javascript - React 中的回调参数如何从子组件传递到父组件？

Question

我一直在 https://reactjs.org/tutorial/tutorial.html 学习 React 教程我过得很好。我有一件事我一直无法理解。这是完整的代码(代码笔:https://codepen.io/gaearon/pen/EmmOqJ?editors=0010):

function Square(props) {  
    return (
      <button className="square"
              onClick={props.onClick}>

          {props.value}
      </button>
    );
}

function calculateWinner(squares) {
    const lines = [  
        [0, 1, 2],
        [3, 4, 5],
        [6, 7, 8],
        [0, 3, 6],
        [1, 4, 7],
        [2, 5, 8],
        [0, 4, 8],
        [2, 4, 6],
    ];

    for (let i = 0; i < lines.length; i++) {
        const [a, b, c] = lines[i];

        if (squares[a] && squares[a] === squares[b] && squares[a] === squares[c]) {
            return squares[a];
        }
    }

    return null;
}

class Board extends React.Component {
    renderSquare(i) {
        return (
            <Square value={this.props.squares[i]} onClick={() => this.props.onClick(i)}/>  
        );
    }

    render() {  
        return (
            <div>
                <div className="status">{status}</div>
                <div className="board-row">
                    {this.renderSquare(0)}
                    {this.renderSquare(1)}
                    {this.renderSquare(2)}
                </div>
                <div className="board-row">
                    {this.renderSquare(3)}
                    {this.renderSquare(4)}
                    {this.renderSquare(5)}
                </div>
                <div className="board-row">
                    {this.renderSquare(6)}
                    {this.renderSquare(7)}
                    {this.renderSquare(8)}
                </div>
            </div>
        );
    }
}

class Game extends React.Component {
    constructor(props) {
        super(props);  

        this.state = {
            history: [{
                squares: Array(9).fill(null),
            }],
            xIsNext: true,
        };
    }

    handleClick(i) {
        const history = this.state.history;
        const current = history[history.length - 1];
        const squares = current.squares.slice();  

        if (calculateWinner(squares) || squares[i]) {  
            return;
        }

        squares[i] = this.state.xIsNext ? 'X' : 'O';

        this.setState({
            history: history.concat([{
                squares: squares,
            }]),  
            xIsNext: !this.state.xIsNext,
        });  
    }

    render() {
        const history = this.state.history;
        const current = history[history.length - 1];
        const winner = calculateWinner(current.squares);

        let status;

        if (winner) {
            status = 'Winner: ' + winner;
        } else {
            status = 'Next player: ' + (this.state.xIsNext ? 'X' : 'O');
        }

        return (
            <div className="game">
                <div className="game-board">
                    <Board  
                        squares={current.squares}
                        onClick={(i) => this.handleClick(i)}  
                    />
                </div>
                <div className="game-info">
                    <div>{status}</div>
                    <ol>{/* TODO */}</ol>
                </div>
            </div>
        );
    }
}

// ========================================

ReactDOM.render(
  <Game />,
  document.getElementById('root')
);

在教程中，他们将回调函数 handleClick(i) 从父 Game 组件传递给子 Board 组件，并从那里到子 Square 组件。我的问题是参数 i 是如何设置的？我的猜测是起点是在 Board 组件中调用 renderSquare(i) 时。从那里我不知道 i 它是如何到达 handleClick(i) 的。它是否存储在“从 Board 传递给Square的 onClick函数对象中”？

Answer 1

My guess is that the starting point is when renderSquare(i) is called in the Board component. From there I am lost as to how i it makes its way to handleClick(i)

你走在正确的轨道上。

在Board.render()内, this.renderSquare(...)被调用号码 1~8 .

和renderSquare有一个 onClick处理程序，this.props.onClick(i) .

renderSquare(i) {
    return (
        <Square value={this.props.squares[i]} onClick={() => this.props.onClick(i)}/>  
    );
}

this.props从父级传递，Game.render() .
所以this.props.onClick来自<Board onClick={...} />

class Game extends React.Component {
    handleClick(i) {
       ...
    }

    render() {
        ...
        return (
            <div className="game">
                <div className="game-board">
                    <Board  
                        squares={current.squares}
                        onClick={(i) => this.handleClick(i)}  
                    />
                </div>
                ...
            </div>
        );
    }
}

所以 this.props.onClick匹配<Board onClick={...} />
它被实现为 (i) => this.handleClick(i) , 其中this.handleClick(i)指Game.handleClick(i) .