feat: snapshots (#8710)

* add failing test

* make it possible to read snapshot props

* refactor sessionStorage stuff a bit

* snapshots

* working

* beef up test

* tidy up

* fix type

* run all tests

* err....

* lint

* account for missing layouts etc

* thank you, past us, for creating accessors for export const automatically

* don't restore if navigation was blocked

* update scroll position at same time as snapshot

* DRY

* types

* docs

* changeset

* add a comment

* only capture snapshot when appropriate

* Update packages/kit/src/runtime/client/client.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

* document caveat around large snapshots

* destroy alternate timelines

* fix

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/little-countries-cough.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: add snapshot mechanism for preserving ephemeral DOM state

documentation/docs/30-advanced/65-snapshots.md

@@ -0,0 +1,33 @@
+---
+title: Snapshots
+---
+
+Ephemeral DOM state — like scroll positions on sidebars, the content of `<input>` elements and so on — is discarded when you navigate from one page to another.
+
+For example, if the user fills out a form but clicks a link before submitting, then hits the browser's back button, the values they filled in will be lost. In cases where it's valuable to preserve that input, you can take a _snapshot_ of DOM state, which can then be restored if the user navigates back.
+
+To do this, export a `snapshot` object with `capture` and `restore` methods from a `+page.svelte` or `+layout.svelte`:
+
+```svelte
+/// file: +page.svelte
+<script>
+	let comment = '';
+
+	/** @type {import('./$types').Snapshot<string>} */
+	export const snapshot = {
+		capture: () => comment,
+		restore: (value) => comment = value
+	};
+</script>
+
+<form method="POST">
+	<textarea bind:value={comment} />
+	<button>Post comment</button>
+</form>
+```
+
+When you navigate away from this page, the `capture` function is called immediately before the page updates, and the returned value is associated with the current entry in the browser's history stack. If you navigate back, the `restore` function is called with the stored value as soon as the page is updated.
+
+The data must be serializable as JSON so that it can be persisted to `sessionStorage`. This allows the state to be restored when the page is reloaded, or when the user navigates back from a different site.
+
+> Avoid returning very large objects from `capture` — once captured, objects will be retained in memory for the duration of the session, and in extreme cases may be too large to persist to `sessionStorage`.

packages/kit/src/core/sync/write_root.js

@@ -21,16 +21,16 @@ export function write_root(manifest_data, output) {
 
 	let l = max_depth;
 
-	let pyramid = `<svelte:component this={components[${l}]} data={data_${l}} {form} />`;
+	let pyramid = `<svelte:component this={constructors[${l}]} bind:this={components[${l}]} data={data_${l}} {form} />`;
 
 	while (l--) {
 		pyramid = `
-			{#if components[${l + 1}]}
-				<svelte:component this={components[${l}]} data={data_${l}}>
+			{#if constructors[${l + 1}]}
+				<svelte:component this={constructors[${l}]} bind:this={components[${l}]} data={data_${l}}>
 					${pyramid.replace(/\n/g, '\n\t\t\t\t\t')}
 				</svelte:component>
 			{:else}
-				<svelte:component this={components[${l}]} data={data_${l}} {form} />
+				<svelte:component this={constructors[${l}]} bind:this={components[${l}]} data={data_${l}} {form} />
 			{/if}
 		`
 			.replace(/^\t\t\t/gm, '')
@@ -49,7 +49,8 @@ export function write_root(manifest_data, output) {
 				export let stores;
 				export let page;
 
-				export let components;
+				export let constructors;
+				export let components = [];
 				export let form;
 				${levels.map((l) => `export let data_${l} = null;`).join('\n\t\t\t\t')}
 

packages/kit/src/core/sync/write_types/index.js

@@ -198,23 +198,26 @@ function update_types(config, routes, route, to_delete = new Set()) {
 
