Angular 2订阅组件或服务？

Question

Context

I have a component HospitalComponent that tries to show a list of hospitals. It uses the readAll method from the HospitalService (that returns an Observable from firebase) :

ngOnInit() {
  this.hospitalService
    .readAll() // Make a firebase call
    .subscribe(hospitals => this.hospitals = hospitals);
}

The route /hospital is plugged to this component HospitalComponent . Each time I reach /hospital , Angular 2 make a new HospitalComponent , and a new call is made to the hospitalService .

Problem

Each time I reach /hospital , the hospital list is showing with delay.

Question

Is it a good practice to retrieve the hospital list from the service constructor itself ? This way, I can manage the refresh list from background, instead of having some delay. I would have in the component :

ngOnInit() {
  this.hospitals = this.hospitalService.readAll();
}

and in the service :

constructor(private hospitalService: HospitalService) {
  hospitalService.subscribe(hospitals => this.hospitals = hospitals);
}

But this implies to manage manually all hospital changes ..

Answer 1

From a technical viewpoint both "solutions" are pretty much equal - since they basically do the same thing it is up to your personal taste if you just want to conside your two solutions.

But in general: Try to avoid manual subscriptions at all.

There are a couple things that you can improve (the following code is based on the assumption, that you'd rather show an outdated list, that is updated in the background than to show a loading-indicator):

• try to avoid manual subscriptions (especially(!!) in components) -> use the async -pipe instead
• try to avoid stateful components (even services if possible) -> use streams instead

Your Service

export class HospitalService {
    allHospitals$: BehaviorSubject<IHospital[]> = new BehaviorSubject<IHospital[]>([]);

    // the fetchAll() method can be called in the constructor, or somewhere else in the application e.g. during startup, this depends on your application-flow, maybe some login is required ect...
    fetchAll(): Observable<IHospital[]> {
        const fetch$: Observable<IHospital[]> = ...get_stuff_from_firebase().share();
        fetch$
            .do(allHospitals => this.allHospitals$.next(allHospitals);
            .subscribe();
        return fetch$; // optional, just in case you'd want to do something with the immediate result(or error) outside the service
    }
}

Your component (=> just inject the service)

constructor(private hospitalService: HospitalService) {
    // nothing to do here
}

The template of the component (=> the async-pipe automatically manages the subscription and unsubscribes automatically as well, so you don't have to worry about memory-leaks ect...)

<div *ngFor="let hospital of (hospitalService.allHospitals$ | async)">
    {{hospital.name}}
</div>

---

A fourth(but much more extended) solutions would be to use a central store like ngrx - So with ngrx basically the part of allHospitals$ would be moved to a centrally managed store-module and you would strictly divide up your application, so that a service will do nothing but fetching and processing data, the store will do nothing but storing and emitting data and a component will do nothing but displaying data.