import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http'
import { Observable } from 'rxjs';
import { finalize } from 'rxjs/operators';
import { NgxSpinnerService } from 'ngx-spinner';

@Injectable()
export class LoaderInterceptor implements HttpInterceptor {
  constructor(private spinner: NgxSpinnerService) { }
  requestCounter = 0;

  // Identifies and handles a http request
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {

    // Handles a http request for showing and hiding the loading panel.
    this.beginRequest();
    return next.handle(req).pipe(
      finalize(() => {
        this.endRequest();
      })
    );
  }

  // Increments the request counter when any api is called and hides the loading panel.
  beginRequest() {
    this.requestCounter = Math.max(this.requestCounter, 0) + 1;
    if (this.requestCounter === 1) {
      this.spinner.show();
    }
  }

  // Decrements the request counter when any api is completed and hides the loading panel.
  endRequest() {
    this.requestCounter = Math.max(this.requestCounter, 1) - 1;
    if (this.requestCounter === 0) {
      this.spinner.hide();
    }
  }
}