7 files changed, 278 insertions, 0 deletions
diff --git a/packages/integrations/preact/src/client-dev.ts b/packages/integrations/preact/src/client-dev.ts
new file mode 100644
index 000000000..9a9edcb3b
--- /dev/null
+++ b/packages/integrations/preact/src/client-dev.ts
@@ -0,0 +1,5 @@
+// @ts-ignore
+import 'preact/debug';
+import clientFn from './client.js';
+
+export default clientFn;
diff --git a/packages/integrations/preact/src/client.ts b/packages/integrations/preact/src/client.ts
new file mode 100644
index 000000000..c7a31d60c
--- /dev/null
+++ b/packages/integrations/preact/src/client.ts
@@ -0,0 +1,29 @@
+import type { SignalLike } from './types';
+import { h, render } from 'preact';
+import StaticHtml from './static-html.js';
+
+const sharedSignalMap: Map<string, SignalLike> = new Map();
+
+export default (element: HTMLElement) =>
+	async (Component: any, props: Record<string, any>, { default: children, ...slotted }: Record<string, any>) => {
+		if (!element.hasAttribute('ssr')) return;
+		for (const [key, value] of Object.entries(slotted)) {
+			props[key] = h(StaticHtml, { value, name: key });
+		}
+		let signalsRaw = element.dataset.preactSignals;
+		if(signalsRaw) {
+			const { signal } = await import('@preact/signals');
+			let signals: Record<string, string> = JSON.parse(element.dataset.preactSignals as string);
+			for(const [propName, signalId] of Object.entries(signals)) {
+				if(!sharedSignalMap.has(signalId)) {
+					const signalValue = signal(props[propName]);
+					sharedSignalMap.set(signalId, signalValue);
+				}
+				props[propName] = sharedSignalMap.get(signalId);
+			}
+		}
+		render(
+			h(Component, props, children != null ? h(StaticHtml, { value: children }) : children),
+			element
+		);
+	};
diff --git a/packages/integrations/preact/src/context.ts b/packages/integrations/preact/src/context.ts
new file mode 100644
index 000000000..73c4402b0
--- /dev/null
+++ b/packages/integrations/preact/src/context.ts
@@ -0,0 +1,32 @@
+import type { RendererContext, SignalLike, PropNameToSignalMap } from './types';
+
+export type Context = {
+	id: string;
+	c: number;
+	signals: Map<SignalLike, string>;
+	propsToSignals: Map<Record<string, any>, PropNameToSignalMap>;
+};
+
+const contexts = new WeakMap<RendererContext['result'], Context>();
+
+export function getContext(result: RendererContext['result']): Context {
+	if (contexts.has(result)) {
+		return contexts.get(result)!;
+	}
+	let ctx = {
+		c: 0,
+		get id() {
+			return 'p' + this.c.toString();
+		},
+		signals: new Map(),
+		propsToSignals: new Map()
+	};
+	contexts.set(result, ctx);
+	return ctx;
+}
+
+export function incrementId(ctx: Context): string {
+	let id = ctx.id;
+	ctx.c++;
+	return id;
+}
diff --git a/packages/integrations/preact/src/server.ts b/packages/integrations/preact/src/server.ts
new file mode 100644
index 000000000..2c1ac738a
--- /dev/null
+++ b/packages/integrations/preact/src/server.ts
@@ -0,0 +1,126 @@
+import type { AstroPreactAttrs, RendererContext } from './types';
+import { h, Component as BaseComponent } from 'preact';
+import render from 'preact-render-to-string';
+import StaticHtml from './static-html.js';
+import { getContext } from './context.js';
+import { restoreSignalsOnProps, serializeSignals } from './signals.js';
+
+const slotName = (str: string) => str.trim().replace(/[-_]([a-z])/g, (_, w) => w.toUpperCase());
+
+let originalConsoleError: typeof console.error;
+let consoleFilterRefs = 0;
+
+function check(this: RendererContext, Component: any, props: Record<string, any>, children: any) {
+	if (typeof Component !== 'function') return false;
+
+	if (Component.prototype != null && typeof Component.prototype.render === 'function') {
+		return BaseComponent.isPrototypeOf(Component);
+	}
+
+	useConsoleFilter();
+
+	try {
+		try {
+			const { html } = renderToStaticMarkup.call(this, Component, props, children);
+			if (typeof html !== 'string') {
+				return false;
+			}
+
+			// There are edge cases (SolidJS) where Preact *might* render a string,
+			// but components would be <undefined></undefined>
+
+			return !/\<undefined\>/.test(html);
+		} catch (err) {
+			return false;
+		}
+	} finally {
+		finishUsingConsoleFilter();
+	}
+}
+
+function renderToStaticMarkup(this: RendererContext, Component: any, props: Record<string, any>, { default: children, ...slotted }: Record<string, any>) {
+	const ctx = getContext(this.result);
+
+	const slots: Record<string, ReturnType<typeof h>> = {};
+	for (const [key, value] of Object.entries(slotted)) {
+		const name = slotName(key);
+		slots[name] = h(StaticHtml, { value, name });
+	}
+
+	// Restore signals back onto props so that they will be passed as-is to components
+	let propsMap = restoreSignalsOnProps(ctx, props);
+
+	const newProps = { ...props, ...slots };
+
+	const attrs: AstroPreactAttrs = {};
+	serializeSignals(ctx, props, attrs, propsMap);
+
+	const html = render(
+		h(Component, newProps, children != null ? h(StaticHtml, { value: children }) : children)
+	);
+	return {
+		attrs,
+		html
+	};
+}
+
+
+/**
+ * Reduces console noise by filtering known non-problematic errors.
