教程：井字棋游戏 | Tutorial: Tic-Tac-Toe

The tutorial is divided into several sections: 教程分成以下几个部分：

• Setup for the tutorial will give you a starting point to follow the tutorial.
• 配置 是一些准备工作。
• Overview will teach you the fundamentals of React: components, props, and state.
• 概览 介绍了 React 的 基础知识：组件、props 和 state。
• Completing the game will teach you the most common techniques in React development.
• 完成游戏 介绍了 React 开发中 最常用的技术。
• Adding time travel will give you a deeper insight into the unique strengths of React.
• 添加“时间旅行” 可以让你更深入地了解 React 的独特优势。

实现的是什么程序？| What are you building?

In this tutorial, you’ll build an interactive tic-tac-toe game with React. 本教程将使用 React 实现一个交互式的井字棋游戏。

You can see what it will look like when you’re finished here: 你可以在下面预览最终成果：

If the code doesn’t make sense to you yet, or if you are unfamiliar with the code’s syntax, don’t worry! The goal of this tutorial is to help you understand React and its syntax. 如果你还不是很明白上面的代码，不用担心！本教程的目的就是帮你理解 React 及其语法。

We recommend that you check out the tic-tac-toe game above before continuing with the tutorial. One of the features that you’ll notice is that there is a numbered list to the right of the game’s board. This list gives you a history of all of the moves that have occurred in the game, and it is updated as the game progresses. 我们建议你在继续本教程之前，先看看上面的井字棋游戏。我们会注意到的一项功能是，棋盘右侧有一个编号列表，它记录了游戏中落子的历史，并随着游戏的进行而更新。

Once you’ve played around with the finished tic-tac-toe game, keep scrolling. You’ll start with a simpler template in this tutorial. Our next step is to set you up so that you can start building the game. 体验完游戏以后，继续阅读本教程吧！我们将从一个更简单的模板开始。下一步将介绍相关配置，以便于你着手实现这个游戏。

配置 | Setup for the tutorial

In the live code editor below, click Fork in the top-right corner to open the editor in a new tab using the website CodeSandbox. CodeSandbox lets you write code in your browser and preview how your users will see the app you’ve created. The new tab should display an empty square and the starter code for this tutorial. 在下面的实时代码编辑器中，单击右上角的 Fork 来在新选项卡中打开 CodeSandbox 编辑器。CodeSandbox 让你能够在浏览器中编写代码并预览效果。一切顺利的话，你应该会看见一个空方块和本教程的初始代码。

概览 | Overview

Now that you’re set up, let’s get an overview of React! 完成配置以后，我们先来大致了解一下 React 吧！

看一下刚刚的代码 | Inspecting the starter code

In CodeSandbox you’ll see three main sections: 在 CodeSandbox 中，你将看到三个主要的部分：

1. The Files section with a list of files like App.js, index.js, styles.css and a folder called public

• Files 部分列出了一些文件：App.js、index.js、styles.css 和 public 文件夹。

2. The code editor where you’ll see the source code of your selected file

• code editor 部分可以看到你所选中文件的源码。

3. The browser section where you’ll see how the code you’ve written will be displayed

• browser 部分可以预览代码的实时结果。

The App.js file should be selected in the Files section. The contents of that file in the code editor should be:

App.js 文件里面的内容应该是这样的：

export default function Square() {
  return <button className="square">X</button>;
}

The browser section should be displaying a square with a X in it like this: browser 部分应该会像下面这样在方块里面显示一个 X：

Now let’s have a look at the files in the starter code. 现在，让我们仔细研究一下这些文件吧。

App.js

The code in App.js creates a component. In React, a component is a piece of reusable code that represents a part of a user interface. Components are used to render, manage, and update the UI elements in your application. Let’s look at the component line by line to see what’s going on: App.js 的代码创建了一个 组件。在 React 中，组件是一段可重用代码，它通常作为 UI 界面的一部分。组件用于渲染、管理和更新应用中的 UI 元素。让我们逐行查看这段代码，看看发生了什么：

export default function Square() {
  return <button className="square">X</button>;
}

The first line defines a function called Square. The export JavaScript keyword makes this function accessible outside of this file. The default keyword tells other files using your code that it’s the main function in your file. 第一行定义了一个名为 Square 的函数。JavaScript 的 export 关键字使此函数可以在此文件之外访问。default 关键字表明它是文件中的主要函数。

export default function Square() {
  return <button className="square">X</button>;
}

The second line returns a button. The return JavaScript keyword means whatever comes after is returned as a value to the caller of the function. <button> is a JSX element. A JSX element is a combination of JavaScript code and HTML tags that describes what you’d like to display. className="square" is a button property or prop that tells CSS how to style the button. X is the text displayed inside of the button and </button> closes the JSX element to indicate that any following content shouldn’t be placed inside the button. 第二行返回一个按钮。JavaScript 的 return 关键字意味着后面的内容都作为值返回给函数的调用者。<button> 是一个 JSX 元素。JSX 元素是 JavaScript 代码和 HTML 标签的组合，用于描述要显示的内容。className="square" 是一个 button 属性，它决定 CSS 如何设置按钮的样式。X 是按钮内显示的文本，</button> 闭合 JSX 元素以表示不应将任何后续内容放置在按钮内。

styles.css

Click on the file labeled styles.css in the Files section of CodeSandbox. This file defines the styles for your React app. The first two CSS selectors (* and body) define the style of large parts of your app while the .square selector defines the style of any component where the className property is set to square. In your code, that would match the button from your Square component in the App.js file. 单击 CodeSandbox 中的 styles.css 文件。该文件定义了 React 应用的样式。前两个 CSS 选择器（* 和 body）定义了应用大部分的样式，而 .square 选择器定义了 className 属性设置为 square 的组件的样式。这与 App.js 文件中的 Square 组件中的按钮是相匹配的。

index.js

Click on the file labeled index.js in the Files section of CodeSandbox. You won’t be editing this file during the tutorial but it is the bridge between the component you created in the App.js file and the web browser. 单击 CodeSandbox 中的 index.js 的文件。在本教程中我们不会编辑此文件，但它是 App.js 文件中创建的组件与 Web 浏览器之间的桥梁。

import { StrictMode } from 'react';
import { createRoot } from 'react-dom/client';
import './styles.css';

import App from './App';

