state changes in `onoutrostart` are not working

[timephy]

Describe the bug

Title says it all, here is an example:

<!-- Parent -->
<script>
	import Component from "./Component.svelte"

	let visible = $state(true)
</script>

<input type="checkbox" bind:checked={visible} />

{#if visible}
	<Component />
{/if}

<!-- Child Component -->
<script lang="ts">
	import { fly } from "svelte/transition"
	
	let status = $state("idle")
	$inspect(status)
</script>

<p transition:fly={{ y: 20, duration: 300 }} onoutrostart={() => (status = 'outro')}>status: {status}</p>

What happens

• status is updated by the onoutrostart listener
• $inspect(status) runs and prints that status = 'outro'
• status: {status} is NOT updated ❌

What I expect to happen

status: {status} should be updated

---

I guess elements that are transitioning out are removed from receiving state updates. However this should only happen when the outro/transition ends, not when it starts.

Reproduction

https://svelte.dev/playground/a221ea9296a1411da4d3fb3563aeb183?version=latest

Logs

System Info

playground

Severity

annoyance

[7nik]

The reactivity stops in the outroing elements by design.

[timephy]

what a bummer.

[7nik]

The problem is that in a case like the following

{#if user}
  <h1 transition:fly>{user.name}</h1>
{/if

reactivity working during outro will throw the error Cannot read properties of undefined (reading 'name').
But maybe it would be nice to opt out it.

[timephy]

Why? Variables from script seem to still exist during outro.
In my example above I used $inspect(status) to print the status after/during the outro. It is not undefined...
Even during onoutroend the variable is still defined.

<script lang="ts">
	import { fly } from "svelte/transition"
	
	let status = $state("idle")
	$inspect(status)
</script>

<p transition:fly={{ y: 20, duration: 300 }} onoutrostart={() => console.log(status)}  onoutroend={() => console.log(status)}>status: {status}</p>

[7nik]

The problem is not in the variable but in its value.

[timephy]

but in its value

Sorry. I meant to say: Even during onoutroend the variable is still defined and its value is still there.

[7nik]

It's not always the case.

[timephy]

Well if you are setting data = null (data = data ? null : { foo: "baz"}), then yes, of course it is going to be null...
I don't see your point

[7nik]

The point is when you want to display some optional data and with a nice transition, during outro data may not be available dispite {#if data !== null}.

[timephy]

Okay I see, you are showing why that design decision was made - and not that it would be impossible/similar. Now I see, thanks for the clarification!