+ *
+ * Performs reference counting to allow parallel usage from async code.
+ *
+ * To stop filtering, please ensure that there always is a matching call
+ * to `finishUsingConsoleFilter` afterwards.
+ */
+function useConsoleFilter() {
+	consoleFilterRefs++;
+
+	if (!originalConsoleError) {
+		// eslint-disable-next-line no-console
+		originalConsoleError = console.error;
+
+		try {
+			// eslint-disable-next-line no-console
+			console.error = filteredConsoleError;
+		} catch (error) {
+			// If we're unable to hook `console.error`, just accept it
+		}
+	}
+}
+
+/**
+ * Indicates that the filter installed by `useConsoleFilter`
+ * is no longer needed by the calling code.
+ */
+function finishUsingConsoleFilter() {
+	consoleFilterRefs--;
+
+	// Note: Instead of reverting `console.error` back to the original
+	// when the reference counter reaches 0, we leave our hook installed
+	// to prevent potential race conditions once `check` is made async
+}
+
+/**
+ * Hook/wrapper function for the global `console.error` function.
+ *
+ * Ignores known non-problematic errors while any code is using the console filter.
+ * Otherwise, simply forwards all arguments to the original function.
+ */
+function filteredConsoleError(msg: string, ...rest: any[]) {
+	if (consoleFilterRefs > 0 && typeof msg === 'string') {
+		// In `check`, we attempt to render JSX components through Preact.
+		// When attempting this on a React component, React may output
+		// the following error, which we can safely filter out:
+		const isKnownReactHookError =
+			msg.includes('Warning: Invalid hook call.') &&
+			msg.includes('https://reactjs.org/link/invalid-hook-call');
+		if (isKnownReactHookError) return;
+	}
+	originalConsoleError(msg, ...rest);
+}
+
+export default {
+	check,
+	renderToStaticMarkup,
+};
diff --git a/packages/integrations/preact/src/signals.ts b/packages/integrations/preact/src/signals.ts
new file mode 100644
index 000000000..db62961aa
--- /dev/null
+++ b/packages/integrations/preact/src/signals.ts
@@ -0,0 +1,48 @@
+import type { AstroPreactAttrs, PropNameToSignalMap, SignalLike } from './types';
+import type { Context } from './context';
+import { incrementId } from './context.js';
+
+function isSignal(x: any): x is SignalLike {
+	return x != null && typeof x === 'object' && typeof x.peek === 'function' && 'value' in x;
+}
+
+export function restoreSignalsOnProps(ctx: Context, props: Record<string, any>) {
+	// Restore signal props that were mutated for serialization
+	let propMap: PropNameToSignalMap;
+	if(ctx.propsToSignals.has(props)) {
+		propMap = ctx.propsToSignals.get(props)!
+	} else {
+		propMap = new Map();
+		ctx.propsToSignals.set(props, propMap);
+	}
+	for(const [key, signal] of propMap) {
+		props[key] = signal;
+	}
+	return propMap;
+}
+
+export function serializeSignals(ctx: Context, props: Record<string, any>, attrs: AstroPreactAttrs, map: PropNameToSignalMap){ 
+		// Check for signals
+		const signals: Record<string, string> = {};
+		for(const [key, value] of Object.entries(props)) {
+			if(isSignal(value)) {
+				// Set the value to the current signal value
+				// This mutates the props on purpose, so that it will be serialized correct.
+				props[key] = value.peek();
+				map.set(key, value);
+	
+				let id: string;
+				if(ctx.signals.has(value)) {
+					id = ctx.signals.get(value)!;
+				} else {
+					id = incrementId(ctx);
+					ctx.signals.set(value, id);
+				}
+				signals[key] = id;
+			}
+		}
+
+		if(Object.keys(signals).length) {
+			attrs['data-preact-signals'] = JSON.stringify(signals);
+		}
+}
diff --git a/packages/integrations/preact/src/static-html.ts b/packages/integrations/preact/src/static-html.ts
new file mode 100644
index 000000000..e474caa5a
--- /dev/null
+++ b/packages/integrations/preact/src/static-html.ts
@@ -0,0 +1,24 @@
+import { h } from 'preact';
+
+/**
+ * Astro passes `children` as a string of HTML, so we need
+ * a wrapper `div` to render that content as VNodes.
+ *
+ * As a bonus, we can signal to Preact that this subtree is
+ * entirely static and will never change via `shouldComponentUpdate`.
+ */
+const StaticHtml = ({ value, name }: { value: string; name?: string; }) => {
+	if (!value) return null;
+	return h('astro-slot', { name, dangerouslySetInnerHTML: { __html: value } });
+};
+
+/**
+ * This tells Preact to opt-out of re-rendering this subtree,
+ * In addition to being a performance optimization,
+ * this also allows other frameworks to attach to `children`.
+ *
+ * See https://preactjs.com/guide/v8/external-dom-mutations
+ */
+StaticHtml.shouldComponentUpdate = () => false;
+
+export default StaticHtml;
diff --git a/packages/integrations/preact/src/types.ts b/packages/integrations/preact/src/types.ts
new file mode 100644
index 000000000..e5058f5f3
--- /dev/null
+++ b/packages/integrations/preact/src/types.ts
@@ -0,0 +1,14 @@
+import type { SSRResult } from 'astro';
+export type RendererContext = {
+	result: SSRResult;
+};
+
+export type SignalLike = {
+	peek(): any;
+};
+
+export type PropNameToSignalMap = Map<string, SignalLike>;
+
+export type AstroPreactAttrs = {
+	['data-preact-signals']?: string
+};