stateful
stateful
As the name stateful implies this operator is useful when you process an Observable which maintains state.
Maintaining state as an Observable source comes with a handful of repetitive as well as use case specific tasks.
It acts like the Observables pipe method.
It accepts RxJS operators and composes them like Observable#pipe and the standalone pipe method.
Furthermore, it takes care of the above mentioned repetitive tasks as listed below.
You will always (aka repetitive) want to ensure that:
- only distinct state changes are emitted
- only defined values are emitted (filter out undefined, which ensures lazy state)
- share and replay custom operations for multiple subscribers (saves performance)
You will sometimes (aka situational) need:
- a subset of the state (derivations)
- compose the state with other Observables or change the Observables behavior
Note that RxState#connect already applies the stateful operator under the hood.
Example
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
import { stateful } from 'rx-angular/state/selections';
const state$: Observable<{ name: string; items: string[] }>;
const derivation$ = state$.pipe(
stateful(
map((state) => state.list.length),
filter((length) => length > 3)
)
);
Signature
function stateful<T>(): MonoTypeOperatorFunction<T>;
Signature
function stateful<T, R>(
optionalDerive: OperatorFunction<T, R>[]
): OperatorFunction<T, T | R>;