最简单的useState实现
看下面的代码:
const Demo = ()=> {
const [n,setN] = React.useState(0)
return <div>
{n}
<button onClick={()=> {setN(n+1)}}>点我+1</button>
</div>
}
const rootElement = document.getElementById("app")
ReactDOM.render(<Demo />, rootElement);
做了哪些事:
首次渲染:render()------得到Demo组件------调用Demo组件------得到虚拟div------创建真实的div
点击button后:调用setN(n+1)------再次render()------得到Demo组件------调用Demo组件------得到虚拟div------使用DOM Diff对比------更新真实的div
往后一次重复第二次的操作
分析:
state:每个组件都应该有自己的数据state,
setN:setN一定会修改数据state,将n+1存入state,然后重新render(),再次渲染
useState:useState肯定会从state读取n的最新值
尝试实现React.useState
let _state
const myUseState = (initState) => {
_state = _state ===undefined ? initState : _state
const setState = (newValue =>{
_state = newValue
render()
})
return [_state,setState]
}
const render =()=>{
ReactDOM.render(<Demo />, rootElement);
}
const Demo = ()=> {
const [n,setN] = myUseState(0)
console.log('n变了');
console.log(`n的值是: ${n}`);
return <div>{n}
<button onClick={()=> {setN(n+1)}}>点我+1</button>
</div>
}
const rootElement = document.getElementById("app")
ReactDOM.render(<Demo />, rootElement);
多点几次,打印的结果看下图
打印的结果也确实对,却又一个很大的问题:如果一个组件用了两个useState怎么办?
let _state
const myUseState = (initState) => {
_state = _state ===undefined ? initState : _state
const setState = (newValue =>{
_state = newValue
render()
})
return [_state,setState]
}
const render =()=>{
ReactDOM.render(<Demo />, rootElement);
}
const Demo = ()=> {
const [n,setN] = myUseState(0)
const [m,setM] = myUseState(0)
return <div>{n}
<button onClick={()=> {setN(n+1)}}>点我+1</button>
<br/>
{m}
<button onClick={()=> {setM(m+1)}}>点我+1</button>
</div>
}
const rootElement = document.getElementById("app")
ReactDOM.render(<Demo />, rootElement);
把上面的代码运行,你会发现,无论你点击哪个button,n和m都会+1,
现在我们来对它改进,_state不能直接让它等于一个值,而是让它等于很多值,只有对象和数组能做到,而对象不太合适,因为_state = {n: 0,m: 0}
,useState()
并不知道变量叫n
还是m
,只能把它做成数组,比如:_state = [0,0]
(前面0是n,后面0是m)
let _state = []
let index = 0
const myUseState = (initState) => {
const currentIndex = index
_state[currentIndex] = _state[currentIndex] ===undefined ? initState : _state[currentIndex]
const setState = (newValue =>{
_state[currentIndex] = newValue
console.log(_state);
render()
})
index += 1
return [_state[currentIndex],setState]
}
const render =()=>{
index = 0
ReactDOM.render(<Demo />, rootElement);
}
const Demo = ()=> {
const [n,setN] = myUseState(0)
const [m,setM] = myUseState(0)
return <div>{n}
<button onClick={()=> {setN(n+1)}}>点我+1</button>
<br/>
{m}
<button onClick={()=> {setM(m+1)}}>点我+1</button>
</div>
}
const rootElement = document.getElementById("app")
ReactDOM.render(<Demo />, rootElement);
最终代码就是上面的代码,用log可以打出_state
的值,我们分别点击button让它们+1,看log出的值
这样,我们自己写的_state
是不是就和React的一模一样?
通过实现简单的useState,能让我们更好地理解useState