react setState

既是同步也是异步
在事件回调和生命周期函数里面是异步

在定时器，原生dom事件，promise中是同步

一个函数里面多次异步setState：

多次次函数模式setState会合并一次

2（执行顺序） ==> 0（输出值）

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 (

{count}

)
}
}