WIP

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

@@ -18,19 +18,20 @@ import {
 	update_reaction,
 	increment_write_version,
 	set_active_effect,
-	handle_error
+	handle_error,
+	flush_sync
 } from '../runtime.js';
 import { equals, safe_equals } from './equality.js';
 import * as e from '../errors.js';
 import * as w from '../warnings.js';
-import { block, destroy_effect, render_effect } from './effects.js';
+import { destroy_effect, render_effect } from './effects.js';
 import { inspect_effects, internal_set, set_inspect_effects, source } from './sources.js';
 import { get_stack } from '../dev/tracing.js';
 import { tracing_mode_flag } from '../../flags/index.js';
-import { capture, suspend } from '../dom/blocks/boundary.js';
+import { capture } from '../dom/blocks/boundary.js';
 import { component_context } from '../context.js';
-import { noop } from '../../shared/utils.js';
 import { UNINITIALIZED } from '../../../constants.js';
+import { active_fork } from './forks.js';
 
 /** @type {Effect | null} */
 export let from_async_derived = null;
@@ -105,16 +106,19 @@ export function async_derived(fn, location) {
 	// only suspend in async deriveds created on initialisation
 	var should_suspend = !active_reaction;
 
-	/** @type {(() => void) | null} */
-	var unsuspend = null;
-
 	render_effect(() => {
 		if (DEV) from_async_derived = active_effect;
-		var current = (promise = fn());
+		promise = fn();
 		if (DEV) from_async_derived = null;
 
 		var restore = capture();
-		if (should_suspend) unsuspend ??= suspend();
+
+		var fork = active_fork;
+
+		if (should_suspend) {
+			// TODO if nearest pending boundary is not ready, attach to the boundary
+			fork?.increment();
+		}
 
 		promise.then(
 			(v) => {
@@ -122,33 +126,36 @@ export function async_derived(fn, location) {
 					return;
 				}
 
-				if (promise === current) {
-					restore();
-					from_async_derived = null;
+				restore();
+				from_async_derived = null;
 
+				if (should_suspend) {
+					fork?.decrement();
+				}
+
+				if (fork !== null) {
+					fork?.enable();
+					flush_sync(() => {
+						internal_set(signal, v);
+					});
+					fork?.disable();
+				} else {
 					internal_set(signal, v);
+				}
 
-					if (DEV && location !== undefined) {
-						recent_async_deriveds.add(signal);
-
-						setTimeout(() => {
-							if (recent_async_deriveds.has(signal)) {
-								w.await_waterfall(location);
-								recent_async_deriveds.delete(signal);
-							}
-						});
-					}
-
-					// TODO we should probably null out active effect here,
-					// rather than inside `restore()`
-					unsuspend?.();
-					unsuspend = null;
+				if (DEV && location !== undefined) {
+					recent_async_deriveds.add(signal);
+
+					setTimeout(() => {
+						if (recent_async_deriveds.has(signal)) {
+							w.await_waterfall(location);
+							recent_async_deriveds.delete(signal);
+						}
+					});
 				}
 			},
 			(e) => {
-				if (promise === current) {
-					handle_error(e, parent, null, parent.ctx);
-				}
+				handle_error(e, parent, null, parent.ctx);
 			}
 		);
 	}, EFFECT_ASYNC | EFFECT_PRESERVED);

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

@@ -0,0 +1,61 @@
+/** @import { Effect, Source } from '#client' */
+
+/** @type {Set<Fork>} */
+const forks = new Set();
+
+/** @type {Fork | null} */
+export let active_fork = null;
+
+let uid = 1;
+
+export class Fork {
+	id = uid++;
+
+	/** @type {Map<Source, any>} */
+	previous = new Map();
+
+	/** @type {Set<Effect>} */
+	skipped_effects = new Set();
+
+	#pending = 0;
+
+	/**
+	 * @param {Source} source
+	 * @param {any} value
+	 */
+	capture(source, value) {
+		if (!this.previous.has(source)) {
+			this.previous.set(source, value);
+		}
+	}
+
+	enable() {
+		active_fork = this;
+		// TODO revert other forks
+	}
+
+	disable() {
+		active_fork = null;
+		// TODO restore state
+	}
+
+	increment() {
+		this.#pending += 1;
+	}
+
+	decrement() {
+		this.#pending -= 1;
+	}
+
+	settled() {
+		return this.#pending === 0;
+	}
+
+	static ensure() {
+		return (active_fork ??= new Fork());
+	}
+
+	static unset() {
+		active_fork = null;
+	}
+}

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

@@ -35,6 +35,7 @@ import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
 import { get_stack } from '../dev/tracing.js';
 import { component_context, is_runes } from '../context.js';
+import { active_fork, Fork } from './forks.js';
 
 export let inspect_effects = new Set();
 
@@ -174,6 +175,9 @@ export function internal_set(source, value) {
 		source.v = value;
 		source.wv = increment_write_version();
 
+		const fork = Fork.ensure();
+		fork.capture(source, old_value);
+
 		if (DEV && tracing_mode_flag) {
 			source.updated = get_stack('UpdatedAt');
 			if (active_effect != null) {
@@ -260,7 +264,7 @@ export function update_pre(source, d = 1) {
  * @param {number} status should be DIRTY or MAYBE_DIRTY
  * @returns {void}
  */
-function mark_reactions(signal, status) {
+export function mark_reactions(signal, status) {
 	var reactions = signal.reactions;
 	if (reactions === null) return;
 
