fix: make prefresh hmr integration easier

The callback to `useMemo` is assumed to be pure during HMR and will be re-triggered. This in turn broke the `useSignal` hooks HMR as they used side-effectful callbacks.

Author: marvinhagemeister

packages/preact/src/index.ts

@@ -1,5 +1,5 @@
 import { options, Component, isValidElement, Fragment } from "preact";
-import { useRef, useMemo, useEffect } from "preact/hooks";
+import { useRef, useEffect } from "preact/hooks";
 import {
 	signal,
 	computed,
@@ -92,7 +92,7 @@ function SignalValue(this: AugmentedComponent, { data }: { data: Signal }) {
 	const currentSignal = useSignal(data);
 	currentSignal.value = data;
 
-	const [isText, s] = useMemo(() => {
+	const cb = () => {
 		let self = this;
 		// mark the parent component as having computeds so it gets optimized
 		let v = this.__v;
@@ -139,7 +139,12 @@ function SignalValue(this: AugmentedComponent, { data }: { data: Signal }) {
 		};
 
 		return [isText, wrappedSignal];
-	}, []);
+	};
+
+	const ref = useRef<ReadonlySignal<any>[] | null>(null);
+	if (ref.current === null) {
+		ref.current = cb();
+	}
 
 	// Rerender the component whenever `data.value` changes from a VNode
 	// to another VNode, from text to a VNode, or from a VNode to text.
@@ -150,6 +155,7 @@ function SignalValue(this: AugmentedComponent, { data }: { data: Signal }) {
 	//
 	// For text-to-text updates, `.peek()` is used to skip full rerenders,
 	// leaving them to the optimized path above.
+	const [isText, s] = ref.current;
 	return isText.value ? s.peek() : s.value;
 }
 SignalValue.displayName = "_st";
@@ -387,17 +393,21 @@ Component.prototype.shouldComponentUpdate = function (
 export function useSignal<T>(value: T, options?: SignalOptions<T>): Signal<T>;
 export function useSignal<T = undefined>(): Signal<T | undefined>;
 export function useSignal<T>(value?: T, options?: SignalOptions<T>) {
-	return useMemo(
-		() => signal<T | undefined>(value, options as SignalOptions),
-		[]
-	);
+	const cb = () => signal<T | undefined>(value, options as SignalOptions);
+	const ref = useRef<Signal<T | undefined> | null>(null);
+	if (!ref.current) ref.current = cb();
+	return ref.current;
 }
 
 export function useComputed<T>(compute: () => T, options?: SignalOptions<T>) {
 	const $compute = useRef(compute);
 	$compute.current = compute;
 	(currentComponent as AugmentedComponent)._updateFlags |= HAS_COMPUTEDS;
-	return useMemo(() => computed<T>(() => $compute.current(), options), []);
+	const ref = useRef<ReadonlySignal<T> | null>(null);
+	if (!ref.current) {
+		ref.current = computed<T>(() => $compute.current(), options);
+	}
+	return ref.current;
 }
 
 function safeRaf(callback: () => void) {