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fix: ensure deferred effects are properly revived after async operations (fixes #17304)fix: ensure deferred effects are properly revived after async operati… #17307

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fix: ensure deferred effects are properly revived after async operations (fixes #17304)fix: ensure deferred effects are properly revived after async operati… #17307

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04b686b
fix: ensure deferred effects are properly revived after async operati…
harshitydv1 Dec 4, 2025
89c2a32
refactor: improve flush condition clarity in revive() method
harshitydv1 Dec 4, 2025
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7 changes: 7 additions & 0 deletions .changeset/fix-async-reactivity-17304.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
---
'svelte': patch
---

fix: ensure deferred effects are properly revived after async operations (fixes #17304)

When using `async` in SvelteKit with preloading data (via `data-sveltekit-preload-data` or `preloadData()`), `$effect` callbacks were not firing even though values were updating. This was because when forks are committed (which SvelteKit's preload uses), the `revive()` method wasn't ensuring the batch was properly activated before scheduling deferred effects. This fix ensures the batch context is correct when reviving effects.
30 changes: 22 additions & 8 deletions packages/svelte/src/internal/client/reactivity/batch.js
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Original file line number Diff line number Diff line change
Expand Up @@ -304,7 +304,7 @@ export class Batch {

dep.f ^= WAS_MARKED;

this.#clear_marked(/** @type {Derived} */ (dep).deps);
this.#clear_marked(/** @type {Derived} */(dep).deps);
}
}

Expand Down Expand Up @@ -483,6 +483,13 @@ export class Batch {
}

revive() {
// Ensure the batch is active before scheduling effects
// This is critical for fork commits (used by SvelteKit's preload)
var was_current = current_batch === this;
if (!was_current) {
this.activate();
}

for (const e of this.#dirty_effects) {
set_signal_status(e, DIRTY);
schedule_effect(e);
Expand All @@ -496,7 +503,14 @@ export class Batch {
this.#dirty_effects = [];
this.#maybe_dirty_effects = [];

this.flush();
// Flush if there are queued root effects, or if there is no pending work left.
if (queued_root_effects.length > 0 || this.#pending === 0) {
this.flush();
}

if (!was_current) {
this.deactivate();
}
}

/** @param {() => void} fn */
Expand Down Expand Up @@ -775,14 +789,14 @@ function mark_effects(value, sources, marked, checked) {
const flags = reaction.f;

if ((flags & DERIVED) !== 0) {
mark_effects(/** @type {Derived} */ (reaction), sources, marked, checked);
mark_effects(/** @type {Derived} */(reaction), sources, marked, checked);
} else if (
(flags & (ASYNC | BLOCK_EFFECT)) !== 0 &&
(flags & DIRTY) === 0 &&
depends_on(reaction, sources, checked)
) {
set_signal_status(reaction, DIRTY);
schedule_effect(/** @type {Effect} */ (reaction));
schedule_effect(/** @type {Effect} */(reaction));
}
}
}
Expand All @@ -802,10 +816,10 @@ function mark_eager_effects(value, effects) {
const flags = reaction.f;

if ((flags & DERIVED) !== 0) {
mark_eager_effects(/** @type {Derived} */ (reaction), effects);
mark_eager_effects(/** @type {Derived} */(reaction), effects);
} else if ((flags & EAGER_EFFECT) !== 0) {
set_signal_status(reaction, DIRTY);
effects.add(/** @type {Effect} */ (reaction));
effects.add(/** @type {Effect} */(reaction));
}
}
}
Expand All @@ -825,8 +839,8 @@ function depends_on(reaction, sources, checked) {
return true;
}

if ((dep.f & DERIVED) !== 0 && depends_on(/** @type {Derived} */ (dep), sources, checked)) {
checked.set(/** @type {Derived} */ (dep), true);
if ((dep.f & DERIVED) !== 0 && depends_on(/** @type {Derived} */(dep), sources, checked)) {
checked.set(/** @type {Derived} */(dep), true);
return true;
}
}
Expand Down