'subscribeKey' gets both the new value and the old value

[ayangweb]

Perhaps subscribeKey can be converted to do something similar to vue's watch?

subscribeKey(xxx, "xxx", (value, oldValue) => {
	// do something
});

[dai-shi]

Thanks for the suggestion.
I have kind of mixed feeling. For one, I rejected adding features like #887.
For another, it's trivial to implement.
But, the fact that subscribeKey is to similar to subscribe which doesn't have the prev value pushes me back.

So, unless we have a huge demand of this feature, I think leaving it as is is reasonable.
And, if we have a huge demand, we should probably explore a new library in valtiojs/* including subscribeKeys.

[ayangweb]

So if you want to add an immediate similar to vue's watch method, also want to implement it in valtiojs/*?

[dai-shi]

https://vuejs.org/guide/essentials/watchers#eager-watchers
Is it about eager callback?
I think it's sync or notifyInSync for us.
https://github.com/pmndrs/valtio?tab=readme-ov-file#update-synchronously

valtio/src/vanilla/utils/subscribeKey.ts

Line 8 in c1a18e2

* notifyInSync: same as the parameter to subscribe(); true disables batching of subscriptions.

[ayangweb]

What I'm trying to say is that the page can trigger the subscribeKey callback function the first time it's opened, whether the value has changed or not

[dai-shi]

Oh, it's probably something I call "fireImmediately". Yeah, it might be nice to support is in your new library.

FYI, this is what I would do:

const callback = () => ...;
subscribe(proxy, callback);
callback(); // fire immediately

[ayangweb]

Looking forward to this feature 🥳