[RxJS] using

RxJS 裡面有許多 Operators 或是建立 Observable 的方法,而這篇文章就是要介紹 using 這一個建立 Observable 的方法

根據官方文件的說明,Using

Creates an Observable that uses a resource which will be disposed at the same time as the Observable.

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using<T>(resourceFactory: () => Unsubscribable | void, observableFactory: (resource: Unsubscribable | void) => ObservableInput<T> | void): Observable<T>

Parameters

Parameters Description
resourceFactory A function which creates any resource object that implements unsubscribe method
observableFactory A function which creates an Observable, that can use injected resource object.

Returns

Observable<T>: An Observable that behaves the same as Observable returned by observableFactory, but which - when completed, errored or unsubscribed - will also call unsubscribe on created resource object.

看不懂沒關係,來看一個簡單的程式碼就了解了

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import { using, Subject, merge, Subscription, interval } from 'rxjs';
import { map, refCount, tap, publish, take } from 'rxjs/operators';

console.clear();

const messages$ = new Subject();
const rejected$ = new Subject();

const source = using(() => {
// resourceFactory
const subscription = new Subscription();
subscription.add(
interval(500).pipe(tap(n => {
if (n === 4) {
// 這行會造成透過 observableFactory 建立出的 observable 發生錯誤而中斷
rejected$.error('oops, something wrong');
}
console.log('inner obs: ' + n)
})).subscribe()
);
subscription.add(() => {
console.log('obs unsubscribe');
})
return subscription;
}, (subscription) => {
// observableFactory
return merge(messages$, rejected$)
}).pipe(
tap((x) => console.log(x)),
publish(),
refCount()
)

interval(1000).pipe(tap(n => messages$.next(n))).subscribe()
const sub = source.pipe(take(5)).subscribe();

執行結果:

1550820374107

當 observableFactory 結束時也會同時執行 resourceFactory 內所回傳的 subscription 物件的 unsubscribe 方法

subscription

subscription.add 裡面除了可以放入其他 subscription 外,也可以放入 function,當 unsubscribe 時,除了會將加到這一個 subscription 內的 subscriptions 給取消註冊外,也會執行所放入的 function,這個在特定情況下十分好用

參考資料