Example - Angular 2+ controller calling data service which in turn calls custom common HTTP service

myHTTPSvc.ts

import { Http, Headers, RequestOptions } from '@angular/http';
import { AsyncSubject } from 'rxjs/AsyncSubject';

@Injectable()
export class myHTTPSvc {
  constructor(private http: Http, private upSvc: MyUserProfileService) {}

  get(url) : Observable<any> {
    let subjectResult = new AsyncSubject();
    this.upSvc.getLoggedInUserDetails()
      .subscribe(details => {
        let headerConfig = this.getHeaderConfig(details);
      
        // here is the actual GET call you want to make
        this.http.get(url, headerConfig)
          .map(data => data.json())
          .subscribe(result => {
            // similar to deferred.resolve(result) we used to do with Angular 1.x
            subjectResult.next(result);
            subjectResult.complete();
          });
      });

    return subjectResult;
  }
  
  private getHeaderConfig(loggedInUserDetails: any): RequestOptions {
    let options = new RequestOptions();
    options.headers = new Headers();
    options.headers.append('license', loggedInUserDetails.licenseInfo);
    // or any details you want to push in message header
    return options;
  }
}