ngrx Load data from server only if the store is em

问题:

I have a static set of data, a list of countries, that are used on some components. This data is loaded upon the ngOnInit() of these components but I'd like to load them only if it's the very first time that I request the data (the store is empty). Every subsequent times I load the component I'd like to simply use the data from the store without "refreshing" it.

How is this achievable using ngrx?

I'm using Effects. This is how my code looks like:

The component:

export class EditPageComponent implements OnInit {
countries$: Observable<Country[]>

constructor(private store: Store<fromContacts.State>) {
    this.countries$ = store.select(fromContacts.getCountriesEntities);
}
ngOnInit() {
   this.store.dispatch(new countries.Load());
}

The Effect:

    @Effect()
      loadCollection$: Observable<Action> = this.actions$
        .ofType(countries.LOAD)
        .switchMap(() =>
          this.countriesService
        .getCountries()
        .map((countriesList: Country[]) => {
          return new countries.LoadSuccess(countriesList);
        })
        .catch(error => of(new countries.LoadFail(error)))
  );

And the reducer:

case countries.LOAD_SUCCESS: {

      const countriesList: Country[] = action.payload;
      const reducedCountries: { [id: string]: Country } = countriesList.reduce((countrs: { [id: string]: Country }, countr: Country) => {
        return Object.assign(countrs, {
            [countr.code]: countr
        });
    }, {});

Thanks, Gab

回答1:

There are different ways of doing this. First of all you can keep a hasLoaded: boolean property in the state. Then you can check this before you make the service get call.

ngOnInit() {
  this.store.select(getHasLoaded)
    .take(1)
    .subscribe(hasLoaded => {
      if (!hasLoaded) this.store.dispatch(new countries.Load()); 
    }
}

Another option is to let your @Effect check the hasLoaded property:

@Effect()
  loadCollection$: Observable<Action> = this.actions$
    .ofType(countries.LOAD)
    .withLatestFrom(this.store.select(getHasLoaded)
    .filter(([ action, hasLoaded ]) => !hasLoaded) // only continue if hasLoaded is false 
    .switchMap(() =>
      this.countriesService
        .getCountries()
        .map((countriesList: Country[]) => {
          return new countries.LoadSuccess(countriesList);
        })
    .catch(error => of(new countries.LoadFail(error)))
);

For this to work you need to provide the store in your Effects constructor.

回答2:

TL;DR

Use take(1) operator in the effect

Don't forget the error handling using catchError and return EMPTY, otherwise when an error occurs, that error will be returned always (timeout, auth error, offline...)

---

I had exactly the same case as you, what I did was adding in the effects the rxjs operator take to fetch the countries only the first time the LoadCountries action was dispatched.

@Effect()
  loadCountries$: Observable<CoreActions> = this.actions$.pipe(
    ofType(CoreActionTypes.LoadCountries),
    mergeMap(() =>
      this.countriesService.getAllCountries().pipe(
        map(c => new LoadCountriesSuccess(c)),
        catchError(() => {
          this.store.dispatch(new LoadCountriesFailed());
          return EMPTY;
        })
      )
    ),
    take(1)
  );

Returning EMPTY inside catchError will complete the observable without passing through the take(1)

回答3:

You can select the countries from the store within effects, if they are represented in the store we ignore the action, if not we fetch the countries.

@Effect()
getOrder = this.actions.pipe(
  ofType<GetOrder>(ActionTypes.GetOrder),
  withLatestFrom(this.store.pipe(select(getOrders))),
  filter(([{payload}, orders]) => !!orders[payload.orderId])
  mergeMap([{payload}] => {
    ...
  })
)

For more info see Start using ngrx/effects for this.

回答4:

My answer is a variation from Hetty de Vries answer's. If you simply want to retrieve the contents of the store without touching the effects you could do something similar to this:

this.store.select(state => state.producer.id).forEach( id => {
...
}

supposing that your store contains a producer object with an attribute id.