我有一个子组件,它位于父组件的循环内。当子组件之一更新父组件的状态时,它会重新渲染所有子组件,因为它是循环的。我怎样才能避免每次迭代的重新渲染。
function Parent() {
const [selectedChild, setSelectedChild] = useState([]);
const onChangeHandle = (event, id) => {
const checked = event.target.checked;
let updatedArray = [...selectedChild];
if(checked){
if(!selectedChild.includes(id)){
updatedArray.push(id);
}
}
else{
var index = updatedArray.indexOf(id)
if (index !== -1) {
updatedArray.splice(index, 1);
}
}
setSelectedChild(updatedArray);
}
const dummy = (id) => {
return selectedChild.includes(id);
}
return (
<div>
<table>
<tbody>
{[1,2,3].map((value, index) => {
return (
<Child
key={index}
index={index}
value={value}
handle={onChangeHandle}
isSelected={dummy}
/>
)
})}
</tbody>
</table>
<div>
{selectedChild}
</div>
</div>)
}
function Child({index, value, handle, isSelected }) {
console.log('rendering')
return (
<tr>
<td>
<input
type="checkbox"
checked={isSelected(index)}
onChange={(event) => handle(event, index)}/>
</td>
<td>hello {index} {value}</td>
</tr>
)
}
export default function App() {
return (
<div className="App">
<Parent />
</div>
);
}
当前行为:在上面的代码中,当我单击子组件之一中的复选框时,它正在更新父组件状态(selectedChild
)。因此循环正在执行,所有子项(所有表行)都在重新渲染。
预期行为:只有特定的行需要重新渲染
Demo: https://codesandbox.io/s/newpro-0pezc https://codesandbox.io/s/newpro-0pezc
为此你可以使用React.memo https://pt-br.reactjs.org/docs/react-api.html#reactmemo如果 props 保持不变,这将记住你的组件。但鉴于您的代码,您需要进行一些额外的更改:
-
你必须申请useCallback
记住 onChangeHandle 函数;
-
要正确记住 onChangeHandle,您需要重构它。你不能通过selectedChild
直接,否则它会记住它的值。使用setSelectedChild
传递一个函数作为参数,该函数接受selectedChild
反而。
-
您的孩子应该收到isSelected
作为布尔值而不是函数。否则 props 将保持不变并且 Child 永远不会更新;
import React, { useState, memo, useCallback } from "react";
function Parent() {
const [selectedChild, setSelectedChild] = useState([]);
const onChangeHandle = useCallback((event, id) => {
setSelectedChild(selectedChild => {
const checked = event.target.checked;
let updatedArray = [...selectedChild];
if (checked) {
if (!selectedChild.includes(id)) {
updatedArray.push(id);
}
} else {
var index = updatedArray.indexOf(id);
if (index !== -1) {
updatedArray.splice(index, 1);
}
}
return updatedArray;
});
}, []);
const dummy = id => {
return selectedChild.includes(id);
};
const renderChildren = () =>
[1, 2, 3].map((value, index) => {
return (
<Child
key={index}
index={index}
value={value}
handle={onChangeHandle}
isSelected={dummy(index)}
/>
);
});
return (
<div>
<table>
<tbody>{renderChildren()}</tbody>
</table>
<div>{selectedChild}</div>
</div>
);
}
const Child = memo(({ index, value, handle, isSelected }) => {
console.log("rendering");
return (
<tr>
<td>
<input
type="checkbox"
checked={isSelected}
onChange={event => handle(event, index)}
/>
</td>
<td>
hello {index} {value}
</td>
</tr>
);
});
export default function App() {
return (
<div className="App">
<Parent />
</div>
);
}
https://stackblitz.com/edit/so-memo-children?file=src/App.js https://stackblitz.com/edit/so-memo-children?file=src/App.js
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