React进阶：setState()的同步、异步、多次调用
setState()的同步、异步、多次调用
一、01_setState()的使用
1.setState()更新状态的2种写法
1). setState(updater, [callback]),
updater为返回stateChange对象的函数: (state, props) => stateChange
接收的state和props被保证为最新的
2). setState(stateChange, [callback])
stateChange为对象,
callback是可选的回调函数, 在状态更新且界面更新后才执行
2. 总结:
对象方式是函数方式的简写方式
如果新状态不依赖于原状态 ===> 使用对象方式
如果新状态依赖于原状态 ===> 使用函数方式
如果需要在setState()后获取最新的状态数据, 在第二个callback函数中读取
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>01_setState()的使用</title>
</head>
<body>
<div id="example"></div>
<script type="text/javascript" src="./js/react.development.js"></script>
<script type="text/javascript" src="./js/react-dom.development.js"></script>
<script type="text/javascript" src="./js/babel.min.js"></script>
<script src="https://cdn.bootcss.com/remarkable/1.7.1/remarkable.min.js"></script>
<script type="text/babel">
class A extends React.Component {
state = {
count: 1
}
test1 = () => {
this.setState(state => ({count: state.count + 1}))
console.log('test1 setState()之后', this.state.count) //输出：1
}
test2 = () => {
/*const count = this.state.count + 1
this.setState({
count
})*/
this.setState({
count: 3
})
console.log('test2 setState()之后', this.state.count) //输出：1
}
test3 = () => {
this.setState(state => ({count: state.count + 1}), () => { // 在状态更新且界面更新之后回调
console.log('test3 setState callback()', this.state.count) //输出2
})
}
render() {
console.log('A render()')
return (
<div>
<h1>A组件: {this.state.count}</h1>
<button onClick={this.test1}>A 测试1</button>
<button onClick={this.test2}>A 测试2</button>
<button onClick={this.test3}>A 测试3</button>
</div>
)
}
}
ReactDOM.render(<A/>, document.getElementById('example'))
</script>
</body>
</html>
二、02_setState()异步OR同步更新
1. setState()更新状态是异步还是同步的?
1). 执行setState()的位置?
在react控制的回调函数中: 生命周期勾子 / react事件监听回调 => 异步
非react控制的异步回调函数中: 定时器回调 / 原生事件监听回调 / promise回调 /... => 同步
2.关于异步的setState()
1). 多次调用, 如何处理?
setState({}): 合并更新一次状态, 只调用一次render()更新界面 ---状态更新和界面更新都合并了
setState(fn): 更新多次状态, 但只调用一次render()更新界面 ---状态更新没有合并, 但界面更新合并了
2). 如何得到异步更新后的状态数据?
在setState()的callback回调函数中
3.为什么在react相关调用中是异步，其它是同步？
在react中用到了【事务】，它存在就是异步，而react之外未用到【事务】所以就是同步。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>02_setState()异步OR同步更新</title>
</head>
<body>
<div id="example"></div>
<script type="text/javascript" src="./js/react.development.js"></script>
<script type="text/javascript" src="./js/react-dom.development.js"></script>
<script type="text/javascript" src="./js/babel.min.js"></script>
<script type="text/babel">
class StateTest extends React.Component {
state = {
count: 0,
}
/*
【异步】react事件监听回调中, setState()是异步更新状态
*/
update1 = () => {
console.log('update1 setState()之前', this.state.count) // =>0
this.setState(state => ({count: state.count + 1}))
console.log('update1 setState()之后', this.state.count) // =>0
}
/*
react生命周期勾子中, setState()是异步更新状态（在其它函数运行时，此处注释掉）
componentDidMount () {
console.log('componentDidMount setState()之前', this.state.count) //=>0
this.setState(state => ({count: state.count + 1}))
console.log('componentDidMount setState()之后', this.state.count) //=>0
}*/
/*
【同步】定时器回调 / 原生事件监听回调 / promise回调 /...
