WIP

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -306,7 +306,7 @@ export class Boundary {
 
 			if (this.#main_effect !== null) {
 				// TODO do we also need to `resume_effect` here?
-				schedule_effect(this.#main_effect);
+				// schedule_effect(this.#main_effect);
 			}
 		}
 	}

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

@@ -138,7 +138,8 @@ export function async_derived(fn, location) {
 						internal_set(signal, v);
 					});
 				} else {
-					internal_set(signal, v);
+					signal.v = v;
+					// internal_set(signal, v);
 				}
 
 				if (DEV && location !== undefined) {

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

@@ -1,5 +1,4 @@
 /** @import { Effect, Source } from '#client' */
-
 import { flush_sync } from '../runtime.js';
 import { internal_set } from './sources.js';
 
@@ -17,48 +16,57 @@ export class Fork {
 	/** @type {Map<Source, any>} */
 	previous = new Map();
 
+	/** @type {Map<Source, any>} */
+	current = 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);
-		}
-	}
-
-	/**
-	 *
-	 * @param {() => void} fn
-	 */
-	flush(fn) {
+	apply() {
 		var values = new Map();
 
+		for (const source of this.previous.keys()) {
+			values.set(source, source.v);
+		}
+
 		for (const fork of forks) {
 			if (fork === this) continue;
 
 			for (const [source, previous] of fork.previous) {
-				if (this.previous.has(source)) continue;
-
-				values.set(source, source.v);
-				source.v = previous;
-				// internal_set(source, previous);
+				if (!values.has(source)) {
+					values.set(source, source.v);
+					// internal_set(source, previous);
+					source.v = previous;
+				}
 			}
 		}
 
-		try {
-			fn();
-		} finally {
+		for (const [source, current] of this.current) {
+			source.v = current;
+			// internal_set(source, current);
+		}
+
+		return () => {
 			for (const [source, value] of values) {
-				// internal_set(source, value);
 				source.v = value;
 			}
+
+			active_fork = null;
+		};
+	}
+
+	/**
+	 * @param {Source} source
+	 * @param {any} value
+	 */
+	capture(source, value) {
+		if (!this.previous.has(source)) {
+			this.previous.set(source, value);
 		}
+
+		this.current.set(source, source.v);
 	}
 
 	remove() {

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

@@ -170,6 +170,8 @@ export function set(source, value) {
  * @returns {V}
  */
 export function internal_set(source, value) {
+	// console.trace('internal_set', source.v, value);
+
 	if (!source.equals(value)) {
 		var old_value = source.v;
 		source.v = value;

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

@@ -695,6 +695,9 @@ function flush_queued_root_effects(root_effects) {
 	var previously_flushing_effect = is_flushing_effect;
 	is_flushing_effect = true;
 
+	var fork = /** @type {Fork} */ (active_fork);
+	var revert = fork.apply();
+
 	try {
 		for (var i = 0; i < length; i++) {
 			var effect = root_effects[i];
@@ -703,17 +706,23 @@ function flush_queued_root_effects(root_effects) {
 				effect.f ^= CLEAN;
 			}
 
-			var fork = /** @type {Fork} */ (active_fork);
 			var collected_effects = process_effects(effect);
 
 			if (fork.settled()) {
 				flush_queued_effects(collected_effects);
-				fork.remove();
 			}
 		}
 	} finally {
 		is_flushing_effect = previously_flushing_effect;
-		Fork.unset();
+
+		// TODO this doesn't seem quite right — may run into
+		// interesting cases where there are multiple roots.
+		// it'll do for now though
+		if (fork.settled()) {
+			fork.remove();
+		}
+
+		revert();
 	}
 }