feat(chapter3): add useRef demo

Demonstrate retaining the previous counter value with a ref while updating state through a shadcn button.

chapter3/src/app/demo/_components/ref.tsx

@@ -0,0 +1,31 @@
+// src/app/demo/_components/ref.tsx
+'use client';
+
+import { type FC, useEffect, useState, useRef } from 'react';
+import { Button } from '@/app/_components/shadcn/ui/button';
+import clsx from 'clsx';
+import $styles from './style.module.css';
+
+const RefDemo: FC = () => {
+    const [count, setCount] = useState(0);
+    const inited = useRef(count);
+    useEffect(() => {
+        if (inited.current !== count) {
+            inited.current = count;
+            console.log('changed');
+        }
+    }, [count]);
+    return (
+        <div className={clsx($styles.container, 'w-80')}>
+            <h2 className="text-center">useRef Demo</h2>
+            <p className="py-5 text-center">{count}</p>
+            <div className="flex justify-around">
+                <Button onClick={() => setCount(Math.ceil(Math.random() * 10))} variant="outline">
+                    变化
+                </Button>
+            </div>
+        </div>
+    );
+};
+
+export default RefDemo;