useEffect()

1. 使用useEffect 如何判断多个值都改变了才执行内部方法? ```javascript import React, { useState, useEffect, useRef } from “react”

const One = () => { const [a, setA] = useState(0) const [b, setB] = useState(0) const ref = useRef({ a, b })

useEffect(() => {
    let { a: prevA, b: prevB } = ref.current
    console.log("更新前:", prevA, prevB)
    console.log("更新后:", a, b)
    if (prevA !== a && prevB !== b) {
        console.log("update!")
        ref.current = { a, b }
    }
}, [a, b])
return (
    <>
        <h1>{a + b}</h1>
        <button onClick={(_) => setA((d) => d + 1)}>Chang A</button>
        <button onClick={(_) => setB((d) => d + 1)}>Chang B</button>
    </>
)

}

export default One

```