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 是

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

1	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

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:

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

範例程式碼

CK's Notepad

[RxJS] using

subscription

參考資料