如何在Svelte中生成派生变量？

<script>
  import { derived } from 'svelte/store';
  import { count } from './store.js';
  
  // 定义计算属性
  export const doubled = derived(count, $count => $count * 2);
</script>

<main>
  <p>The count value is {$count}</p>
  <p>The doubled value is {$doubled}</p>
</main>

<script>
  import { count } from './store.js';
  import { get } from 'svelte/store';
  import { onDestroy } from 'svelte';

  let doubled;

  // 订阅依赖项
  const unsubscribe = count.subscribe(($count) => {
    doubled = $count * 2;
  });

  // 当组件销毁时取消订阅
  onDestroy(() => {
    unsubscribe();
  });
</script>

<main>
  <p>The count value is {$count}</p>
  <p>The doubled value is {doubled}</p>
</main>