小主您可能经常需要从一个状态 衍生 另一个状态，为此贴心的Svelte提供了$derived符文(rune):

Often, you will need to derive state from other state. For this, we have the $derived rune:

let numbers = $state([1, 2, 3, 4]);
let total = $derived(numbers.reduce((t, n) => t + n, 0));

我们可以像往常一样在标签中使用它:

We can now use this in our markup:

<p>{numbers.join(' + ')} = {total}</p>

使用$derived声明的表达式只有在它的依赖状态（在这个例子中就是numbers）更新时才会重新计算。不同于普通状态，衍生状态都是只读的。

The expression inside the $derived declaration will be re-evaluated whenever its dependencies (in this case, just numbers) are updated. Unlike normal state, derived state is read-only.

是不是跟React中的useCallback, useMemo类似啊？！小主。