请问react中useEffect useRef useState useMemo有什么区别与不同？
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1、useState
返回一个 state，以及更新 state 的函数。
语法:
const [n, setN] = React.useState(0); 0是n的默认值,setN是操作n的函数
在初始渲染期间，返回的状态 (state) 与传入的第一个参数 (initialState) 值相同。setState 函数用于更新 state。它接收一个新的 state 值并将组件的一次重新渲染加入队列。
函数式更新 如果新的 state 需要通过使用先前的 state 计算得出，那么可以将函数传递给 setState。该函数将接收先前的 state，并返回一个更新后的值。下面的计数器组件示例展示了 setState 的两种用法：<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
</head>
<body>
<div id="box"></div>
<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<script src="https://cdn.bootcss.com/babel-standalone/7.0.0-beta.3/babel.js"></script>
<script type="text/babel">
const {
useState,
useEffect,
useMemo,
useRef
} = React
function Compont() {
const [state,
setState] = useState('初始值');
return ( <div> < buttonBfwOnClick= {(e) => setState("我的值改变了")} >
click Me! {
state
} </button> < buttonBfwOnClick= {(e) => setState("bfw")} >
click Me! {
state
} </button> </div>
)
}
ReactDOM.render( < Compont/>, document.getElementById('box'))
</script>
</body>
</html>2、useEffect
监听第二个参数[]的state的变化,执行第一个参数
useEffect第二个参数为空数组的时候,相当于componentDidMount
useEffect在第一个参数(函数主体里return相当于componentWillUnmount)<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
</head>
<body>
<div id="box"></div>
<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<script src="https://cdn.bootcss.com/babel-standalone/7.0.0-beta.3/babel.js"></script>
<script type="text/babel">
const {
useState,
useEffect,
useMemo,
useRef
} = React
function Compont() {
const [time,
settime] = useState(Date());
let timer = 0;
useEffect(() => {
timer = setInterval(() => {
settime(Date())
}, 1000);
return ()=> {
clearInterval(timer)
}
}, []);
return time
}
ReactDOM.render( <Compont/>, document.getElementById('box'))
</script>
</body>
</html>
如果在第一个参数里存在定时器记得在return的时候清除掉,不清除掉会抱错
devScripts.js:5035 Warning: Can’t perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.
函数名字首字母记得大写否则会报这个错
“useEffect” is called in function “dateTime” that is neither a React function component nor a custom React Hook
useEffect监听变量的时候,初始值也会监听到,所以,做个判断,可以减少一次不必要的渲染
useEffect(() => {
if(state){
console.log(state,'state')
}
}, [state])监听多个变量的时候,任意一个变量发生变化都会执行
3、useRef
useRef 返回一个可变的 ref 对象，其 .current 属性被初始化为传入的参数（initialValue）。返回的 ref 对象在组件的整个生命周期内保持不变。 一个常见的用例便是命令式地访问子组件：<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
</head>
<body>
<div id="box"></div>
<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<script src="https://cdn.bootcss.com/babel-standalone/7.0.0-beta.3/babel.js"></script>
<script type="text/babel">
const {useRef} = React
function Compont() {
const username = useRef()
const password = useRef()
const handleClick = () => {
console.log('用户名：' + username.current.value)
console.log('密码：' + password.current.value)
}
return(
<div>
用户名：<input type="text" ref={username} /> <br/>
密码：<input type="password" ref={password} /> <br/>
<buttonBfwOnClick={handleClick}>点击登入</button>
</div>
)
}
ReactDOM.render( <Compont/>, document.getElementById('box'))
</script>
</body>
</html>4、useMemo
大概就是使用最新的数据,返回新的Dom节点 使用的时候就像state一样使用 {变量名} 就可以了 好处:可以避免不必要的重复渲染,也不怕拿不到最新的数据去渲染
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
</head>
<body>
<div id="box"></div>
<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<script src="https://cdn.bootcss.com/babel-standalone/7.0.0-beta.3/babel.js"></script>
<script type="text/babel">
const { useState, useEffect,useMemo,useRef } = React
function App() {
const [name, setName] = useState('名称')
const [content,setContent] = useState('内容')
return (
<div>
<buttonBfwOnClick={() => setName(new Date().getTime())}>name</button>
<buttonBfwOnClick={() => setContent(new Date().getTime())}>content</button>
<Button name={name}>{content}</Button> </div>
)
}
function Button({ name, children }) {
function changeName(name) {
console.log('11')
return name + '改变name的方法'
}
const otherName = useMemo(()=>changeName(name),[name])
return ( <p>
<div>{otherName}</div>
<div>{children}</div> </p>
)
}
ReactDOM.render( <App/>, document.getElementById('box'))
</script>
</body>
</html>by thinkfuture
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