在 setState 更新状态之前调用渲染

Question

有什么方法可以确保在setState 实际更新状态之后调用render？据我所知，setState 是异步的。我尝试将render 作为setState 的回调，但它仍然会在状态更新之前呈现。

HTML 代码：

<div id="root"></div>

反应代码：

class Table extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            squares: []
        };
        this.create = this.create.bind(this);
    }

    create() {
        console.log('squares before: ' + this.state.squares);
        var a = [];
        for (var i = 0; i < 16; i++) {
            a.push(undefined);
        }
        i = Math.floor(Math.random() * 8);
        a[i] = 2;
        i = Math.floor(Math.random() * 8) + 8;
        a[i] = 2;
        console.log('new squares: ' + squares);
        console.log('non-updated squares: ' + this.state.squares);
        this.setState({
            squares: a
        }, console.log('updated squares: ' + this.state.squares);
    }

    componentWillMount() {
        this.create();
    }

    render() {
        console.log('rendering');
        return (<div>
                <table className="table">
                    <tbody>
                        <tr>
                            <td className="td"><Number number={this.state.squares[0]}/></td>
                            <td className="td"><Number number={this.state.squares[1]}/></td>
                            <td className="td"><Number number={this.state.squares[2]}/></td>
                            <td className="td"><Number number={this.state.squares[3]}/></td>
                        </tr>
                        <tr>
                            <td className="td"><Number number={this.state.squares[4]}/></td>
                            <td className="td"><Number number={this.state.squares[5]}/></td>
                            <td className="td"><Number number={this.state.squares[6]}/></td>
                            <td className="td"><Number number={this.state.squares[7]}/></td>
                        </tr>
                        <tr>
                            <td className="td"><Number number={this.state.squares[8]}/></td>
                            <td className="td"><Number number={this.state.squares[9]}/></td>
                            <td className="td"><Number number={this.state.squares[10]}/></td>
                            <td className="td"><Number number={this.state.squares[11]}/></td>
                        </tr>
                        <tr>
                            <td className="td"><Number number={this.state.squares[12]}/></td>
                            <td className="td"><Number number={this.state.squares[13]}/></td>
                            <td className="td"><Number number={this.state.squares[14]}/></td>
                            <td className="td"><Number number={this.state.squares[15]}/></td>
                        </tr>
                    </tbody>
                </table>
            </div>);
    }
}

class Game extends React.Component {
    newGame() {
        this.refs.table.create();
    }

    render() {
        return (<div>
            <Table ref="table"/>
            <button id="newGame" onClick={this.newGame.bind(this)}>New Game</button>
        </div>);
    }
}

基本上，这里发生的事情是首先记录“渲染”，然后是“更新方块：...”，如下所示：

squares before: [undefined, 2, undefined, undefined, undefined, 2]
new squares: [undefined, undefined, 2, undefined, 2, undefined]
non-updated squares: [undefined, 2, undefined, undefined, undefined, 2]
rendering
updated squares: [undefined, undefined, 2, undefined, 2, undefined]

我尝试对setStaterender进行回调，但第二次渲染将在第一次渲染之后但在setState更新state之前渲染：

this.setState({
  squares: a
}, this.render);

它记录了：

squares before: [undefined, 2, undefined, undefined, undefined, 2]
new squares: [undefined, undefined, 2, undefined, 2, undefined]
non-updated squares: [undefined, 2, undefined, undefined, undefined, 2]
rendering
rendering

任何帮助将不胜感激。提前致谢！

Answer 1

componentWillMount 中的synchronous setState 不会触发重新渲染，而是反映初始渲染本身的变化。然而，setState 的回调是在渲染方法之后调用的。

如果您在渲染方法中记录state squares，它将在初始渲染本身中记录正确的结果。

P.S. componentWillMount 在最新版本中已弃用，最好将当前逻辑放在构造函数中，而不是 componentWillMount

Working demo

Answer 2

使用shouldComponentUpdate(object nextProps, object nextState)，进行渲染，每次返回true，都会进行渲染