Lines 1-5 bring all the necessary pieces together: 第 1-5 行将所有必要的部分组合在一起：

• React
• React
• React’s library to talk to web browsers (React DOM)
• React 与 Web 浏览器对话的库（React DOM）
• the styles for your components
• 组件的样式
• the component you created in App.js.
• App.js 里面创建的组件

其他文件将它们组合在一起，并将最终成果注入 public 文件夹里面的 index.html 中。

构建棋盘 | Building the board

Let’s get back to App.js. This is where you’ll spend the rest of the tutorial. 让我们回到 App.js。接下来我们将专注于这个文件。

Currently the board is only a single square, but you need nine! If you just try and copy paste your square to make two squares like this: 目前棋盘只有一个方块，但你需要九个！如果你只是想着复制粘贴来制作两个像这样的方块：

export default function Square() {
  return <button className="square">X</button><button className="square">X</button>;
}

You’ll get this error: 你将会得到如下错误：

React components need to return a single JSX element and not multiple adjacent JSX elements like two buttons. To fix this you can use Fragments (<> and </>) to wrap multiple adjacent JSX elements like this: React 组件必须返回单个 JSX 元素，不能像两个按钮那样返回多个相邻的 JSX 元素。要解决此问题，可以使用 Fragment（<> 与 </>）包裹多个相邻的 JSX 元素，如下所示：

export default function Square() {
  return (
    <>
      <button className="square">X</button>
      <button className="square">X</button>
    </>
  );
}

Now you should see: 现在你应该可以看见：

great! now you just need to copy-paste a few times to add nine squares and… 非常棒！现在你只需要通过复制粘贴来添加九个方块，然后……

Oh no! The squares are all in a single line, not in a grid like you need for our board. To fix this you’ll need to group your squares into rows with divs and add some CSS classes. While you’re at it, you’ll give each square a number to make sure you know where each square is displayed. 但事与愿违的是这些方块并没有排列成网格，而是都在一条线上。要解决此问题，需要使用 div 将方块分到每一行中并添加一些 CSS 样式。当你这样做的时候，需要给每个方块一个数字，以确保你知道每个方块的位置。

In the App.js file, update the Square component to look like this: App.js 文件中，Square 组件看起来像这样：

export default function Square() {
  return (
    <>
      <div className="board-row">
        <button className="square">1</button>
        <button className="square">2</button>
        <button className="square">3</button>
      </div>
      <div className="board-row">
        <button className="square">4</button>
        <button className="square">5</button>
        <button className="square">6</button>
      </div>
      <div className="board-row">
        <button className="square">7</button>
        <button className="square">8</button>
        <button className="square">9</button>
      </div>
    </>
  );
}

The CSS defined in styles.css styles the divs with the className of board-row. Now that you’ve grouped your components into rows with the styled divs you have your tic-tac-toe board: 借助 styles.css 中定义的 board-row 样式，我们将组件分到每一行的 div 中。最终完成了井字棋棋盘：

But you now have a problem. Your component named Square, really isn’t a square anymore. Let’s fix that by changing the name to Board: 但是现在有个问题，名为 Square 的组件实际上不再是方块了。让我们通过将名称更改为 Board 来解决这个问题：

export default function Board() {
  //...
}

At this point your code should look something like this: 此时你的代码应如下所示：

通过 props 传递数据 | Passing data through props

Next, you’ll want to change the value of a square from empty to “X” when the user clicks on the square. With how you’ve built the board so far you would need to copy-paste the code that updates the square nine times (once for each square you have)! Instead of copy-pasting, React’s component architecture allows you to create a reusable component to avoid messy, duplicated code. 接下来，当用户单击方块时，我们要将方块的值从空更改为“X”。根据目前构建的棋盘，你需要复制并粘贴九次更新方块的代码（每个方块都需要一次）！但是，React 的组件架构可以创建可重用的组件，以避免混乱、重复的代码。

First, you are going to copy the line defining your first square (<button className="square">1</button>) from your Board component into a new Square component: 首先，要将定义第一个方块（<button className="square">1</button>）的这行代码从 Board 组件复制到新的 Square 组件中：

function Square() {
  return <button className="square">1</button>;
}

export default function Board() {
  // ...
}

Then you’ll update the Board component to render that Square component using JSX syntax: 然后，更新 Board 组件并使用 JSX 语法渲染 Square 组件：

// ...
export default function Board() {
  return (
    <>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
    </>
  );
}

Note how unlike the browser divs, your own components Board and Square must start with a capital letter. 需要注意的是，这并不像 div，这些你自己的组件如 Board 和 Square，必须以大写字母开头。

让我们来看一看效果： Let’s take a look:

Oh no! You lost the numbered squares you had before. Now each square says “1”. To fix this, you will use props to pass the value each square should have from the parent component (Board) to its child (Square). 哦不！你失去了你以前有正确编号的方块。现在每个方块都写着“1”。要解决此问题，需要使用 props 将每个方块应有的值从父组件（Board）传递到其子组件（Square）。

Update the Square component to read the value prop that you’ll pass from the Board: 更新 Square 组件，读取从 Board 传递的 value props：

function Square({ value }) {
  return <button className="square">1</button>;
}

function Square({ value }) indicates the Square component can be passed a prop called value. function Square({ value }) 表示可以向 Square 组件传递一个名为 value 的 props。

Now you want to display that value instead of 1 inside every square. Try doing it like this: 现在你如果想要显示对应的 value 而不是 1，可以试一下像下面这样：

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

Oops, this is not what you wanted: 糟糕！这还不是你想要的：

You wanted to render the JavaScript variable called value from your component, not the word “value”. To “escape into JavaScript” from JSX, you need curly braces. Add curly braces around value in JSX like so: 我们需要从组件中渲染名为 value 的 JavaScript 变量，而不是“value”这个词。要从 JSX“转义到 JavaScript”，你需要使用大括号。在 JSX 中的 value 周围添加大括号，如下所示：