 	// These could also be placed in our public types, but it would bloat them unnecessarily and we may want to change these in the future
 	if (route.layout || route.leaf) {
-		// If T extends the empty object, void is also allowed as a return type
-		declarations.push(`type MaybeWithVoid<T> = {} extends T ? T | void : T;`);
-		// Returns the key of the object whose values are required.
 		declarations.push(
-			`export type RequiredKeys<T> = { [K in keyof T]-?: {} extends { [P in K]: T[K] } ? never : K; }[keyof T];`
-		);
-		// Helper type to get the correct output type for load functions. It should be passed the parent type to check what types from App.PageData are still required.
-		// If none, void is also allowed as a return type.
-		declarations.push(
-			`type OutputDataShape<T> = MaybeWithVoid<Omit<App.PageData, RequiredKeys<T>> & Partial<Pick<App.PageData, keyof T & keyof App.PageData>> & Record<string, any>>`
-		);
-		// null & {} == null, we need to prevent that in some situations
-		declarations.push(`type EnsureDefined<T> = T extends null | undefined ? {} : T;`);
-		// Takes a union type and returns a union type where each type also has all properties
-		// of all possible types (typed as undefined), making accessing them more ergonomic
-		declarations.push(
-			`type OptionalUnion<U extends Record<string, any>, A extends keyof U = U extends U ? keyof U : never> = U extends unknown ? { [P in Exclude<A, keyof U>]?: never } & U : never;`
+			// If T extends the empty object, void is also allowed as a return type
+			`type MaybeWithVoid<T> = {} extends T ? T | void : T;`,
+
+			// Returns the key of the object whose values are required.
+			`export type RequiredKeys<T> = { [K in keyof T]-?: {} extends { [P in K]: T[K] } ? never : K; }[keyof T];`,
+
+			// Helper type to get the correct output type for load functions. It should be passed the parent type to check what types from App.PageData are still required.
+			// If none, void is also allowed as a return type.
+			`type OutputDataShape<T> = MaybeWithVoid<Omit<App.PageData, RequiredKeys<T>> & Partial<Pick<App.PageData, keyof T & keyof App.PageData>> & Record<string, any>>`,
+
+			// null & {} == null, we need to prevent that in some situations
+			`type EnsureDefined<T> = T extends null | undefined ? {} : T;`,
+
+			// Takes a union type and returns a union type where each type also has all properties
+			// of all possible types (typed as undefined), making accessing them more ergonomic
+			`type OptionalUnion<U extends Record<string, any>, A extends keyof U = U extends U ? keyof U : never> = U extends unknown ? { [P in Exclude<A, keyof U>]?: never } & U : never;`,
+
+			// Re-export `Snapshot` from @sveltejs/kit — in future we could use this to infer <T> from the return type of `snapshot.capture`
+			`export type Snapshot<T = any> = Kit.Snapshot<T>;`
 		);
 	}
 

packages/kit/src/runtime/client/client.js

@@ -15,6 +15,7 @@ import {
 	is_external_url,
 	scroll_state
 } from './utils.js';
+import * as storage from './session-storage.js';
 import {
 	lock_fetch,
 	unlock_fetch,
@@ -31,7 +32,7 @@ import { HttpError, Redirect } from '../control.js';
 import { stores } from './singletons.js';
 import { unwrap_promises } from '../../utils/promises.js';
 import * as devalue from 'devalue';
-import { INDEX_KEY, PRELOAD_PRIORITIES, SCROLL_KEY } from './constants.js';
+import { INDEX_KEY, PRELOAD_PRIORITIES, SCROLL_KEY, SNAPSHOT_KEY } from './constants.js';
 import { validate_common_exports } from '../../utils/exports.js';
 import { compact } from '../../utils/array.js';
 
@@ -52,12 +53,10 @@ default_error_loader();
 
 /** @typedef {{ x: number, y: number }} ScrollPosition */
 /** @type {Record<number, ScrollPosition>} */
-let scroll_positions = {};
-try {
-	scroll_positions = JSON.parse(sessionStorage[SCROLL_KEY]);
-} catch {
-	// do nothing
-}
+const scroll_positions = storage.get(SCROLL_KEY) ?? {};
+
+/** @type {Record<string, any[]>} */
+const snapshots = storage.get(SNAPSHOT_KEY) ?? {};
 
 /** @param {number} index */
 function update_scroll_positions(index) {
@@ -75,6 +74,14 @@ export function create_client({ target }) {
 	/** @type {Array<((url: URL) => boolean)>} */
 	const invalidated = [];
 
