Angular HttpClient Architecture and Best Practices
Angular is a popular JavaScript front-end framework used for building web applications. By default, Angular provides an HttpClient module that allows developers to make HTTP requests to the server, fetch data, and send data back to the server. In this article, we will be discussing the architecture of Angular HttpClient module and best practices to follow while using it.
What is HttpClient?
HttpClient is an Angular module that allows you to make HTTP requests to the server via a simple API. The HttpClient module provides methods for performing GET, POST, PUT, DELETE requests and more. The Angular team made it available as a replacement for the deprecated Http module that was available in Angular versions 2-4.
Architecture of HttpClient
The architecture of HttpClient can be broken down into two main parts:
-
- The `@angular/common/http` namespace:
import { HttpClient } from '@angular/common/http';
- The `HttpClient` class:
constructor(private http: HttpClient) { }
The @angular/common/http
namespace includes all the classes required for making HTTP requests, such as HttpClient, HttpHeaders, HttpParams, etc. The HttpClient
class provides methods for performing the actual HTTP request, as well as handling the response.
Importing HttpClient
To import HttpClient into an Angular component, you can use the following code:
import { HttpClient } from '@angular/common/http';
@Injectable({
providedIn: 'root'
})
export class MyService {
constructor(private http: HttpClient) { }
getData() {
return this.http.get('http://example.com/data');
}
}
In the code above, we import the HttpClient
class from the @angular/common/http
namespace, and then inject it into our MyService
class constructor. This allows us to use the HttpClient
to make an HTTP GET request to http://example.com/data
and fetch the data.
Using HttpHeaders with HttpClient
While making HTTP requests, you can also include headers in your requests. The HttpHeaders
class allows you to create and set headers for HTTP requests:
import { HttpClient, HttpHeaders } from '@angular/common/http';
@Injectable({
providedIn: 'root'
})
export class MyService {
constructor(private http: HttpClient) { }
postData(data: any) {
const headers = new HttpHeaders({
'Content-Type': 'application/json'
});
return this.http.post('http://example.com/data', data, { headers });
}
}
The code above includes the HttpHeaders
class and creates a new header with a Content-Type
of application/json
. This header is then set in the post
request options and sent with the data to http://example.com/data
.
Best Practices for using HttpClient
Now that you know the basics of the HttpClient
module, let's take a look at some best practices that should be followed when using it:
Always Use Observable object
The HttpClient
module returns an Observable
object by default. Observables can be subscribed to from the component, and can provide multiple values over time, unlike Promises that retrive a single value. This provides more flexibility for resolving values over an extended period of time. For exmaple:
export class MyComponent implements OnInit {
items$: Observable<any[]>;
constructor(private http: HttpClient) { }
ngOnInit() {
this.items$ = this.http.get<any[]>('http://example.com/data');
}
}
In the code above, we use the Observable
object returned by the HttpClient
module and assign it to a property called items$
, which can be subscribed to from the component. This provides more flexibility over resolving values over an extended period of time.
Handle Errors
When making HTTP requests, it is important to handle errors that may occur. This can be done using the catchError
operator:
import { HttpClient } from '@angular/common/http';
import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class MyService {
constructor(private http: HttpClient) { }
getData() {
return this.http.get('http://example.com/data').pipe(
catchError(err => {
console.error(err);
return throwError(err);
})
);
}
}
In the code above, we use the catchError
operator provided by the RxJS library to catch any error that may occur when making the HTTP request. We then log the error to the console and return it using the throwError
operator.
Implement a caching strategy
When making repeated HTTP requests, it can be beneficial to implement a caching strategy. This can help reduce the number of requests made to the server and improve the performance of our application.
One way to implement caching is to use Angular's built-in cacheInterceptor
. This can be done by creating a new HttpCacheInterceptor
class:
import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpResponse, HttpHandler, HttpEvent } from '@angular/common/http';
import { Observable, of } from 'rxjs';
import { tap } from 'rxjs/operators';
@Injectable()
export class HttpCacheInterceptor implements HttpInterceptor {
private cache = new Map<string, HttpResponse<any>>();
intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
// try to retrieve a cached response
const cachedResponse = this.cache.get(req.url);
// return cached response
if (cachedResponse) {
return of(cachedResponse);
}
// send request to server and cache response
return next.handle(req).pipe(
tap(event => {
if (event instanceof HttpResponse) {
this.cache.set(req.url, event);
}
})
);
}
}
In the code above, we create a new HttpCacheInterceptor
class that intercepts HTTP requests and responses. The cache
variable is a Map
object that stores cached responses. The intercept
method first checks whether a cached response exists for the requested URL. If it exists, it returns the cached response, otherwise it sends the request to the server, caches the response, and returns it.
Testing HttpClient requests
When testing components that make HTTP requests using the HttpClient
module, it is important to mock the HTTP requests. This can be done by providing a mock implementation of the HttpClient
:
describe('MyComponent', () => {
let component: MyComponent;
let fixture: ComponentFixture<MyComponent>;
let httpMock: HttpTestingController;
beforeEach(async(() => {
TestBed.configureTestingModule({
imports: [HttpClientTestingModule],
declarations: [MyComponent]
})
.compileComponents();
}));
beforeEach(() => {
fixture = TestBed.createComponent(MyComponent);
component = fixture.componentInstance;
httpMock = TestBed.inject(HttpTestingController);
});
afterEach(() => {
httpMock.verify();
});
it('should get data', () => {
const mockData = [
{ id: 1, name: 'John' },
{ id: 2, name: 'Doe' }
];
component.getData();
const req = httpMock.expectOne('http://example.com/data');
req.flush(mockData);
expect(component.items).toEqual(mockData);
});
});
In the code above, we create a new test for the MyComponent
component. We first import the HttpClientTestingModule
into the test module, and then create a mock implementation of the HttpClient
using the HttpTestingController
. We then create a new test that checks whether the getData
method fetches data from the server and parses it correctly.
Conclusion
The Angular HttpClient
module provides a simple and powerful API for making HTTP requests in your Angular application. With the information we have discussed in this article, you should now be able to use the HttpClient
module with confidence and follow the best practices to make your application more scalable and maintainable.
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