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

For now, you should see an empty board: 现在，你应该会看到一个空的棋盘了：

This is because the Board component hasn’t passed the value prop to each Square component it renders yet. To fix it you’ll add the value prop to each Square component rendered by the Board component: 这是因为 Board 组件还没有将 value props 传递给它渲染的每个 Square 组件。要修复这个问题，需要向 Board 组件里面的每个 Square 组件添加 value props：

export default function Board() {
  return (
    <>
      <div className="board-row">
        <Square value="1" />
        <Square value="2" />
        <Square value="3" />
      </div>
      <div className="board-row">
        <Square value="4" />
        <Square value="5" />
        <Square value="6" />
      </div>
      <div className="board-row">
        <Square value="7" />
        <Square value="8" />
        <Square value="9" />
      </div>
    </>
  );
}

Now you should see a grid of numbers again: 现在你应该能再次看到数字网格：

Your updated code should look like this: 更新后的代码应该是这样：

创建一个具有交互性的组件 | Making an interactive component

Let’s fill the Square component with an X when you click it. Declare a function called handleClick inside of the Square. Then, add onClick to the props of the button JSX element returned from the Square: 当你单击它的时候，Square 组件需要显示“X”。在 Square 内部声明一个名为 handleClick 的函数。然后，将 onClick 添加到由 Square 返回的 JSX 元素的 button 的 props 中：

function Square({ value }) {
  function handleClick() {
    console.log('clicked!');
  }

  return (
    <button
      className="square"
      onClick={handleClick}
    >
      {value}
    </button>
  );
}

If you click on a square now, you should see a log saying "clicked!" in the Console tab at the bottom of the Browser section in CodeSandbox. Clicking the square more than once will log "clicked!" again. Repeated console logs with the same message will not create more lines in the console. Instead, you will see an incrementing counter next to your first "clicked!" log. 如果现在单击一个方块，你应该会看到一条日志，上面写着 "clicked!"。在 CodeSandbox 中 Browser 部分底部的 Console 选项卡中。多次单击方块将再次记录 "clicked!"。具有相同消息的重复控制台日志不会在控制台中重复创建。而你会在第一次 "clicked!" 旁边看到一个递增的计数器。

As a next step, you want the Square component to “remember” that it got clicked, and fill it with an “X” mark. To “remember” things, components use state. 下一步，我们希望 Square 组件能够“记住”它被单击过，并用“X”填充它。为了“记住”一些东西，组件使用 state。

React provides a special function called useState that you can call from your component to let it “remember” things. Let’s store the current value of the Square in state, and change it when the Square is clicked. React 提供了一个名为 useState 的特殊函数，可以从组件中调用它来让它“记住”一些东西。让我们将 Square 的当前值存储在 state 中，并在单击 Square 时更改它。

Import useState at the top of the file. Remove the value prop from the Square component. Instead, add a new line at the start of the Square that calls useState. Have it return a state variable called value: 在文件的顶部导入 useState。从 Square 组件中移除 value props。在调用 useState 的 Square 的开头添加一个新行。让它返回一个名为 value 的 state 变量：

import { useState } from 'react';

function Square() {
  const [value, setValue] = useState(null);

  function handleClick() {
    //...

value stores the value and setValue is a function that can be used to change the value. The null passed to useState is used as the initial value for this state variable, so value here starts off equal to null. value 存储值，而 setValue 是可用于更改值的函数。传递给 useState 的 null 用作这个 state 变量的初始值，因此此处 value 的值开始时等于 null。

Since the Square component no longer accepts props anymore, you’ll remove the value prop from all nine of the Square components created by the Board component: 由于 Square 组件不再接受 props，我们从 Board 组件创建的所有九个 Square 组件中删除 value props：

// ...
export default function Board() {
  return (
    <>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
    </>
  );
}

Now you’ll change Square to display an “X” when clicked. Replace the console.log("clicked!"); event handler with setValue('X');. Now your Square component looks like this: 现在将更改 Square 以在单击时显示“X”。不再使用 console.log("clicked!"); 而使用 setValue('X'); 的事件处理程序。现在你的 Square 组件看起来像这样：

function Square() {
  const [value, setValue] = useState(null);

  function handleClick() {
    setValue('X');
  }

  return (
    <button
      className="square"
      onClick={handleClick}
    >
      {value}
    </button>
  );
}

By calling this set function from an onClick handler, you’re telling React to re-render that Square whenever its <button> is clicked. After the update, the Square’s value will be 'X', so you’ll see the “X” on the game board. Click on any Square, and “X” should show up: 通过从 onClick 处理程序调用此 set 函数，你告诉 React 在单击其 <button> 时要重新渲染该 Square。更新后，方块的值将为“X”，因此会在棋盘上看到“X”。点击任意方块，“X”应该出现：

Each Square has its own state: the value stored in each Square is completely independent of the others. When you call a set function in a component, React automatically updates the child components inside too. 每个 Square 都有自己的 state：存储在每个 Square 中的 value 完全独立于其他的 Square。当你在组件中调用 set 函数时，React 也会自动更新内部的子组件。

After you’ve made the above changes, your code will look like this: 完成上述更改后，代码将如下所示：

React 开发者工具 | React Developer Tools

React DevTools let you check the props and the state of your React components. You can find the React DevTools tab at the bottom of the browser section in CodeSandbox: React 开发者工具可以检查 React 组件的 props 和 state。可以在 CodeSandbox 的 Browser 部分底部找到 React DevTools 选项卡：

To inspect a particular component on the screen, use the button in the top left corner of React DevTools: 要检查屏幕上的特定组件，请使用 React 开发者工具左上角的按钮：

完成这个游戏 | Completing the game

By this point, you have all the basic building blocks for your tic-tac-toe game. To have a complete game, you now need to alternate placing “X”s and “O”s on the board, and you need a way to determine a winner. 至此，你已经拥有井字棋游戏的所有基本构建块。要玩完整的游戏，你现在需要在棋盘上交替放置“X”和“O”，并且你需要一种确定获胜者的方法。

