Add watched and unwatched

[JoviDeCroock]

Resolves #351
Resolves #428

This adds a second argument to signal which are the options, currently this adds two properties, watched and unwatched. These will most likely serve third party libraries a lot more than first party consumers.

• watched will trigger when the first subscriber subscribes to a signal
• unwatched will trigger when the last subscriber unsubscribes from a signal

The second parameter can in the future be used for other RFC's in our pipeline like the equality function. The addition of this parameter is in line with the TC39 proposal for browser-native signals https://github.com/tc39/proposal-signals

I opted not to add this to computed for the time being as the signal itself is the atom while computed is more of a derived value. Not sure if a utility like this would be useful there. In the TC39 proposal they also add this to computed, so might as well for parity.

Note that calling the watcher on the subscribe is slightly distinct from how we normally treat signals, i.e. we calculate the computed when its value is accessed for the first time.

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Latest commit: ee34c3c

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@preact/signals	Minor
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[github-actions]

Size Change: +197 B (+0.24%)

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[andrewiggins]

Is this mostly to align with the signal proposal? If so, perhaps we can wait for that proposal to go to Stage 2 or 3 and we have more confidence in the API landing before we implement (just to avoid any churn).

if we have some use cases we'd like to use this for now, it does seems reasonable to me.

[mxck]

Is this mostly to align with the signal proposal? If so, perhaps we can wait for that proposal to go to Stage 2 or 3 and we have more confidence in the API landing before we implement (just to avoid any churn).

if we have some use cases we'd like to use this for now, it does seems reasonable to me.

Hi! I’m one of the people who requested this feature.

The main use case is for subscriptions over WebSocket or data polling using interval/timeout, but only when the signal is mounted/used.

I previously asked about how we could achieve a setup where data updates occur only if the signal is actively in use. Here’s the link to my question in discussions: link.