reactjs - 状态变量在功能组件中有错误的值
问题描述
我试图在表单中添加和删除输入框。我将用 useState 钩子控制这个盒子。我对添加状态变量没有任何问题,但是当我想从状态变量中删除时,状态变量的值错误。
import React, { useState } from 'react';
export default function Addable() {
const [element, setElement] = useState([]);
function add() {
const list = [...element];
const length = element.length;
list.push(
<React.Fragment>
<input key={length} />
<button onClick={()=>{_delete(length)}}>delete</button>
</React.Fragment>,
);
setElement(list);
}
function _delete(index) {
//****element has wrong value here****//
const list = [...element];
list.splice(index, 1);
setElement(list);
}
return (
<React.Fragment>
<button onClick={add}>add Element</button>
{element}
</React.Fragment>
);
}
解决方案
切片函数不会改变list
数组
const animals = ['ant', 'bison', 'camel', 'duck', 'elephant'];
animals.slice(2)
console.log(animals);
您可能想改用splice,以防止关闭问题,传递一个函数来settlement
设置当前状态的状态
function _delete(index) {
//****element has wrong value here****//
console.log(element);
setElement((currentEl) => {
const list = [...currentEl];
list.splice(index, 1);
return list
});
}
注意:如果使用不受控制的输入组件,则需要为输入生成唯一的 id 并将其设置为键。因为 React 会检查 key 以知道要删除哪个元素。例如,你的元素长度为 5,然后你删除元素索引2
,所以现在元素长度为 4,但索引 2 仍然存在,所以 React 不知道你删除索引 2(因为键 2 仍然存在)所以它会移动 3到 2、4 到 3..检查这个以获得更好的解释和演示
你需要创建一个唯一的id,uuid作为选择
function add() {
const list = [...element];
const length = element.length;
const uniqueId = uuid()
list.push(
{id: uniqueId
comp: () => (<React.Fragment>
<input key={uniqueId} />
// index is length-1
<button onClick={()=>{_delete(uniqueId)}}>delete</button>
</React.Fragment>)
}
);
setElement(list);
}
function _delete(elementId) {
// Because we set new state on current state
// it's better to pass an function into setElement like this to prevent some problem with closure
setElement(currently =>{
return currently.filter(el => el.id !== elementId );
});
}
return (
<React.Fragment>
<button onClick={add}>add Element</button>
{element.map(el => el.comp())}
</React.Fragment>
);