状态提升 | Lifting state up

Currently, each Square component maintains a part of the game’s state. To check for a winner in a tic-tac-toe game, the Board would need to somehow know the state of each of the 9 Square components. 目前，每个 Square 组件都维护着游戏 state 的一部分。要检查井字棋游戏中的赢家，Board 需要以某种方式知道 9 个 Square 组件中每个组件的 state。

How would you approach that? At first, you might guess that the Board needs to “ask” each Square for that Square’s state. Although this approach is technically possible in React, we discourage it because the code becomes difficult to understand, susceptible to bugs, and hard to refactor. Instead, the best approach is to store the game’s state in the parent Board component instead of in each Square. The Board component can tell each Square what to display by passing a prop, like you did when you passed a number to each Square. 你会如何处理？起初，你可能会猜测 Board 需要向每个 Square“询问”Square 的 state。尽管这种方法在 React 中在技术上是可行的，但我们不鼓励这样做，因为代码变得难以理解、容易出现错误并且难以重构。相反，最好的方法是将游戏的 state 存储在 Board 父组件中，而不是每个 Square 中。Board 组件可以通过传递一个 props 来告诉每个 Square 显示什么，就像你将数字传递给每个 Square 时所做的那样。

To collect data from multiple children, or to have two child components communicate with each other, declare the shared state in their parent component instead. The parent component can pass that state back down to the children via props. This keeps the child components in sync with each other and with their parent. 要从多个子组件收集数据，或让两个子组件相互通信，请改为在其父组件中声明共享 state。父组件可以通过 props 将该 state 传回给子组件。这使子组件彼此同步并与其父组件保持同步。

Lifting state into a parent component is common when React components are refactored. 重构 React 组件时，将状态提升到父组件中很常见。

Let’s take this opportunity to try it out. Edit the Board component so that it declares a state variable named squares that defaults to an array of 9 nulls corresponding to the 9 squares: 让我们借此机会尝试一下。编辑 Board 组件，使其声明一个名为 squares 的 state 变量，该变量默认为对应于 9 个方块的 9 个空值数组：

// ...
export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));
  return (
    // ...
  );
}

Array(9).fill(null) creates an array with nine elements and sets each of them to null. The useState() call around it declares a squares state variable that’s initially set to that array. Each entry in the array corresponds to the value of a square. When you fill the board in later, the squares array will look like this: Array(9).fill(null) 创建了一个包含九个元素的数组，并将它们中的每一个都设置为 null。包裹它的 useState() 声明了一个初始设置为该数组的 squares state 变量。数组中的每个元素对应于一个 square 的值。当你稍后填写棋盘时，squares 数组将如下所示：

['O', null, 'X', 'X', 'X', 'O', 'O', null, null]

Now your Board component needs to pass the value prop down to each Square that it renders: 现在你的 Board 组件需要将 value props 向下传递给它渲染的每个 Square：

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));
  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} />
        <Square value={squares[1]} />
        <Square value={squares[2]} />
      </div>
      <div className="board-row">
        <Square value={squares[3]} />
        <Square value={squares[4]} />
        <Square value={squares[5]} />
      </div>
      <div className="board-row">
        <Square value={squares[6]} />
        <Square value={squares[7]} />
        <Square value={squares[8]} />
      </div>
    </>
  );
}

Next, you’ll edit the Square component to receive the value prop from the Board component. This will require removing the Square component’s own stateful tracking of value and the button’s onClick prop: 接下来，你将编辑 Square 组件，以从 Board 组件接收 value props。这将需要删除 Square 组件自己的 value state 和按钮的 onClick props：

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

At this point you should see an empty tic-tac-toe board: 此时你应该看到一个空的井字棋棋盘：

And your code should look like this: 你的代码应该是这样的：

Each Square will now receive a value prop that will either be 'X', 'O', or null for empty squares. 现在，每个 Square 都会收到一个 value props，对于空方块，该 props 将是 'X'、'O' 或 null。

Next, you need to change what happens when a Square is clicked. The Board component now maintains which squares are filled. You’ll need to create a way for the Square to update the Board’s state. Since state is private to a component that defines it, you cannot update the Board’s state directly from Square. 接下来，你需要更改单击 Square 时发生的情况。Board 组件现在维护已经填充过的方块。你需要为 Square 创建一种更新 Board state 的方法。由于 state 对于定义它的组件是私有的，因此你不能直接从 Square 更新 Board 的 state。

Instead, you’ll pass down a function from the Board component to the Square component, and you’ll have Square call that function when a square is clicked. You’ll start with the function that the Square component will call when it is clicked. You’ll call that function onSquareClick: 你将从 Board 组件向下传递一个函数到 Square 组件，然后让 Square 在单击方块时调用该函数。我们将从单击 Square 组件时将调用的函数开始。调用该函数 onSquareClick：

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

Next, you’ll add the onSquareClick function to the Square component’s props: 接下来，将 onSquareClick 函数添加到 Square 组件的 props 中：

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

Now you’ll connect the onSquareClick prop to a function in the Board component that you’ll name handleClick. To connect onSquareClick to handleClick you’ll pass a function to the onSquareClick prop of the first Square component: 现在，你将把 onSquareClick props 连接到 Board 组件中的一个函数，命名为 handleClick。要将 onSquareClick 连接到 handleClick，需要将一个函数传递给第一个 Square 组件的 onSquareClick props：

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));

  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} onSquareClick={handleClick} />
        //...
  );
}

Lastly, you will define the handleClick function inside the Board component to update the squares array holding your board’s state: 最后，你将在 Board 组件内定义 handleClick 函数来更新并保存棋盘 state 的 squares 数组：

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));

  function handleClick() {
    const nextSquares = squares.slice();
    nextSquares[0] = "X";
    setSquares(nextSquares);
  }

  return (
    // ...
  )
}

The handleClick function creates a copy of the squares array (nextSquares) with the JavaScript slice() Array method. Then, handleClick updates the nextSquares array to add X to the first ([0] index) square. handleClick 函数使用 JavaScript 数组的 slice() 方法创建 squares 数组（nextSquares）的副本。然后，handleClick 更新 nextSquares 数组，将 X 添加到第一个（[0] 索引）方块。

