Angular2: convert array to Observable

Question

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');
  }
}

Solution 1

You're right there. If you already have the data in memory, you can use of observable (equivalent of return/just in RxJS 4).

getContacts() {

    if(this.contacts != null) 
    {
        return Observable.of(this.contacts);
    } 
    else 
    {
        return this.http.get(url)
            .map(res => <Contact[]> res.json())
            .do(contacts => this.contacts = contacts)
            .catch(this.handleError);
    }
}

Solution 2

import { of } from 'rxjs';
    
return of(this.contacts);

Solution 3

Some people like me want it differently, which is from string[] into Observable<string>.

This is an example which involves the conversion:

import { from } from 'rxjs/observable/from';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/toArray';

const ids = ['x12', 'y81'];
let userUrls: string[];

from(ids) // Converting string[] into Observable<string>
  .map(id => 'http://localhost:8080/users/' + id)
  .toArray()
  .subscribe(urls => userUrls = urls);

Hopefully it will help some others.

Solution 4

In angular7 its enough to just put the of(). Whatever you put inside of() will be changed to observable. Here, this.contacts is converted to observable.

import { of } from 'rxjs';

getContacts() {
    if (this.contacts != null) {
        return of(this.contacts);
    }
}

Solution 5

Quick solution here:

getSomething(): Observable < Object[] > {
    return new Observable(observable => {
        this.http.get('example.com').subscribe(results => {
            observable.next(results.json());
            observable.complete();
        });
    });
}

View more solutions

Comments

[Evan Plaice]

This is probably a dumb question but couldn't you just cache the observable itself and call .next() rather than caching the state and recreating the observable on each call?

[Mathius17]

Good point, I'll try it. Though it my not be the best for when I want to iterate the array to return a single contact by a specific key

[Sal]

import 'rxjs/add/observable/of';

[David Sagang]

@Eric Martinez, thanks for your answer. Where do you get the of from. I imported that from 'rxjs' however it is not in the Observable.