Fetch data once with Observables in Angular 2

Question

I have a service, what is used several times from a lot of my Angular 2 components. It fetches customer data from a Web API and returns an Observable:

getCus         


        

Answer 1

1) You can simply save downloaded data in your service:

export class CustomersService {
  protected _customers: Array;

  constructor(public http: Http) {}

  public getCustomers(): Observable> {
    return new Observable(observer => {
      if (this._customers) {
        observer.next(this._customers);
        return observer.complete();
      }
      this.http
        .get(this.baseURI + this.url)
        .map((r: Response) => (r.json() as Array))
        .subscribe((customers: Array) => {
          this._customers = customers;
          observer.next(this.customers);
          observer.complete();
        });
    });
  }
}

2) Shorter approach taking refresh parameter:

export class CustomersService {
  protected _customers: Array;

  constructor(public http: Http) {}

  public getCustomers(refresh?: boolean): Observable> {
    if (!refresh && this._customers) {
      return Observable.of(this._customers);
    }
    return this.http
            .get(this.baseURI + this.url)
            .map((c: Response) => (c.json() as Array))
            .do((customers: Array) => {
                this._customers = customers;
            });
    });
  }
}

3) Taking advantage of ReplaySubject:

export class CustomersService {
  protected _customers$: ReplaySubject> = new ReplaySubject(1);
  protected _customersInitialized: boolean;

  constructor(public http: Http) {}

  public getCustomers(refresh?: boolean): Observable> {
    if (refresh || !this._customersInitialized) {
      this._customersInitialized = true;
      this.http
        .get(this.baseURI + this.url)
        .map((c: Response) => (c.json() as Array))
        .subscribe((customers: Array) => {
          this._customers$.next(customers);
        });
    }
    return this._customers$.asObservable().skip(+refresh).distinctUntilChanged();
  }
}

And then:

this.customersService.getCustomers()
    .subscribe(customers => this.customers = customers);

You can also expose the always up-to-date customers field from SomeService for read only purposes (like displaying in the templates) this way:

public get customers(): ReadonlyArray {
  return this._customers;
}