Calling the setSquares function lets React know the state of the component has changed. This will trigger a re-render of the components that use the squares state (Board) as well as its child components (the Square components that make up the board). 调用 setSquares 函数让 React 知道组件的 state 已经改变。这将触发使用 squares state 的组件（Board）及其子组件（构成棋盘的 Square 组件）的重新渲染。

Now you can add X’s to the board… but only to the upper left square. Your handleClick function is hardcoded to update the index for the upper left square (0). Let’s update handleClick to be able to update any square. Add an argument i to the handleClick function that takes the index of the square to update: 现在你可以将 X 添加到棋盘上……但只能添加到左上角的方块。你的 handleClick 函数被硬编码为更新左上角方块（ 0）的索引。让我们更新 handleClick 以便能够更新任何方块。将参数 i 添加到 handleClick 函数，该函数采用要更新的 square 索引：

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));

  function handleClick(i) {
    const nextSquares = squares.slice();
    nextSquares[i] = "X";
    setSquares(nextSquares);
  }

  return (
    // ...
  )
}

Next, you will need to pass that i to handleClick. You could try to set the onSquareClick prop of square to be handleClick(0) directly in the JSX like this, but it won’t work: 接下来，你需要将 i 传递给 handleClick。你可以尝试像这样在 JSX 中直接将 square 的 onSquareClick props 设置为 handleClick(0)，但这是行不通的：

<Square value={squares[0]} onSquareClick={handleClick(0)} />

Here is why this doesn’t work. The handleClick(0) call will be a part of rendering the board component. Because handleClick(0) alters the state of the board component by calling setSquares, your entire board component will be re-rendered again. But this runs handleClick(0) again, leading to an infinite loop: 为什么会是这样呢？handleClick(0) 调用将成为渲染 Board 组件的一部分。因为 handleClick(0) 通过调用 setSquares 改变了棋盘组件的 state，所以你的整个棋盘组件将再次重新渲染。但这再次运行了 handleClick(0)，导致无限循环：

Why didn’t this problem happen earlier? 为什么这个问题没有早点发生？

When you were passing onSquareClick={handleClick}, you were passing the handleClick function down as a prop. You were not calling it! But now you are calling that function right away—notice the parentheses in handleClick(0)—and that’s why it runs too early. You don’t want to call handleClick until the user clicks! 当你传递 onSquareClick={handleClick} 时，你将 handleClick 函数作为 props 向下传递。你不是在调用它！但是现在你立即调用了该函数——注意 handleClick(0) 中的括号——这就是它运行得太早的原因。你不想在用户点击之前调用 handleClick！

You could fix this by creating a function like handleFirstSquareClick that calls handleClick(0), a function like handleSecondSquareClick that calls handleClick(1), and so on. You would pass (rather than call) these functions down as props like onSquareClick={handleFirstSquareClick}. This would solve the infinite loop. 你可以通过创建调用 handleClick(0) 的函数（如 handleFirstSquareClick）、调用 handleClick(1) 的函数（如 handleSecondSquareClick）等来修复。你可以将这些函数作为 onSquareClick={handleFirstSquareClick} 之类的 props 传递（而不是调用）。这将解决无限循环的问题。

However, defining nine different functions and giving each of them a name is too verbose. Instead, let’s do this: 但是，定义九个不同的函数并为每个函数命名过于冗余。让我们这样做：

export default function Board() {
  // ...
  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} onSquareClick={() => handleClick(0)} />
        // ...
  );
}

Notice the new () => syntax. Here, () => handleClick(0) is an arrow function, which is a shorter way to define functions. When the square is clicked, the code after the => “arrow” will run, calling handleClick(0). 注意新的 () => 语法。这里，() => handleClick(0) 是一个箭头函数，它是定义函数的一种较短的方式。单击方块时，=>“箭头”之后的代码将运行，调用 handleClick(0)。

Now you need to update the other eight squares to call handleClick from the arrow functions you pass. Make sure that the argument for each call of the handleClick corresponds to the index of the correct square: 现在你需要更新其他八个方块以从你传递的箭头函数中调用 handleClick。确保 handleClick 的每次调用的参数对应于正确的 square 索引：

export default function Board() {
  // ...
  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} onSquareClick={() => handleClick(0)} />
        <Square value={squares[1]} onSquareClick={() => handleClick(1)} />
        <Square value={squares[2]} onSquareClick={() => handleClick(2)} />
      </div>
      <div className="board-row">
        <Square value={squares[3]} onSquareClick={() => handleClick(3)} />
        <Square value={squares[4]} onSquareClick={() => handleClick(4)} />
        <Square value={squares[5]} onSquareClick={() => handleClick(5)} />
      </div>
      <div className="board-row">
        <Square value={squares[6]} onSquareClick={() => handleClick(6)} />
        <Square value={squares[7]} onSquareClick={() => handleClick(7)} />
        <Square value={squares[8]} onSquareClick={() => handleClick(8)} />
      </div>
    </>
  );
};

Now you can again add X’s to any square on the board by clicking on them: 现在你可以再次通过单击将 X 添加到棋盘的方块上：

But this time all the state management is handled by the Board component! 但是这次所有的 state 管理都由 Board 组件处理！

This is what your code should look like: 你的代码应该是这样的：

Now that your state handling is in the Board component, the parent Board component passes props to the child Square components so that they can be displayed correctly. When clicking on a Square, the child Square component now asks the parent Board component to update the state of the board. When the Board’s state changes, both the Board component and every child Square re-renders automatically. Keeping the state of all squares in the Board component will allow it to determine the winner in the future. 现在，我们在 Board 组件中处理 state， Board 父组件将 props 传递给 Square 子组件，以便它们可以正确显示。单击 Square 时， Square 子组件现在要求 Board 父组件更新棋盘的 state。当 Board 的 state 改变时，Board 组件和每个子 Square 都会自动重新渲染。保存 Board 组件中所有方块的 state 将使得它可以确定未来的赢家。

Let’s recap what happens when a user clicks the top left square on your board to add an X to it: 让我们回顾一下当用户单击你的棋盘左上角的方块以向其添加 X 时会发生什么：

1. Clicking on the upper left square runs the function that the button received as its onClick prop from the Square. The Square component received that function as its onSquareClick prop from the Board. The Board component defined that function directly in the JSX. It calls handleClick with an argument of 0.