*/
update2 = () => {//同步
setTimeout(() => {
console.log('setTimeout setState()之前', this.state.count) // 0
this.setState(state => ({count: state.count + 1}))
console.log('setTimeout setState()之后', this.state.count) // 1
},1000) //1秒后执行
}
update3 = () => {//同步
const h2 = this.refs.count //根据ref找到h2，即页面显示的数字
h2.onclick = () => { //点过更新按钮之后，再点h2里的数字才执行
console.log('onclick setState()之前', this.state.count) // 0
this.setState(state => ({count: state.count + 1}))
console.log('onclick setState()之后', this.state.count) // 1
}
}
update4 = () => {//同步
Promise.resolve().then(value => {
console.log('Promise setState()之前', this.state.count) //0
this.setState(state => ({count: state.count + 1}))
console.log('Promise setState()之后', this.state.count) //1
})
}
update5 = () => {
//setState(fn): 更新多次状态, 但只调用一次render()更新界面--状态更新没有合并, 但界面更新合并了
console.log('onclick setState()之前', this.state.count) // 0
this.setState(state => ({count: state.count + 1}))
console.log('onclick setState()之后', this.state.count) // 0
console.log('onclick setState()之前2', this.state.count) // 0
this.setState(state => ({count: state.count + 1}))
console.log('onclick setState()之后2', this.state.count) // 0
}
update6 = () => {
//setState({}):合并更新一次状态, 只调用一次render()更新界面--状态更新和界面更新都合并了
console.log('onclick setState()之前', this.state.count) // 0
this.setState({count: this.state.count + 1})
console.log('onclick setState()之后', this.state.count) // 0
console.log('onclick setState()之前2', this.state.count) // 0
this.setState({count: this.state.count + 1})
console.log('onclick setState()之后2', this.state.count) // 0
}
update7 = () => {//setState({}) setState(fn)5-6混合：
console.log('onclick setState()之前', this.state.count) // 0
this.setState({count: this.state.count + 1})
console.log('onclick setState()之后', this.state.count) // 0
console.log('onclick setState()之前2', this.state.count) // 0
this.setState(state => ({count: state.count + 1}))
console.log('onclick setState()之后2', this.state.count) // 0
}
update8 = () => {//利用回调显示state更新后的值
console.log('onclick setState()之前', this.state.count) //0
this.setState({count: this.state.count + 1},
()=>{console.log('onclick setState()之后', this.state.count)}) //1
}
render() {
const {count} = this.state
console.log('render()', count)
return (
<div>
<h2 ref='count'>{count}</h2>
<button onClick={this.update1}>更新1</button> ---
<button onClick={this.update2}>更新2</button>
<button onClick={this.update3}>更新3</button>
<button onClick={this.update4}>更新4</button> ---
<button onClick={this.update5}>更新5</button>
<button onClick={this.update6}>更新6</button>
<button onClick={this.update7}>更新7</button>
<button onClick={this.update8}>更新8</button>
</div>
)
}
}
ReactDOM.render(<StateTest/>, document.getElementById('example')) // 渲染组件标签, 内部会调用组件标签对象的render()虚拟DOM
</script>
</body>
</html>
三、setState()面试题
2（执行顺序） ==> 0（输出值）
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>03_setState()面试题</title>
</head>
<body>
<div id="example"></div>
<script type="text/javascript" src="./js/react.development.js"></script>
<script type="text/javascript" src="./js/react-dom.development.js"></script>
<script type="text/javascript" src="./js/babel.min.js"></script>
<script type="text/babel">
class StateTest extends React.Component {
state = {
count: 0,
}
componentDidMount() {
this.setState({count: this.state.count + 1})
this.setState({count: this.state.count + 1})
console.log('[1]',this.state.count) // 2 ==> 0
this.setState(state => ({count: state.count + 1}))
this.setState(state => ({count: state.count + 1}))
console.log('[2]',this.state.count) // 3 ==> 0
setTimeout(() => {
this.setState({count: this.state.count + 1})
console.log('timeout', this.state.count) // 10 ==> 6
this.setState({count: this.state.count + 1})
console.log('timeout', this.state.count) // 12 ==> 7
}, 0)
Promise.resolve().then(value => {
this.setState({count: this.state.count + 1})
console.log('promise', this.state.count) // 6 ==>4
this.setState({count: this.state.count + 1})
console.log('promise', this.state.count) // 8 ==> 5
})
}
render() {
const count = this.state.count
console.log('render', count) // 1 ==> 0 4 ==>3 5 ==>4 7 ==>5 9 ==>6 11 ==>7
return (
<div>
<p>{count}</p>
</div>
)
}
}
ReactDOM.render(<StateTest/>, document.getElementById('example'))
</script>
</body>
</html>
源码： https://github.com/chenxi188/admin-client_final/tree/master/test