"Enhancing HTTP Requests in Ionic: A Guide to Using Interceptors"

What is an HTTP Interceptor?

An HTTP Interceptor is a mechanism that allows you to modify HTTP requests and responses globally before they reach your app’s code or are returned to the server. This is useful for attaching authentication tokens, logging, handling errors, or caching responses without having to modify every request individually.

Why Use an Interceptor in Ionic?

Interceptors offer several advantages when used in an Ionic application:

• Global Request Modification: Add headers or parameters (e.g., Authorization token) to every outgoing HTTP request.

• Centralized Error Handling: Catch errors in all HTTP requests, allowing a unified error response strategy.

• Logging Requests and Responses: Useful for debugging purposes by logging details of HTTP requests and responses.

• Loading Indicator: Automatically show and hide loading spinners for all network requests.

• Caching: Cache certain responses to reduce unnecessary network calls.

Implementing an HTTP Interceptor in Ionic

Step 1: Create the Interceptor Service

ionic generate service services/http-interceptor

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class HttpInterceptorService implements HttpInterceptor {

 intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  // Clone the request to add the new header
  const clonedRequest = req.clone({
   headers: req.headers.set('Authorization', 'Bearer YOUR_TOKEN_HERE')
  });

  // Pass the cloned request instead of the original request to the next handle
  return next.handle(clonedRequest);
 }
}

Step 2: Register the Interceptor

Once you’ve created the interceptor service, you’ll need to register it in your app module. Open app.module.ts and add the interceptor to the providers array.

import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';
import { HttpInterceptorService } from './services/http-interceptor.service';

@NgModule({
 declarations: [...],
 imports: [
  HttpClientModule, // Make sure HttpClientModule is imported
  ...
 ],
 providers: [
  {
   provide: HTTP_INTERCEPTORS,
   useClass: HttpInterceptorService,
   multi: true // This is important to allow multiple interceptors
  }
 ],
 bootstrap: [AppComponent]
})
export class AppModule { }

Step 3: Handling Responses and Errors

The beauty of interceptors is that they work both ways – for outgoing requests and incoming responses. You can handle errors globally by modifying the intercept method in your service.

import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
 const clonedRequest = req.clone({
  headers: req.headers.set('Authorization', 'Bearer YOUR_TOKEN_HERE')
 });

 return next.handle(clonedRequest).pipe(
  catchError(error => {
   // Handle error here
   console.error('Error occurred:', error);
   return throwError(error);
  })
 );
}

Step 4: Displaying a Global Loader

import { Injectable } from '@angular/core';
import { LoadingController } from '@ionic/angular';

@Injectable({
 providedIn: 'root'
})
export class LoaderService {
 private loading: HTMLIonLoadingElement;

 constructor(private loadingController: LoadingController) {}

 async present() {
  this.loading = await this.loadingController.create({
   message: 'Loading...',
  });
  await this.loading.present();
 }

 async dismiss() {
  if (this.loading) {
   await this.loading.dismiss();
  }
 }
}

Use Cases for HTTP Interceptors in Ionic

1. Authentication: Automatically attach JWT tokens or API keys to each request.

2. Error Handling: Show a unified error message to the user when a network error occurs.

3. Logging: Log request/response details for debugging or monitoring.

4. Data Transformation: Modify the incoming data format to suit your application needs.

Best Practices for Using Interceptors

• Modularity: Keep your interceptor logic clean and modular by separating concerns such as error handling, authentication, and logging.

• Performance: Avoid making your interceptors too heavy by performing only necessary actions.

• Token Expiry: Handle token expiration by refreshing the token in the interceptor when you detect a 401 Unauthorized error.