• 单击左上角的方块运行 button 从 Square 接收到的 onClick props 的函数。Square 组件从 Board 通过 onSquareClick props 接收到该函数。Board 组件直接在 JSX 中定义了该函数。它使用参数 0 调用 handleClick。

2. handleClick uses the argument (0) to update the first element of the squares array from null to X.

• handleClick 使用参数（0）将 squares 数组的第一个元素从 null 更新为 X。

3. The squares state of the Board component was updated, so the Board and all of its children re-render. This causes the value prop of the Square component with index 0 to change from null to X.

• Board 组件的 squares state 已更新，因此 Board 及其所有子组件都将重新渲染。这会导致索引为 0 的 Square 组件的 value props 从 null 更改为 X。

In the end the user sees that the upper left square has changed from empty to having a X after clicking it. 最后，用户看到左上角的方块在单击后从空变为 X。

为什么不变性很重要 | Why immutability is important

Note how in handleClick, you call .slice() to create a copy of the squares array instead of modifying the existing array. To explain why, we need to discuss immutability and why immutability is important to learn. 请注意在 handleClick 中，你调用了 .slice() 来创建 squares 数组的副本而不是修改现有数组。为了解释原因，我们需要讨论不变性以及为什么学习不变性很重要。

There are generally two approaches to changing data. The first approach is to mutate the data by directly changing the data’s values. The second approach is to replace the data with a new copy which has the desired changes. Here is what it would look like if you mutated the squares array: 通常有两种更改数据的方法。第一种方法是通过直接更改数据的值来改变数据。第二种方法是使用具有所需变化的新副本替换数据。如果你改变 squares 数组，它会是这样的：

const squares = [null, null, null, null, null, null, null, null, null];
squares[0] = 'X';
// Now `squares` is ["X", null, null, null, null, null, null, null, null];

And here is what it would look like if you changed data without mutating the squares array: 如果你在不改变 squares 数组的情况下更改数据，它会是这样的：

const squares = [null, null, null, null, null, null, null, null, null];
const nextSquares = ['X', null, null, null, null, null, null, null, null];
// Now `squares` is unchanged, but `nextSquares` first element is 'X' rather than `null`

The result is the same but by not mutating (changing the underlying data) directly, you gain several benefits. 结果是一样的，但通过不直接改变（改变底层数据），你可以获得几个好处。

Immutability makes complex features much easier to implement. Later in this tutorial, you will implement a “time travel” feature that lets you review the game’s history and “jump back” to past moves. This functionality isn’t specific to games—an ability to undo and redo certain actions is a common requirement for apps. Avoiding direct data mutation lets you keep previous versions of the data intact, and reuse them later. 不变性使复杂的功能更容易实现。在本教程的后面，你将实现一个“时间旅行”功能，让你回顾游戏的历史并“跳回”到过去的动作。此功能并非特定于游戏——撤消和重做某些操作的能力是应用程序的常见要求。避免数据直接突变可以让你保持以前版本的数据完好无损，并在以后重用它们。

There is also another benefit of immutability. By default, all child components re-render automatically when the state of a parent component changes. This includes even the child components that weren’t affected by the change. Although re-rendering is not by itself noticeable to the user (you shouldn’t actively try to avoid it!), you might want to skip re-rendering a part of the tree that clearly wasn’t affected by it for performance reasons. Immutability makes it very cheap for components to compare whether their data has changed or not. You can learn more about how React chooses when to re-render a component in the memo API reference. 不变性还有另一个好处。默认情况下，当父组件的 state 发生变化时，所有子组件都会自动重新渲染。这甚至包括未受变化影响的子组件。尽管重新渲染本身不会引起用户注意（你不应该主动尝试避免它！），但出于性能原因，你可能希望跳过重新渲染显然不受其影响的树的一部分。不变性使得组件比较其数据是否已更改的成本非常低。你可以在 memo API 参考 中了解更多关于 React 如何选择何时重新渲染组件的信息。

交替落子 | Taking turns

It’s now time to fix a major defect in this tic-tac-toe game: the “O”s cannot be marked on the board. 现在是时候修复这个井字棋游戏的一个主要缺陷了：棋盘上无法标记“O”。

You’ll set the first move to be “X” by default. Let’s keep track of this by adding another piece of state to the Board component: 默认情况下，你会将第一步设置为“X”。让我们通过向 Board 组件添加另一个 state 来跟踪这一点：

function Board() {
  const [xIsNext, setXIsNext] = useState(true);
  const [squares, setSquares] = useState(Array(9).fill(null));

  // ...
}

Each time a player moves, xIsNext (a boolean) will be flipped to determine which player goes next and the game’s state will be saved. You’ll update the Board’s handleClick function to flip the value of xIsNext: 每次玩家落子时，xIsNext（一个布尔值）将被翻转以确定下一个玩家，游戏 state 将被保存。你将更新 Board 的 handleClick 函数以翻转 xIsNext 的值：

export default function Board() {
  const [xIsNext, setXIsNext] = useState(true);
  const [squares, setSquares] = useState(Array(9).fill(null));

  function handleClick(i) {
    const nextSquares = squares.slice();
    if (xIsNext) {
      nextSquares[i] = "X";
    } else {
      nextSquares[i] = "O";
    }
    setSquares(nextSquares);
    setXIsNext(!xIsNext);
  }

  return (
    //...
  );
}

Now, as you click on different squares, they will alternate between X and O, as they should! 现在，当你点击不同的方块时，它们会在 X 和 O 之间交替，这是它们应该做的！

But wait, there’s a problem. Try clicking on the same square multiple times: 但是等等，有一个问题。尝试多次点击同一个方块：

The X is overwritten by an O! While this would add a very interesting twist to the game, we’re going to stick to the original rules for now.

