将值分配给组件状态时，为什么 console.log 会打印以前的状态?

When value is assigned to components state, why console.log prints the previous state?(将值分配给组件状态时，为什么 console.log 会打印以前的状态?)

问题描述

我正在将数字值从 Numbers 组件发送到 Main 组件.在我将 Main 组件中的该值设置为该组件的状态之前，一切正常.

var Numbers = React.createClass({处理点击:函数(数字){this.props.num(数字)},渲染:函数(){返回 (
<button onClick={this.handleClick.bind(null, 1)}>1</button><button onClick={this.handleClick.bind(null, 2)}>2</button><button onClick={this.handleClick.bind(null, 3)}>3</button></div>)}})var Main = React.createClass({获取初始状态:函数(){返回 {数量:0}},处理回调:函数(编号){console.log("数字就在这里:" + num);这个.setState({数:数})console.log("但是这里错了(前一个数字):" + this.state.number)},渲染:函数(){返回 (<数字 num={this.handleCallback}/>//<SomeComponent number={this.state.number}/>)}});React.render(<Main/>, document.getElementById('container'));

为什么会这样?handleCallback 函数中的第二个 console.log 打印前一个数字，而不是 num 参数中的数字.我需要正确的数字才能处于我的状态，因为我将立即将其作为 SomeComponent 组件中的道具发送.

https://jsfiddle.net/69z2wepo/13000/

来自 文档:

setState() 不会立即改变 this.state 而是创建一个等待状态转换.调用 this 后访问 this.state方法可能会返回现有值.没有保证对 setState 的调用和调用的同步操作可能进行批处理以获得性能提升.

如果要在调用 setState 后打印更改，请使用可选的回调参数:

this.setState({数:数}， 功能 () {控制台.log(this.state.number);});

I'm sending values of numbers from Numbers component to Main component. Everything is working fine until I set that value in my Main component to that component's state.

var Numbers = React.createClass({
    handleClick: function(number){
        this.props.num(number)
    },
    render: function(){
        return (
            <div>
                <button onClick={this.handleClick.bind(null, 1)}>1</button>
                <button onClick={this.handleClick.bind(null, 2)}>2</button>
                <button onClick={this.handleClick.bind(null, 3)}>3</button>
            </div>
        )
    }
})

var Main = React.createClass({
    getInitialState: function(){
        return {
            number: 0
        }
    },
    handleCallback: function(num){
        console.log("number is right here: " + num);
        this.setState({
            number: num
        })
        console.log("but wrong here (previous number): " + this.state.number)
    },
    render: function() {
        return (
            <Numbers num={this.handleCallback} />
            //<SomeComponent number={this.state.number} />
        )
    }
});

React.render(<Main />, document.getElementById('container'));

Why is it behaving like this? Second console.log in handleCallback function prints the previous number, not the number which is in num parameter. I need right number to be in my state, because I'm going to send it immediately as an props in my SomeComponent component.

https://jsfiddle.net/69z2wepo/13000/

From the docs:

setState() does not immediately mutate this.state but creates a pending state transition. Accessing this.state after calling this method can potentially return the existing value. There is no guarantee of synchronous operation of calls to setState and calls may be batched for performance gains.

If you want to print the change after a call to setState, use the optional callback parameter:

this.setState({
    number: num
}, function () {
    console.log(this.state.number);
});

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