I have a component that gets the data from a service via http, the problem is that I don't want to hit the API backend every time I show this component. I want my service to check if the data is in memory, if it is, return an observable with the array in memory, and if not, make the http request.

My component

import {Component, OnInit } from 'angular2/core';
import {Router} from 'angular2/router';

import {Contact} from './contact';
import {ContactService} from './contact.service';

@Component({
  selector: 'contacts',
  templateUrl: 'app/contacts/contacts.component.html'
})
export class ContactsComponent implements OnInit {

  contacts: Contact[];
  errorMessage: string;

  constructor(
    private router: Router,
    private contactService: ContactService) { }

  ngOnInit() {
    this.getContacts();
  }

  getContacts() {
    this.contactService.getContacts()
                       .subscribe(
                         contacts => this.contacts = contacts,
                         error =>  this.errorMessage = <any>error
                       );
  }
}

My service

import {Injectable} from 'angular2/core';
import {Http, Response, Headers, RequestOptions} from 'angular2/http';
import {Contact} from './contact';
import {Observable} from 'rxjs/Observable';

@Injectable()
export class ContactService {

  private contacts: Array<Contact> = null;

  constructor (private http: Http) {

  }

  getContacts() {
    // Check first if contacts == null
    // if not, return Observable(this.contacts)? <-- How to?

    return this.http.get(url)
                    .map(res => <Contact[]> res.json())
                    .do(contacts => {
                      this.contacts = contacts;
                      console.log(contacts);
                    }) // eyeball results in the console
                    .catch(this.handleError);
  }


  private handleError (error: Response) {
    // in a real world app, we may send the server to some remote logging infrastructure
    // instead of just logging it to the console
    console.error(error);
    return Observable.throw(error.json().error || 'Server error');
  }
}