rewrite.handler.ts

import {InjectionToken, Injector} from '@angular/core';
import {HttpBackend, HttpEvent, HttpInterceptor, HttpReques, HttpHandler, HttpRequest} from '@angular/common/http';
import { Observable } from 'rxjs';

/**
* `HttpHandler` which applies an `HttpInterceptor` to an `HttpRequest`.
*/
export class MyHttpInterceptorHandler implements HttpHandler {
  constructor(private next: HttpHandler, private interceptor: HttpInterceptor) {}

  handle(req: HttpRequest<any>): Observable<HttpEvent<any>> {
    return this.interceptor.intercept(req, this.next);
  }
}

export class MyHandlerService implements HttpHandler {

  private chain: HttpHandler | null = null;

  constructor(private backend: HttpBackend, private injector: Injector, private interceptors: InjectionToken<HttpInterceptor[]>) { }

  handle(req: HttpRequest<any>): Observable<HttpEvent<any>> {
    if (this.chain === null) {
      const interceptors = this.injector.get(this.interceptors, []);
      this.chain = interceptors.reduceRight(
        (next, interceptor) => new MyHttpInterceptorHandler(next, interceptor), this.backend);
    }
    return this.chain.handle(req);
  }
}