Discussion on: Observables are promises on steroids

View post

Hi! Andy here, the author of observable-fns and threads.js.

Just stumbled upon this blog post. Nice job!

About the cancellation: Observables are kind-of cancellable – you can unsubscribe from them.

Observables are "cold" by default which means every new subscriber will get their own fresh instance of this observable, which also means that the code in new Observable(/* ... */) will be run on every .subscribe() call (!). If a subscriber unsubscribes, the upstream subscription is terminated (cancellation) – just that there may be numerous more subscriptions of the same sort.

You can make observables "hot" by using the multicast() function. Then there will only ever be one upstream subscription that all downstream subscribers share. You can unsubscribe, too, just that the single shared upstream subscription will live on until the last subscriber has unsubscribed.

What does that mean for your use case?

Use multicast() 😉
Then enjoy .unsubscribe() acting as cancellation
Profit $$$

Hope that helps 🙂

Pacharapol Withayasakpunt • May 16 '20 • Edited

How do I listen to .once('all-unsubscribed') event? Like this one package - p-cancelable.

Otherwise, it is possible to use Subject, as in thread.js? (I wouldn't want to use Worker, as I write to SQLite as well, and it is to supposed to be written from multiple threads.)

Andy Wermke • May 16 '20 • Edited

How do I listen to .once('all-unsubscribed') event?

function subscribeToThings() {
  return multicast(new Observable(observer => {
    // <Subscribe to things>

    const terminateSubscription = () => {
      // <-- THIS IS THE "all-unsubscribed" CODE
      // Remember? multicast() makes that all downstream subscriptions are bundled into one,
      // so this is the callback function that will be run when this bundled subscription is terminated
    }

    return terminateSubscription
  }))
}

subscribeToThings().subscribe(/* ... */)

Otherwise, it is possible to use Subject, as in thread.js?

Sure, who would stop you? ;)

Pacharapol Withayasakpunt • May 17 '20

Currently, I use the signature,

Observable<{
      value?: QueryItemPartial
      i: number
      cancelFunction: CallbackType
    }>

Will it work?

Andy Wermke • May 17 '20

That doesn't make much sense. Why would you want to emit a new cancellation function for every new value?

You shouldn't have this non-standard custom cancellation approach when there is already a standardized one (observable.subscribe(), subscription.unsubscribe()). Keep in mind that emitting functions is generally considered to be an anti-pattern.

Maybe the observable-fns readme lacks a link to some documentation about observables in general… I guess this is one of your first times using observables? (No offense – I'm seriously interested :) )

Pacharapol Withayasakpunt • May 17 '20

Yes. Only promises and eventemitters.

Should I start with RxJS?

Andy Wermke • May 17 '20

Yeah, maybe that's the best to get started. You should definitely start with RxJS's documentation – it's pretty good!

Which library you then use doesn't really matter too much as their APIs are very similar to almost identical.

Pacharapol Withayasakpunt • May 19 '20

const terminateSubscription = () =>

Do you actually mean

new Observable((obs) => {
  return terminateSubscription
  // I actually use
  // return () => { isUnsubscribed = true }
})

Andy Wermke • May 19 '20

Yes, my bad! I updated my code sample accordingly.