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WICG / observable

Observable API proposal
https://wicg.github.io/observable/
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Ref Counted Observable discussion #178

Open benlesh opened 4 hours ago

benlesh commented 4 hours ago

After TPAC last week, it was determined that the ideal type of observable might be a ref-counted observable. The ref-counted observable will function roughly as follows:

  1. The observable is "cold" until the first subscriber.
  2. When the first subscriber subscribes, the subscription logic is executed.
  3. Every additional subscriber will share that subscription (it's "hot").
  4. When all subscribers abort their subscriptions, the ref-count drops to 0, and teardown/unsubscription logic is executed, returning the observable to a "cold" state.

The idea behind this type is to address concerns from one issue (#170), where an RxJS user thought it was confusing that sometimes observables have side effects, and sometimes they didn't. This would guarantee that for N subscribers, there would be at most one side effect.

benlesh commented 4 hours ago

This type exists in RxJS-land as the result of share(). The only two issues I can think of, off the top of my head, that will come up with this design are:

  1. When ref-count drops to zero, it's often desirable to have the teardown wait for some small amount of time to make sure that no new subscribers join shortly after. An example of this would be an observable that was shared amongst multiple web components. It's not uncommon that during a single render, all components may be removed or unmounted, then replaced with all new component instances that were going to subscribe to the same thing. In this case, the ref count would drop to zero, everything would teardown, then moments later new components would mount and subscribe, and the subscription needs to start all over again. To that end, RxJS has made this configurable.
  2. A very common use case for observables is to have this "share" behavior, but to emit to new subscribers the most recent value that was seen. For example, imagine you have a sensor that collects data once every 10 seconds. Ideally, new subscribers don't have to wait for 9.5 seconds, if the last bit of data showed up 0.5 seconds ago. RxJS has also made this configurable, but we don't have configuration for this in this new design.