consider implementing the following:

<p *ngFor="let a of data | async">{{a}}</p>

Make sure to utilize the async pipe as shown above.

Refer to Angular's documentation

The async pipe subscribes to an Observable or Promise, returning the most recent value emitted. Upon receiving a new value, the async pipe triggers a check for changes in the component. When the component is destroyed, the async pipe unsubscribes automatically to prevent memory leaks.

Remember to call getdata() in an appropriate location, typically within the ngOnInit() method.

Additional tip: Managing responses from Observable subscriptions

If there is a need to receive only one response from an Observable subscription, you can achieve this by using the take operator.

import { take } from 'rxjs/operators';

[...]

someService.getObservable(...)
           .pipe( take(1) ) // <-- take first response
           .subscribe( (r) => { ... } );

What is the process for unsubscribing through an HTTP post request within an Angular application

Answer №1

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