snapshot maps

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

@@ -16,3 +16,4 @@ export const EFFECT_RAN = 1 << 13;
 export const EFFECT_TRANSPARENT = 1 << 14;
 
 export const STATE_SYMBOL = Symbol('$state');
+export const STATE_SNAPSHOT_SYMBOL = Symbol('$state.snapshot');

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

@@ -1,4 +1,4 @@
-import { STATE_SYMBOL } from '../constants.js';
+import { STATE_SNAPSHOT_SYMBOL, STATE_SYMBOL } from '../constants.js';
 import { array_prototype, get_prototype_of, is_array, object_prototype } from '../utils.js';
 
 /**
@@ -13,17 +13,22 @@ export function snapshot(value, deep = false, values = new Map()) {
 		return value;
 	}
 
+	var unwrapped = /** @type {T} */ (values.get(value));
+	if (unwrapped !== undefined) {
+		return unwrapped;
+	}
+
+	if (STATE_SNAPSHOT_SYMBOL in value) {
+		// @ts-expect-error
+		return value[STATE_SNAPSHOT_SYMBOL](deep);
+	}
+
 	var proto = get_prototype_of(value);
 
 	if (
 		(proto === object_prototype || proto === array_prototype) &&
 		(deep || STATE_SYMBOL in value)
 	) {
-		var unwrapped = /** @type {T} */ (values.get(value));
-		if (unwrapped !== undefined) {
-			return unwrapped;
-		}
-
 		if (is_array(value)) {
 			var length = value.length;
 			var array = Array(length);

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

@@ -15,7 +15,8 @@ import {
 	BRANCH_EFFECT,
 	STATE_SYMBOL,
 	BLOCK_EFFECT,
-	ROOT_EFFECT
+	ROOT_EFFECT,
+	STATE_SNAPSHOT_SYMBOL
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { add_owner } from './dev/ownership.js';
@@ -1137,6 +1138,12 @@ export function deep_read(value, visited = new Set()) {
 		!visited.has(value)
 	) {
 		visited.add(value);
+
+		if (STATE_SNAPSHOT_SYMBOL in value) {
+			value[STATE_SNAPSHOT_SYMBOL](true);
+			return;
+		}
+
 		for (let key in value) {
 			try {
 				deep_read(value[key], visited);
@@ -1144,6 +1151,7 @@ export function deep_read(value, visited = new Set()) {
 				// continue
 			}
 		}
+
 		const proto = get_prototype_of(value);
 		if (
 			proto !== Object.prototype &&

packages/svelte/src/reactivity/map.js

@@ -3,6 +3,8 @@ import { source, set } from '../internal/client/reactivity/sources.js';
 import { get } from '../internal/client/runtime.js';
 import { UNINITIALIZED } from '../constants.js';
 import { map } from './utils.js';
+import { STATE_SNAPSHOT_SYMBOL } from '../internal/client/constants.js';
+import { snapshot } from '../internal/client/reactivity/snapshot.js';
 
 /**
  * @template K
@@ -155,4 +157,15 @@ export class ReactiveMap extends Map {
 	get size() {
 		return get(this.#size);
 	}
+
+	/** @param {boolean} deep */
+	[STATE_SNAPSHOT_SYMBOL](deep) {
+		return new Map(
+			map(
+				this.#sources.entries(),
+				([key, source]) => /** @type {[K, V]} */ ([key, snapshot(get(source), deep)]),
+				'Map Iterator'
+			)
+		);
+	}
 }