X 被 O 覆盖！虽然这会给游戏带来非常有趣的变化，但我们现在将坚持原来的规则。

When you mark a square with a X or an O you aren’t first checking to see if the square already has a X or O value. You can fix this by returning early. You’ll check to see if the square already has a X or an O. If the square is already filled, you will return in the handleClick function early—before it tries to update the board state. 当你用 X 或 O 标记方块时，你没有检查该方块是否已经具有 X 或 O 值。你可以通过提早返回来解决此问题。我们将检查方块是否已经有 X 或 O。如果方块已经填满，你将尽早在 handleClick 函数中 return——在它尝试更新棋盘 state 之前。

function handleClick(i) {
  if (squares[i]) {
    return;
  }
  const nextSquares = squares.slice();
  //...
}

Now you can only add X’s or O’s to empty squares! Here is what your code should look like at this point: 现在你只能将 X 或 O 添加到空方块中！此时你的代码应该如下所示：

宣布获胜者 | Declaring a winner

Now that the players can take turns, you’ll want to show when the game is won and there are no more turns to make. To do this you’ll add a helper function called calculateWinner that takes an array of 9 squares, checks for a winner and returns 'X', 'O', or null as appropriate. Don’t worry too much about the calculateWinner function; it’s not specific to React: 现在你可以轮流对战了，接下来我们应该显示游戏何时获胜。为此，你将添加一个名为 calculateWinner 的辅助函数，它接受 9 个方块的数组，检查获胜者并根据需要返回 'X'、'O' 或 null。不要太担心 calculateWinner 函数；它不是 React 才会有的：

export default function Board() {
  //...
}

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;
}

You will call calculateWinner(squares) in the Board component’s handleClick function to check if a player has won. You can perform this check at the same time you check if a user has clicked a square that already has a X or and O. We’d like to return early in both cases: 你将在 Board 组件的 handleClick 函数中调用 calculateWinner(squares) 来检查玩家是否获胜。你可以在检查用户是否单击了已经具有 X 或 O 的方块的同时执行此检查。在这两种情况下，我们都希望尽早返回：

function handleClick(i) {
  if (squares[i] || calculateWinner(squares)) {
    return;
  }
  const nextSquares = squares.slice();
  //...
}

To let the players know when the game is over, you can display text such as “Winner: X” or “Winner: O”. To do that you’ll add a status section to the Board component. The status will display the winner if the game is over and if the game is ongoing you’ll display which player’s turn is next: 为了让玩家知道游戏何时结束，你可以显示“获胜者：X”或“获胜者：O”等文字。为此，你需要将 status 部分添加到 Board 组件。如果游戏结束，将显示获胜者，如果游戏正在进行，你将显示下一轮将会是哪个玩家：

export default function Board() {
  // ...
  const winner = calculateWinner(squares);
  let status;
  if (winner) {
    status = "Winner: " + winner;
  } else {
    status = "Next player: " + (xIsNext ? "X" : "O");
  }

  return (
    <>
      <div className="status">{status}</div>
      <div className="board-row">
        // ...
  )
}

Congratulations! You now have a working tic-tac-toe game. And you’ve just learned the basics of React too. So you are the real winner here. Here is what the code should look like: 恭喜！你现在有一个可以运行的井字棋游戏。你也学习了 React 的基础知识。所以你是这里真正的赢家。代码应该如下所示：

添加时间旅行 | Adding time travel

As a final exercise, let’s make it possible to “go back in time” to the previous moves in the game. 作为最后的练习，让我们能够“回到”到游戏中之前的动作。

存储落子历史 | Storing a history of moves

If you mutated the squares array, implementing time travel would be very difficult. 如果你改变了 squares 数组，实现时间旅行将非常困难。

However, you used slice() to create a new copy of the squares array after every move, and treated it as immutable. This will allow you to store every past version of the squares array, and navigate between the turns that have already happened. 但是，你在每次落子后都使用 slice() 创建 squares 数组的新副本，并将其视为不可变的。这将允许你存储 squares 数组的每个过去的版本，并在已经发生的轮次之间“来回”。

You’ll store the past squares arrays in another array called history, which you’ll store as a new state variable. The history array represents all board states, from the first to the last move, and has a shape like this: 把过去的 squares 数组存储在另一个名为 history 的数组中，把它存储为一个新的 state 变量。history 数组表示所有棋盘的 state，从第一步到最后一步，其形状如下：

[
  // Before first move
  [null, null, null, null, null, null, null, null, null],
  // After first move
  [null, null, null, null, 'X', null, null, null, null],
  // After second move
  [null, null, null, null, 'X', null, null, null, 'O'],
  // ...
]

再一次“状态提升” | Lifting state up, again

You will now write a new top-level component called Game to display a list of past moves. That’s where you will place the history state that contains the entire game history. 你现在将编写一个名为 Game 的新顶级组件来显示过去的着法列表。这就是放置包含整个游戏历史的 history state 的地方。

Placing the history state into the Game component will let you remove the squares state from its child Board component. Just like you “lifted state up” from the Square component into the Board component, you will now lift it up from the Board into the top-level Game component. This gives the Game component full control over the Board’s data and lets it instruct the Board to render previous turns from the history. 将 history state 放入 Game 组件将使你可以从其 Board 子组件中删除 squares state。就像你将 state 从 Square 组件“提升”到 Board 组件一样，你现在将把它从 Board 提升到顶层 Game 组件。这使 Game 组件可以完全控制 Board 的数据，并使它让 Board 渲染来自 history 的之前的回合。

First, add a Game component with export default. Have it render the Board component and some markup: 首先，添加一个带有 export default 的 Game 组件。让它渲染 Board 组件和一些标签：

function Board() {
  // ...
}

export default function Game() {
  return (
    <div className="game">
      <div className="game-board">
        <Board />
      </div>
      <div className="game-info">
        <ol>{/*TODO*/}</ol>
      </div>
    </div>
  );
}

Note that you are removing the export default keywords before the function Board() { declaration and adding them before the function Game() { declaration. This tells your index.js file to use the Game component as the top-level component instead of your Board component. The additional divs returned by the Game component are making room for the game information you’ll add to the board later. 请注意，你要删除 function Board() { 声明之前的 export default 关键字，并将它们添加到 function Game() { 声明之前。这会告诉你的 index.js 文件使用 Game 组件而不是你的 Board 组件作为顶层组件。Game 组件返回的额外 div 正在为你稍后添加到棋盘的游戏信息腾出空间。

Add some state to the Game component to track which player is next and the history of moves: 向 Game 组件添加一些 state 以跟踪下一个玩家和落子历史：

export default function Game() {
  const [xIsNext, setXIsNext] = useState(true);
  const [history, setHistory] = useState([Array(9).fill(null)]);
  // ...

