How to Cancel Angular

To implement a full cancellation instruction for HTTP requests in an Angular application, you can use HTTP interceptors along with RxJS operators. Here’s a concise guide:

Overview

HTTP request cancellation in Angular is useful for optimizing performance by preventing unnecessary network traffic. This is particularly beneficial when users make multiple similar requests in a short timeframe or change their actions mid-request.

Implementation Steps

1. Create an Interceptor: Develop an HTTP interceptor that can cancel ongoing requests. This interceptor will use a cache to track and manage requests.

2. Use RxJS Operators: Utilize RxJS operators like takeUntil to subscribe to an observable that cancels the request when a new similar request is made.

3. Add to Providers: Include the interceptor in your application module’s providers.

4. Automate Cancellation: Use switchMap or similar operators to automatically cancel previous requests when a new one is initiated.

5. Component-Level Handling: Manage subscriptions at the component level to ensure cleanup when components are destroyed.

Example Code

typescript
import { Injectable } from ‘@angular/core’;
import { HttpRequest, HttpHandler, HttpEvent, HttpInterceptor } from ‘@angular/common/http’;
import { Observable, Subject } from ‘rxjs’;
import { tap, takeUntil } from ‘rxjs/operators’;

@Injectable()
export class CancelSameApisInterceptor implements HttpInterceptor {
private cache = new Map<string, Subject>();

intercept(request: HttpRequest, next: HttpHandler): Observable<HttpEvent> {
if (request.method !== ‘GET’) {
return next.handle(request);
}

const url = request.url;

const cachedResponse = this.cache.get(url);
if (cachedResponse) {
  cachedResponse.next();
}

const cancelRequests$ = new Subject<void>();
this.cache.set(url, cancelRequests$);

const newRequest = next.handle(request).pipe(
  takeUntil(cancelRequests$),
  tap((event) => {
    if (event instanceof HttpResponse) {
      this.cache.delete(url);
    }
  })
);

return newRequest;

}
}

Adding to Providers

In your app.module.ts, add the interceptor to the providers:

typescript
import { HTTP_INTERCEPTORS } from ‘@angular/common/http’;
import { CancelSameApisInterceptor } from ‘./cancel-same-apis.interceptor’;

@NgModule({
providers: [
{ provide: HTTP_INTERCEPTORS, useClass: CancelSameApisInterceptor, multi: true }
],
// Other configurations
})
export class AppModule {}

Component-Level Handling

For component-level handling, ensure that subscriptions are properly cleaned up when components are destroyed:

typescript
import { Component, OnDestroy } from ‘@angular/core’;
import { Subscription } from ‘rxjs’;

@Component({
selector: ‘app-example’,
templateUrl: ‘./example.component.html’
})
export class ExampleComponent implements OnDestroy {
private subscriptions: Subscription[] = [];

ngOnDestroy(): void {
this.subscriptions.forEach(sub => sub.unsubscribe());
}

// Example usage
fetchData() {
const subscription = this.http.get(‘https://example.com/data’).subscribe();
this.subscriptions.push(subscription);
}
}

This approach ensures efficient cancellation of repeated or unnecessary HTTP requests in Angular applications.