alternative approach

Author: Rich-Harris

packages/svelte/src/internal/client/constants.js

@@ -20,6 +20,7 @@ export const LEGACY_DERIVED_PROP = 1 << 17;
 export const INSPECT_EFFECT = 1 << 18;
 export const HEAD_EFFECT = 1 << 19;
 export const EFFECT_HAS_DERIVED = 1 << 20;
+export const EFFECT_IS_UPDATING = 1 << 21;
 
 export const STATE_SYMBOL = Symbol('$state');
 export const STATE_SYMBOL_METADATA = Symbol('$state metadata');

packages/svelte/src/internal/client/dom/elements/transitions.js

@@ -289,11 +289,11 @@ export function transition(flags, element, get_fn, get_params) {
 		var run = is_global;
 
 		if (!run) {
-			var block = /** @type {Effect | null} */ (e.p);
+			var block = /** @type {Effect | null} */ (e.parent);
 
 			// skip over transparent blocks (e.g. snippets, else-if blocks)
 			while (block && (block.f & EFFECT_TRANSPARENT) !== 0) {
-				while ((block = block.p)) {
+				while ((block = block.parent)) {
 					if ((block.f & BLOCK_EFFECT) !== 0) break;
 				}
 			}

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -51,7 +51,7 @@ export function derived(fn) {
 		rv: 0,
 		v: /** @type {V} */ (null),
 		wv: 0,
-		p: parent_derived ?? active_effect
+		parent: parent_derived ?? active_effect
 	};
 
 	if (DEV && tracing_mode_flag) {
@@ -101,12 +101,12 @@ let stack = [];
  * @returns {Effect | null}
  */
 function get_derived_parent_effect(derived) {
-	var parent = derived.p;
+	var parent = derived.parent;
 	while (parent !== null) {
 		if ((parent.f & DERIVED) === 0) {
 			return /** @type {Effect} */ (parent);
 		}
-		parent = parent.p;
+		parent = parent.parent;
 	}
 	return null;
 }

packages/svelte/src/internal/client/reactivity/effects.js

@@ -87,7 +87,7 @@ function create_effect(type, fn, sync, push = true) {
 	if (DEV) {
 		// Ensure the parent is never an inspect effect
 		while (parent !== null && (parent.f & INSPECT_EFFECT) !== 0) {
-			parent = parent.p;
+			parent = parent.parent;
 		}
 	}
 
@@ -102,7 +102,7 @@ function create_effect(type, fn, sync, push = true) {
 		fn,
 		last: null,
 		next: null,
-		p: parent,
+		parent: parent,
 		prev: null,
 		teardown: null,
 		transitions: null,
@@ -393,7 +393,7 @@ export function destroy_effect_children(signal, remove_dom = false) {
 
 		if ((effect.f & ROOT_EFFECT) !== 0) {
 			// this is now an independent root
-			effect.p = null;
+			effect.parent = null;
 		} else {
 			destroy_effect(effect, remove_dom);
 		}
@@ -456,7 +456,7 @@ export function destroy_effect(effect, remove_dom = true) {
 
 	execute_effect_teardown(effect);
 
-	var parent = effect.p;
+	var parent = effect.parent;
 
 	// If the parent doesn't have any children, then skip this work altogether
 	if (parent !== null && parent.first !== null) {
@@ -486,7 +486,7 @@ export function destroy_effect(effect, remove_dom = true) {
  * @param {Effect} effect
  */
 export function unlink_effect(effect) {
-	var parent = effect.p;
+	var parent = effect.parent;
 	var prev = effect.prev;
 	var next = effect.next;
 

packages/svelte/src/internal/client/reactivity/sources.js

@@ -25,7 +25,9 @@ import {
 	UNOWNED,
 	MAYBE_DIRTY,
 	BLOCK_EFFECT,
-	ROOT_EFFECT
+	ROOT_EFFECT,
+	EFFECT,
+	EFFECT_IS_UPDATING
 } from '../constants.js';
 import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
@@ -36,6 +38,14 @@ import { proxy } from '../proxy.js';
 export let inspect_effects = new Set();
 export const old_values = new Map();
 
+/** @type {Source[] | null} */
+export let reaction_sources = null;
+
+/** @param {Source[] | null} v */
+export function set_reaction_sources(v) {
+	reaction_sources = v;
+}
+
 /**
  * @param {Set<any>} v
  */
@@ -62,6 +72,14 @@ export function source(v, stack) {
 		wv: 0
 	};
 
+	if (active_reaction !== null && active_reaction.f & EFFECT_IS_UPDATING) {
+		if (reaction_sources === null) {
+			reaction_sources = [signal];
+		} else {
+			reaction_sources.push(signal);
+		}
+	}
+
 	if (DEV && tracing_mode_flag) {
 		signal.created = stack ?? get_stack('CreatedAt');
 		signal.debug = null;
@@ -118,7 +136,7 @@ export function set(source, value, should_proxy = false) {
 		!untracking &&
 		is_runes() &&
 		(active_reaction.f & (DERIVED | BLOCK_EFFECT)) !== 0 &&
-		active_reaction !== source.p
+		!reaction_sources?.includes(source)
 	) {
 		e.state_unsafe_mutation();
 	}

