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reactjs - 为什么调用这个变量会错过它包含的数组？

问题描述

在 React 的在线教程中，您将使用以下代码创建井字游戏：

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';

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

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

  }

  render() {
    return (
      <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),
        }],
        stepNumber:0,
        xIsNext: true,
    };
  }

  handleClick(i) {
    const history = this.state.history.slice(0, this.state.stepNumber + 1);
    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,
        }]),
        stepNumber: history.length,
        xIsNext: !this.state.xIsNext,
    }); 
  }
  jumpTo(step) {
    this.setState({
      stepNumber: step,
      xIsNext: (step % 2) === 0,
    });
  }

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

    const moves = history.map((step, move) => {
      const desc = move ?
        'Go to move #' + move :
        'Go to game start';
      return(
        <li key={move}>
          <button onClick={() => this.jumpTo(move)}>{desc}</button>
        </li>
      );
    });

    let status;
    if(winner){
        status = 'Winner: ' + winner;
    } else{
        if(this.state.stepNumber === 9){
            status = 'Draw!';
        } 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>{moves}</ol>
        </div>
      </div>
    );
  }
}

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;
  }

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

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

我的问题很简单。在 Board 类中，当调用this.props.squares[i]. 为什么我们错过了 squares 所在的历史数组？这个电话不应该是this.props.history.squares[i]吗？

标签： reactjs

解决方案

让我们将其分解为您与一些变量一起使用的声明：

const history = [{ squares: Array(9).fill(null) }]
// history = [{ squares: [null, null, null, ...] }]
const current = history[0]
// current = { squares: [null, null, null, ...] }

现在Board你正在传递：

<Board
  squares = {current.squares}
  // squares = [null, null, null, ...]

所以本质上，你只是将一个传递Array给你的道具，并且可以通过this.props.squares[i]

如果你想使用this.props.history.squares[i]，首先你应该使用this.props.history[step].squares[i]（两者history都是squares数组），其次 - 你应该通过

// Game
  <Board 
    squares = {history}
    ...

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