+	/**
+	 * An array of the `+layout.svelte` and `+page.svelte` component instances
+	 * that currently live on the page — used for capturing and restoring snapshots.
+	 * It's updated/manipulated through `bind:this` in `Root.svelte`.
+	 * @type {import('svelte').SvelteComponent[]}
+	 */
+	const components = [];
+
 	/** @type {{id: string, promise: Promise<import('./types').NavigationResult>} | null} */
 	let load_cache = null;
 
@@ -158,6 +165,20 @@ export function create_client({ target }) {
 		await update(intent, url, []);
 	}
 
+	/** @param {number} index */
+	function capture_snapshot(index) {
+		if (components.some((c) => c?.snapshot)) {
+			snapshots[index] = components.map((c) => c?.snapshot?.capture());
+		}
+	}
+
+	/** @param {number} index */
+	function restore_snapshot(index) {
+		snapshots[index]?.forEach((value, i) => {
+			components[i]?.snapshot?.restore(value);
+		});
+	}
+
 	/**
 	 * @param {string | URL} url
 	 * @param {{ noScroll?: boolean; replaceState?: boolean; keepFocus?: boolean; state?: any; invalidateAll?: boolean }} opts
@@ -238,11 +259,20 @@ export function create_client({ target }) {
 	 * @param {import('./types').NavigationIntent | undefined} intent
 	 * @param {URL} url
 	 * @param {string[]} redirect_chain
+	 * @param {number} [previous_history_index]
 	 * @param {{hash?: string, scroll: { x: number, y: number } | null, keepfocus: boolean, details: { replaceState: boolean, state: any } | null}} [opts]
 	 * @param {{}} [nav_token] To distinguish between different navigation events and determine the latest. Needed for example for redirects to keep the original token
 	 * @param {() => void} [callback]
 	 */
-	async function update(intent, url, redirect_chain, opts, nav_token = {}, callback) {
+	async function update(
+		intent,
+		url,
+		redirect_chain,
+		previous_history_index,
+		opts,
+		nav_token = {},
+		callback
+	) {
 		token = nav_token;
 		let navigation_result = intent && (await load_route(intent));
 
@@ -301,11 +331,28 @@ export function create_client({ target }) {
 
 		updating = true;
 
+		// `previous_history_index` will be undefined for invalidation
+		if (previous_history_index) {
+			update_scroll_positions(previous_history_index);
+			capture_snapshot(previous_history_index);
+		}
+
 		if (opts && opts.details) {
 			const { details } = opts;
 			const change = details.replaceState ? 0 : 1;
 			details.state[INDEX_KEY] = current_history_index += change;
 			history[details.replaceState ? 'replaceState' : 'pushState'](details.state, '', url);
+
+			if (!details.replaceState) {
+				// if we navigated back, then pushed a new state, we can
+				// release memory by pruning the scroll/snapshot lookup
+				let i = current_history_index + 1;
+				while (snapshots[i] || scroll_positions[i]) {
+					delete snapshots[i];
+					delete scroll_positions[i];
+					i += 1;
+				}
+			}
 		}
 
 		// reset preload synchronously after the history state has been set to avoid race conditions
@@ -384,10 +431,12 @@ export function create_client({ target }) {
 
 		root = new Root({
 			target,
-			props: { ...result.props, stores },
+			props: { ...result.props, stores, components },
 			hydrate: true
 		});
 
+		restore_snapshot(current_history_index);
+
 		/** @type {import('types').AfterNavigate} */
 		const navigation = {
 			from: null,
@@ -445,7 +494,7 @@ export function create_client({ target }) {
 			},
 			props: {
 				// @ts-ignore Somehow it's getting SvelteComponent and SvelteComponentDev mixed up
-				components: compact(branch).map((branch_node) => branch_node.node.component)
+				constructors: compact(branch).map((branch_node) => branch_node.node.component)
 			}
 		};
 
@@ -1091,7 +1140,8 @@ export function create_client({ target }) {
 			return;
 		}
 
