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Angular2 - Loading indicator for Http service

I wanted to notifiy the user, whenever I am doing an AJAX request in Angular2. To achieve this in an existing project, the goal was, to edit as little files as possible. The solution to this problem is very straightforward: The class Http needs to be extended which allows own code to be used. First, I needed to create the EventHttpService with the following code: 

import { Http } from '@angular/http';
export class EventHttpService extends Http{}
So far this class does nothing, it just extends the Http class, providing the same functionality. Therefore I needed to override all the methods which should perform a different action. Below is an example for the get method, with all the required imports. Of course, the whole class needs to be decorated with Injectable() so that we can inject it via Dependency Injection. 
import { Injectable } from '@angular/core';
import { Http, RequestOptions, RequestOptionsArgs, Response, ConnectionBackend } from '@angular/http';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class EventHttpService extends Http {
	public get(url: string, options?: RequestOptionsArgs) : Observable<Response> {
			// Own code before the request is performed
			var response = super.get(url, options);
			response.subscribe(null, error => {
				// Own code on error
			}, () => {
				// Own code on success
			});
			return response;
	}
}
The comments in the source code above mark the points where the developer can implement own code. I used it, to increment and decrement a counter. When the counter starts from 0, an injected loading bar is shown and when the counter finally reaches 0, the loading bar is hidden. So far, the class does nothing, since we are not using it. The right place to use it, is app.module.ts: 
@NgModule({
  imports:      [],
  declarations: [],
  providers: [
    HTTP_PROVIDERS,
    provide(Http, {
      useFactory: (xhrBackend: XHRBackend, requestOptions: Request) => new EventHttpService(xhrBackend, requestOptions),
      deps: [XHRBackend, RequestOptions]})
    ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }
With this example, whenever Http.get is used, the EventHttpService is used automatically, executing my custom code. If you want to inject other services into Http, this is also straightforward, just add these to the constructor of the EventHttpService so that it looks as follows. In my case, I am using the awesome ng2-slim-loading-bar to show a loading bar, when the first AJAX request is performed. 
public constructor(_backend: ConnectionBackend, _defaultOptions: RequestOptions, private loadingBar: SlimLoadingBarService) {
	super(_backend, _defaultOptions);
}
Below is the complete code of the event-http.service.ts: 
import { Injectable } from '@angular/core';
import { Http, RequestOptions, RequestOptionsArgs, Response, ConnectionBackend } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import { SlimLoadingBarService } from 'ng2-slim-loading-bar';

@Injectable()
export class EventHttpService extends Http {
	private currentRequests: number = 0;

	public constructor(_backend: ConnectionBackend, _defaultOptions: RequestOptions, private loadingBar: SlimLoadingBarService) {
		super(_backend, _defaultOptions);
	}

	public get(url: string, options?: RequestOptionsArgs) : Observable<Response> {
		this.incrementRequestCount();
		var response = super.get(url, options);
		response.subscribe(null, error => {
			this.decrementRequestCount();
		}, () => {
			this.decrementRequestCount();
		});
		return response;
	}

	private decrementRequestCount() {
		if (--this.currentRequests == 0) {
			this.loadingBar.complete();
		}
	}

	private incrementRequestCount() {
		if (this.currentRequests++ == 0) {
			this.loadingBar.start();
		}
	}
}

About the author

is an experienced Software Engineer with a Master's degree in Computer Science. He started this website in late 2015, mostly as a digital business card. He is interested in Go, Python, Ruby, SQL- and NoSQL-databases, machine learning and AI and is experienced in building scalable, distributed systems and micro-services at multiple larger and smaller companies.

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