@@ -271,9 +275,6 @@ function mark_reactions(signal, status) {
 		var reaction = reactions[i];
 		var flags = reaction.f;
 
-		// Skip any effects that are already dirty
-		if ((flags & DIRTY) !== 0) continue;
-
 		// In legacy mode, skip the current effect to prevent infinite loops
 		if (!runes && reaction === active_effect) continue;
 
@@ -285,13 +286,10 @@ function mark_reactions(signal, status) {
 
 		set_signal_status(reaction, status);
 
-		// If the signal a) was previously clean or b) is an unowned derived, then mark it
-		if ((flags & (CLEAN | UNOWNED)) !== 0) {
-			if ((flags & DERIVED) !== 0) {
-				mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
-			} else {
-				schedule_effect(/** @type {Effect} */ (reaction));
-			}
+		if ((flags & DERIVED) !== 0) {
+			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
+		} else {
+			schedule_effect(/** @type {Effect} */ (reaction));
 		}
 	}
 }

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

@@ -53,6 +53,8 @@ import {
 import { Boundary } from './dom/blocks/boundary.js';
 import * as w from './warnings.js';
 import { is_firefox } from './dom/operations.js';
+import { active_fork, Fork } from './reactivity/forks.js';
+import { log_effect_tree } from './dev/debug.js';
 
 const FLUSH_MICROTASK = 0;
 const FLUSH_SYNC = 1;
@@ -702,10 +704,14 @@ function flush_queued_root_effects(root_effects) {
 			}
 
 			var collected_effects = process_effects(effect);
-			flush_queued_effects(collected_effects);
+
+			if (/** @type {Fork} */ (active_fork).settled()) {
+				flush_queued_effects(collected_effects);
+			}
 		}
 	} finally {
 		is_flushing_effect = previously_flushing_effect;
+		Fork.unset();
 	}
 }
 
@@ -805,14 +811,16 @@ export function schedule_effect(signal) {
  * effects to be flushed.
  *
  * @param {Effect} effect
- * @param {Effect[]} effects
- * @param {Boundary} [boundary]
  * @returns {Effect[]}
  */
-function process_effects(effect, effects = [], boundary) {
+function process_effects(effect) {
 	var current_effect = effect.first;
 
-	var current_effect = effect.first;
+	/** @type {Effect[]} */
+	var render_effects = [];
+
+	/** @type {Effect[]} */
+	var effects = [];
 
 	main_loop: while (current_effect !== null) {
 		var flags = current_effect.f;
@@ -820,64 +828,24 @@ function process_effects(effect, effects = [], boundary) {
 		var is_skippable_branch = is_branch && (flags & CLEAN) !== 0;
 		var sibling = current_effect.next;
 
-		if (!is_skippable_branch && (flags & INERT) === 0) {
-			if (boundary !== undefined && (flags & (BLOCK_EFFECT | BRANCH_EFFECT | EFFECT_ASYNC)) === 0) {
-				// Inside a boundary, defer everything except block/branch effects
-				boundary.add_effect(current_effect);
-			} else if ((flags & BOUNDARY_EFFECT) !== 0) {
-				var b = /** @type {Boundary} */ (current_effect.b);
+		var skip =
+			is_skippable_branch ||
+			(flags & INERT) !== 0 ||
+			active_fork?.skipped_effects.has(current_effect);
 
-				process_effects(current_effect, effects, b);
-
-				if (!b.suspended) {
-					// no more async work to happen
-					b.commit();
+		if (!skip) {
+			if ((flags & (BLOCK_EFFECT | EFFECT_ASYNC)) !== 0) {
+				if (check_dirtiness(current_effect)) {
+					update_effect(current_effect);
 				}
 			} else if ((flags & RENDER_EFFECT) !== 0) {
 				if (is_branch) {
 					current_effect.f ^= CLEAN;
 				} else {
-					// Ensure we set the effect to be the active reaction
-					// to ensure that unowned deriveds are correctly tracked
-					// because we're flushing the current effect
-					var previous_active_reaction = active_reaction;
-					try {
-						active_reaction = current_effect;
-						if (check_dirtiness(current_effect)) {
-							update_effect(current_effect);
-						}
-					} catch (error) {
-						handle_error(error, current_effect, null, current_effect.ctx);
-					} finally {
-						active_reaction = previous_active_reaction;
-					}
-				}
-
-				var child = current_effect.first;
-
-				if (child !== null) {
-					current_effect = child;
-					continue;
+					render_effects.push(current_effect);
 				}
 			} else if ((flags & EFFECT) !== 0) {
 				effects.push(current_effect);
-			} else if (is_branch) {
-				current_effect.f ^= CLEAN;
-			} else {
-				// Ensure we set the effect to be the active reaction
-				// to ensure that unowned deriveds are correctly tracked
-				// because we're flushing the current effect
-				var previous_active_reaction = active_reaction;
-				try {
-					active_reaction = current_effect;
-					if (check_dirtiness(current_effect)) {
-						update_effect(current_effect);
-					}
-				} catch (error) {
-					handle_error(error, current_effect, null, current_effect.ctx);
-				} finally {
-					active_reaction = previous_active_reaction;
-				}
 			}
 
 			var child = current_effect.first;
@@ -908,7 +876,7 @@ function process_effects(effect, effects = [], boundary) {
 		current_effect = sibling;
 	}
 
-	return effects;
+	return [...render_effects, ...effects];
 }
 
 /**