-		update_scroll_positions(current_history_index);
+		// store this before calling `accepted()`, which may change the index
+		const previous_history_index = current_history_index;
 
 		accepted();
 
@@ -1105,6 +1155,7 @@ export function create_client({ target }) {
 			intent,
 			url,
 			redirect_chain,
+			previous_history_index,
 			{
 				scroll,
 				keepfocus,
@@ -1385,12 +1436,10 @@ export function create_client({ target }) {
 			addEventListener('visibilitychange', () => {
 				if (document.visibilityState === 'hidden') {
 					update_scroll_positions(current_history_index);
+					storage.set(SCROLL_KEY, scroll_positions);
 
-					try {
-						sessionStorage[SCROLL_KEY] = JSON.stringify(scroll_positions);
-					} catch {
-						// do nothing
-					}
+					capture_snapshot(current_history_index);
+					storage.set(SNAPSHOT_KEY, snapshots);
 				}
 			});
 
@@ -1537,7 +1586,7 @@ export function create_client({ target }) {
 				});
 			});
 
-			addEventListener('popstate', (event) => {
+			addEventListener('popstate', async (event) => {
 				if (event.state?.[INDEX_KEY]) {
 					// if a popstate-driven navigation is cancelled, we need to counteract it
 					// with history.go, which means we end up back here, hence this check
@@ -1555,8 +1604,9 @@ export function create_client({ target }) {
 					}
 
 					const delta = event.state[INDEX_KEY] - current_history_index;
+					let blocked = false;
 
-					navigate({
+					await navigate({
 						url: new URL(location.href),
 						scroll,
 						keepfocus: false,
@@ -1567,10 +1617,15 @@ export function create_client({ target }) {
 						},
 						blocked: () => {
 							history.go(-delta);
+							blocked = true;
 						},
 						type: 'popstate',
 						delta
 					});
+
+					if (!blocked) {
+						restore_snapshot(current_history_index);
+					}
 				}
 			});
 

packages/kit/src/runtime/client/constants.js

@@ -1,3 +1,4 @@
+export const SNAPSHOT_KEY = 'sveltekit:snapshot';
 export const SCROLL_KEY = 'sveltekit:scroll';
 export const INDEX_KEY = 'sveltekit:index';
 

packages/kit/src/runtime/client/session-storage.js

@@ -0,0 +1,25 @@
+/**
+ * Read a value from `sessionStorage`
+ * @param {string} key
+ */
+export function get(key) {
+	try {
+		return JSON.parse(sessionStorage[key]);
+	} catch {
+		// do nothing
+	}
+}
+
+/**
+ * Write a value to `sessionStorage`
+ * @param {string} key
+ * @param {any} value
+ */
+export function set(key, value) {
+	const json = JSON.stringify(value);
+	try {
+		sessionStorage[key] = json;
+	} catch {
+		// do nothing
+	}
+}

packages/kit/src/runtime/server/page/render.js

@@ -90,7 +90,7 @@ export async function render_response({
 				navigating: writable(null),
 				updated
 			},
-			components: await Promise.all(branch.map(({ node }) => node.component())),
+			constructors: await Promise.all(branch.map(({ node }) => node.component())),
 			form: form_value
 		};
 

packages/kit/test/apps/basics/src/routes/snapshot/+layout.svelte

@@ -0,0 +1,5 @@
+<a href="/snapshot/a">a</a>
+<a href="/snapshot/b">b</a>
+<a href="/snapshot/c" data-sveltekit-reload>c</a>
+
+<slot />

packages/kit/test/apps/basics/src/routes/snapshot/a/+page.svelte

@@ -0,0 +1,11 @@
+<script>
+	let message = '';
+
+	/** @type {import('./$types').Snapshot<string>} */
+	export const snapshot = {
+		capture: () => message,
+		restore: (snapshot) => (message = snapshot)
+	};
+</script>
+
+<input bind:value={message} />

packages/kit/test/apps/basics/src/routes/snapshot/b/+page.svelte

@@ -0,0 +1 @@
+<h1>b</h1>

packages/kit/test/apps/basics/src/routes/snapshot/c/+page.svelte

@@ -0,0 +1 @@
+<h1>c</h1>