Make $$.on_hmr public (or a way to detect HMR update)

[adiguba]

Hello,

I'm working on some change on the way Svelte manage the event-handler via the on:event directive.
My changes work fine as long as the code is not reloaded via HMR...

In fact my code process the handlers added to the component, via .$on() (called by on:event={handler}), who were stored on cmp.$$.callbacks.
But when the code is refreshed via HMR, the component is fully recreated with props, and cmp.$$.callbacks is updated, but I have no way to detect that, so my code cannot (re)process the handlers correctly.

I need a way to know that the component was reloaded in order to reprocess cmp.$$.callback.

I see this comment about the undocumented hook cmp.$$.on_hmr : #57 (comment)
But it seem it didn't work on my case, because this field is initialized when svelte-hmr instrument the component (line 216 of svelte-hooks.js), and it will overwritte my own hook initialized when my component is created :

    targetCmp.$$.on_hmr = []

I edited this line of svelte-hooks.js in order to keep my hooks :

   targetCmp.$$.on_hmr = targetCmp.$$.on_hmr || [];

It's seem to work for me, but I don't know how Svelte-HMR works nor the impact of this change.

Or is there another way to detect code reload ?

Thanks

[adiguba]

My bad I feel so stupid.
I just need to add the on_hmr hook AFTER the component was constructed, not inside the constructor.

So sometinh like that :

	let comp = new Comp(...);
	comp.$$.on_hmr && comp.$$.on_hmr.push( (_: any) => {
		return (c: SvelteComponent) => restart_all_callback(c);
	});

Just one question before closing : do $$.on_hmr is safe to use ?

[dominikg]

Not sure which changes to svelte you are talking about. If you intend to send a PR to svelte itself, is there a feature request, open issue or PR you can link to?

In general $$ is not meant for public use or part of a stable api for public consumption. Please see vite docs for how to listen to update events: https://vitejs.dev/guide/api-hmr.html#hot-on-event-cb

That being said this is dev only, so you could do introspection like in your example and assume that if on_hmr exists it works the way it currently does. It may break the dev workflow but not production builds if svelte/svelte-hmr change in ways that make it incompatible.

[adiguba]

Thanks for your response.
This code is only generated on dev mode.

The issue is here : sveltejs/svelte#2837
I will try to send a PR later...

[rixo]

@adiguba There's no immediate plan to make $$.on_hmr public, but there's no plan to remove or change it either.

Make sure you're subscribed to #57, as we'll issue some notice there if for some reason it comes to change.

In any case, if your usage of it is to support a feature that gets included in Svelte itself, you can be sure we won't change it without proper synchronisation to make sure everything keeps working.

[dominikg]

also please note that svelte-hmr is currently a dev time addon over svelte. svelte itself is completely unaware of svelte-hmr and in its current major version (svelte3) is should stay that way.

A future major version of svelte may incorporate hmr features directly so svelte-hmr as a separate package will no longer be needed.
For svelte3, new features would have to be supported by svelte-hmr by improving it's proxy or other means.

Given this and the fact that svelte on:* support might need further discussions i encourage you to get in touch on svelte discord #contributing before investing a lot of work in a PR.

[adiguba]

Thanks for your advices !