packages/svelte/src/internal/client/reactivity/types.d.ts

@@ -5,8 +5,6 @@ export interface Signal {
 	f: number;
 	/** Write version */
 	wv: number;
-	/** Parent effect or derived */
-	p: Reaction | null;
 }
 
 export interface Value<V = unknown> extends Signal {
@@ -40,6 +38,8 @@ export interface Derived<V = unknown> extends Value<V>, Reaction {
 	fn: () => V;
 	/** Effects created inside this signal */
 	effects: null | Effect[];
+	/** Parent effect or derived */
+	parent: Effect | Derived | null;
 }
 
 export interface Effect extends Reaction {
@@ -66,7 +66,7 @@ export interface Effect extends Reaction {
 	/** Last child effect created inside this signal */
 	last: null | Effect;
 	/** Parent effect */
-	p: Effect | null;
+	parent: Effect | null;
 	/** Dev only */
 	component_function?: any;
 }

packages/svelte/src/internal/client/runtime.js

@@ -22,10 +22,16 @@ import {
 	ROOT_EFFECT,
 	LEGACY_DERIVED_PROP,
 	DISCONNECTED,
-	BOUNDARY_EFFECT
+	BOUNDARY_EFFECT,
+	EFFECT_IS_UPDATING
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
-import { internal_set, old_values } from './reactivity/sources.js';
+import {
+	internal_set,
+	old_values,
+	reaction_sources,
+	set_reaction_sources
+} from './reactivity/sources.js';
 import { destroy_derived_effects, update_derived } from './reactivity/deriveds.js';
 import * as e from './errors.js';
 import { FILENAME } from '../../constants.js';
@@ -161,7 +167,7 @@ export function check_dirtiness(reaction) {
 			// then we need to re-connect the reaction to the dependency
 			if (is_disconnected || is_unowned_connected) {
 				var derived = /** @type {Derived} */ (reaction);
-				var parent = derived.p;
+				var parent = derived.parent;
 
 				for (i = 0; i < length; i++) {
 					dependency = dependencies[i];
@@ -228,7 +234,7 @@ function propagate_error(error, effect) {
 			}
 		}
 
-		current = current.p;
+		current = current.parent;
 	}
 
 	is_throwing_error = false;
@@ -240,7 +246,8 @@ function propagate_error(error, effect) {
  */
 function should_rethrow_error(effect) {
 	return (
-		(effect.f & DESTROYED) === 0 && (effect.p === null || (effect.p.f & BOUNDARY_EFFECT) === 0)
+		(effect.f & DESTROYED) === 0 &&
+		(effect.parent === null || (effect.parent.f & BOUNDARY_EFFECT) === 0)
 	);
 }
 
@@ -352,7 +359,8 @@ function schedule_possible_effect_self_invalidation(signal, effect, root = true)
 
 	for (var i = 0; i < reactions.length; i++) {
 		var reaction = reactions[i];
-		if (signal.p === reaction) continue;
+
+		if (reaction_sources?.includes(signal)) continue;
 
 		if ((reaction.f & DERIVED) !== 0) {
 			schedule_possible_effect_self_invalidation(/** @type {Derived} */ (reaction), effect, false);
@@ -380,8 +388,11 @@ export function update_reaction(reaction) {
 	var previous_skip_reaction = skip_reaction;
 	var previous_component_context = component_context;
 	var previous_untracking = untracking;
+	var previous_reaction_sources = reaction_sources;
+
 	var flags = reaction.f;
 
+	set_reaction_sources(null);
 	new_deps = /** @type {null | Value[]} */ (null);
 	skipped_deps = 0;
 	untracked_writes = null;
@@ -393,6 +404,8 @@ export function update_reaction(reaction) {
 	untracking = false;
 	read_version++;
 
+	reaction.f |= EFFECT_IS_UPDATING;
+
 	try {
 		var result = /** @type {Function} */ (0, reaction.fn)();
 		var deps = reaction.deps;
@@ -464,6 +477,9 @@ export function update_reaction(reaction) {
 		skip_reaction = previous_skip_reaction;
 		set_component_context(previous_component_context);
 		untracking = previous_untracking;
+		set_reaction_sources(previous_reaction_sources);
+
+		reaction.f ^= EFFECT_IS_UPDATING;
 	}
 }
 
@@ -719,8 +735,8 @@ export function schedule_effect(signal) {
 
 	var effect = (last_scheduled_effect = signal);
 
-	while (effect.p !== null) {
-		effect = effect.p;
+	while (effect.parent !== null) {
+		effect = effect.parent;
 		var flags = effect.f;
 
 		if ((flags & (ROOT_EFFECT | BRANCH_EFFECT)) !== 0) {
@@ -785,12 +801,12 @@ function process_effects(root) {
 			}
 		}
 
-		var parent = effect.p;
+		var parent = effect.parent;
 		effect = effect.next;
 
 		while (effect === null && parent !== null) {
 			effect = parent.next;
-			parent = parent.p;
+			parent = parent.parent;
 		}
 	}
 
@@ -873,7 +889,7 @@ export function get(signal) {
 		/** @type {Derived} */ (signal).effects === null
 	) {
 		var derived = /** @type {Derived} */ (signal);
-		var parent = derived.p;
+		var parent = derived.parent;
 
 		if (parent !== null && (parent.f & UNOWNED) === 0) {
 			// If the derived is owned by another derived then mark it as unowned