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在通过遍历数组来创建组件的时候react往往提示你要为组件设置一个唯一key值。很多情况下一般就直接用索引来当这个值了,方便又简单如:
this.state.cells.map(({name,x,y},index) =>
<Cell name={name} key={index} x={x} y={y} onRemove={(t) => this.onRemove(t)}/>
)
但这里有个问题,假如这个数组是动态可变的,你会发现简单的只设置key=index会带来不必要的重绘开支。现在来看看这种情况。
我们有两个组件,画布容器Spacer,独立组件Cell,容器中有添加按钮,每点一次就添加一个Cell,Cell就一个label用来区分,还有删除按钮来移除该Cell的简单组件。
这两个组件的代码如下:
import React, { PropTypes } from 'react';
import Cell from './Cell.jsx';
import _ from 'lodash';
class Spacer extends React.Component {
constructor(props) {
super(props);
this.state = {
cells: [],
};
}
addCell() {
this.state.cells.push({
name: _.uniqueId('label'),
x: _.random(1000),
y: _.random(800),
});
this.setState({cells: this.state.cells});
}
onRemove(name) {
const index = _.findIndex(this.state.cells, 'name', name);
const cells = this.state.cells.splice(index,1);
this.setState({cells: this.state.cells});
}
render() {
console.log('>>> Spacer:render', this.state.cells);
return (
<div style={{
width: '100%',
height: '100%',
padding: '10px',
}}>
<button onClick={() => this.addCell()}>添加</button>
{
this.state.cells.map(({name,x,y},index) =>
<Cell name={name} key={index} x={x} y={y} onRemove={(t) => this.onRemove(t)}/>
)
}
</div>
);
}
}
Spacer.propTypes = {
foo: PropTypes.string.isRequired,
};
Spacer.defaultProps = {
foo: 'bar',
};
export default Spacer;
import React, { PropTypes } from 'react';
class Cell extends React.Component {
constructor(props) {
super(props);
this.state = {
name: 'bar',
};
}
shouldComponentUpdate(nextProps, nextState) {
console.log('>>> Cell:shouldComponentUpdate',this.props.name,nextProps.name);
return this.props.name !== nextProps.name ||
this.props.x !== nextProps.x ||
this.props.y !== nextProps.y;
}
componentDidUpdate(prevProps, prevState) {
console.log('>>> Cell:componentDidUpdate');
}
render() {
const { name, x, y, onRemove } = this.props;
console.log('>>> Cell:render',name);
return (
<div style={{
position: 'absolute',
padding: '4px',
width: '100px',
height: '100px',
border: '1px solid',
left: x + 'px',
top: y + 'px',
}}>
{name}
<div style={{
width: '100%',
height: '20px',
cursor: 'pointer',
background: '#aabbcc',
}} onClick={() => onRemove(name)}>
删除
</div>
</div>
);
}
}
Cell.propTypes = {
x: PropTypes.number.isRequired,
y: PropTypes.number.isRequired,
name: PropTypes.string.isRequired,
onRemove: PropTypes.func.isRequired,
};
Cell.defaultProps = {
name: 'bar',
};
export default Cell;
假如现在你添加了四个组件:c1,c2,c3,c4,打开控制台,因为Cell组件中我们在shouldComponentUpdate方法里加了判断只有组件名字或者坐标发生了变化才重绘,因此添加时c1,c2,c3,c4格子只重绘一次。
现在你从最末起删除c4,控制没有提示有组件重绘,符合咱们预期。留下c1,c2,c3。
你再从中间开始删除,你删除c2,那么你会发现c3重绘了一次?
为啥会发生这样的关联?删除c2跟c3有啥关系?
在Spacer中遍历cells数组时给每个Cell设置的key值是index!!!,react会根据key来判断是否重构和删除,因为c2删除了c3的index值上升了一位,因此实际上c3的props用的还是c2的,但于此同时又手动设置一遍c3的props,这对c3来说相当于一次props更新,于是就又重绘了一次。假如现在还有这四个组件,我们删除掉c1,则c2,c3,c4将各自重绘一遍。
为避免这种无必要的重绘,只需把key的值和name保持一直即可,设置key=name,那么一切即正常了。
this.state.cells.map(({name,x,y},index) =>
<Cell name={name} key={name} x={x} y={y} onRemove={(t) => this.onRemove(t)}/>
)
对于动态数组来说,谨慎设置key尤为重要,这个东西你设错了也没大关系不会影响功能,就是没设置也顶多在控制台打印个警告,但是数组一大,还是很影响性能的。
使用CSS Module也有两个月了,整体上感觉良好。因为是用的webpack导入样式,有个小问题:
引用第三方库时如果他需要引用自己的css文件,那么就不能用webpack导入
有两个解决方法:
目前采用的是第二种方式,so far so good。
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