Notice how [Array(9).fill(null)] is an array with a single item, which itself is an array of 9 nulls. 请注意，[Array(9).fill(null)] 是一个包含单个元素的数组，它本身是一个包含 9 个 null 的数组。

To render the squares for the current move, you’ll want to read the last squares array from the history. You don’t need useState for this—you already have enough information to calculate it during rendering: 要渲染当前落子的方块，你需要从 history 中读取最后一个 squares 数组。你不需要 useState——你已经有足够的信息可以在渲染过程中计算它：

export default function Game() {
  const [xIsNext, setXIsNext] = useState(true);
  const [history, setHistory] = useState([Array(9).fill(null)]);
  const currentSquares = history[history.length - 1];
  // ...

Next, create a handlePlay function inside the Game component that will be called by the Board component to update the game. Pass xIsNext, currentSquares and handlePlay as props to the Board component: 接下来，在 Game 组件中创建一个 handlePlay 函数，Board 组件将调用该函数来更新游戏。将 xIsNext、currentSquares 和 handlePlay 作为 props 传递给 Board 组件：

export default function Game() {
  const [xIsNext, setXIsNext] = useState(true);
  const [history, setHistory] = useState([Array(9).fill(null)]);
  const currentSquares = history[history.length - 1];

  function handlePlay(nextSquares) {
    // TODO
  }

  return (
    <div className="game">
      <div className="game-board">
        <Board xIsNext={xIsNext} squares={currentSquares} onPlay={handlePlay} />
        //...
  )
}

Let’s make the Board component fully controlled by the props it receives. Change the Board component to take three props: xIsNext, squares, and a new onPlay function that Board can call with the updated squares array when a player makes a move. Next, remove the first two lines of the Board function that call useState: 让 Board 组件完全由它接收到的 props 控制。更改 Board 组件以采用三个 props：xIsNext、squares和一个新的 onPlay 函数，当玩家落子时，Board 可以使用更新的 square 数组调用该函数。接下来，删除调用 useState 的 Board 函数的前两行：

function Board({ xIsNext, squares, onPlay }) {
  function handleClick(i) {
    //...
  }
  // ...
}

Now replace the setSquares and setXIsNext calls in handleClick in the Board component with a single call to your new onPlay function so the Game component can update the Board when the user clicks a square: 现在，将 Board 组件里面的 handleClick 中的 setSquares 和 setXIsNext 调用替换为对新 onPlay 函数的一次调用，这样 Game 组件就可以在用户单击方块时更新 Board：

function Board({ xIsNext, squares, onPlay }) {
  function handleClick(i) {
    if (calculateWinner(squares) || squares[i]) {
      return;
    }
    const nextSquares = squares.slice();
    if (xIsNext) {
      nextSquares[i] = "X";
    } else {
      nextSquares[i] = "O";
    }
    onPlay(nextSquares);
  }
  //...
}

The Board component is fully controlled by the props passed to it by the Game component. You need to implement the handlePlay function in the Game component to get the game working again. Board 组件完全由 Game 组件传递给它的 props 控制。你需要在 Game 组件中实现 handlePlay 函数才能使游戏重新运行。

What should handlePlay do when called? Remember that Board used to call setSquares with an updated array; now it passes the updated squares array to onPlay. handlePlay 被调用应该做什么？请记住，Board 以前使用更新后的数组调用 setSquares；现在它将更新后的 squares 数组传递给 onPlay。

The handlePlay function needs to update Game’s state to trigger a re-render, but you don’t have a setSquares function that you can call any more—you’re now using the history state variable to store this information. You’ll want to update history by appending the updated squares array as a new history entry. You also want to toggle xIsNext, just as Board used to do: handlePlay 函数需要更新 Game 的 state 以触发重新渲染，但是你没有可以再调用的 setSquares 函数——你现在正在使用 history state 变量来存储这些信息。你需要追加更新后的 squares 数组来更新 history 作为新的历史入口。你还需要切换 xIsNext，就像 Board 过去所做的那样：

export default function Game() {
  //...
  function handlePlay(nextSquares) {
    setHistory([...history, nextSquares]);
    setXIsNext(!xIsNext);
  }
  //...
}

Here, [...history, nextSquares] creates a new array that contains all the items in history, followed by nextSquares. (You can read the ...history spread syntax as “enumerate all the items in history”.) 在这里，[...history, nextSquares] 创建了一个新数组，其中包含 history 中的所有元素，后跟 nextSquares。（你可以将 ...history 展开语法理解为“枚举 history 中的所有元素”。）

For example, if history is [[null,null,null], ["X",null,null]] and nextSquares is ["X",null,"O"], then the new [...history, nextSquares] array will be [[null,null,null], ["X",null,null], ["X",null,"O"]]. 例如，如果 history 是 [[null,null,null], ["X",null,null]]，nextSquares 是 ["X",null,"O"]，那么新的 [...history, nextSquares] 数组就是 [[null,null,null], ["X",null,null], ["X",null,"O"]]。

At this point, you’ve moved the state to live in the Game component, and the UI should be fully working, just as it was before the refactor. Here is what the code should look like at this point: 此时，你已将 state 移至 Game 组件中，并且 UI 应该完全正常工作，就像重构之前一样。这是此时代码的样子：