create-react-app初探

本文做者:IMWeb IMWeb團隊 原文出處:IMWeb社區 未經贊成,禁止轉載css

create-react-app是一個react的cli腳手架+構建器,咱們能夠基於CRA零配置直接上手開發一個react的SPA應用。 經過3種方式快速建立一個React SPA應用:html

  1. npm init with initializer (npm 6.1+)
  2. npx with generator (npm 5.2+)
  3. yarn create with initializer (yarn 0.25+)

例如咱們新建一個叫my-app的SPA:node

my-app
├── README.md
├── node_modules
├── package.json
├── .gitignore
├── public
│   ├── favicon.ico
│   ├── index.html
│   └── manifest.json
└── src
    ├── App.css
    ├── App.js
    ├── App.test.js
    ├── index.css
    ├── index.js
    ├── logo.svg
    └── serviceWorker.js
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經過添加參數生成ts支持:react

npx create-react-app my-app --typescript
# or
yarn create react-app my-app --typescript
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固然,若是咱們是把一個CRA已經生成的js項目改爲支持ts,能夠:webpack

npm install --save typescript @types/node @types/react @types/react-dom @types/jest
# or
yarn add typescript @types/node @types/react @types/react-dom @types/jest
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而後,將.js文件後綴改爲.ts重啓development server便可。git

CRA還能幹嗎

CRA除了能幫咱們構建出一個React的SPA項目(generator),充當腳手架的做用。還能爲咱們在項目開發,編譯時進行構建,充當builder的做用。能夠看到生成的項目中的package.json包含不少命令:github

  1. react-scripts start啓動開發模式下的一個dev-server,並支持代碼修改時的Hot Reload
  2. react-scripts build使用webpack進行編譯打包,生成生產模式下的全部腳本,靜態資源
  3. react-scripts test執行全部測試用例,完成對咱們每一個模塊質量的保證

這裏,咱們針對start這條線進行追蹤,探查CRA實現的原理。入口爲create-react-app/packages/react-scripts/bin/react-scripts.js,這個腳本會在react-scripts中設置到package.json的bin字段中去,也就是說這個package能夠做爲可執行的nodejs腳本,經過cli方式在nodejs宿主環境中。這個入口腳本很是簡單,這裏只列出主要的一個switch分支:web

switch (script) {
  case 'build':
  case 'eject':
  case 'start':
  case 'test': {
    const result = spawn.sync(
      'node',
      nodeArgs
        .concat(require.resolve('../scripts/' + script))
        .concat(args.slice(scriptIndex + 1)),
      { stdio: 'inherit' }
    );
    if (result.signal) {
      if (result.signal === 'SIGKILL') {
        console.log(
          'The build failed because the process exited too early. ' +
            'This probably means the system ran out of memory or someone called ' +
            '`kill -9` on the process.'
        );
      } else if (result.signal === 'SIGTERM') {
        console.log(
          'The build failed because the process exited too early. ' +
            'Someone might have called `kill` or `killall`, or the system could ' +
            'be shutting down.'
        );
      }
      process.exit(1);
    }
    process.exit(result.status);
    break;
  }
  default:
    console.log('Unknown script "' + script + '".');
    console.log('Perhaps you need to update react-scripts?');
    console.log(
      'See: https://facebook.github.io/create-react-app/docs/updating-to-new-releases'
    );
    break;
}
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能夠看到,當根據不一樣command,會分別resolve不一樣的js腳本,執行不一樣的任務,這裏咱們繼續看require('../scripts/start')chrome

// Do this as the first thing so that any code reading it knows the right env.
process.env.BABEL_ENV = 'development';
process.env.NODE_ENV = 'development';
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由於是開發模式,因此這裏把babel,node的環境變量都設置爲development,而後是全局錯誤的捕獲,這些都是一個cli腳本一般的處理方式:typescript

// Makes the script crash on unhandled rejections instead of silently
// ignoring them. In the future, promise rejections that are not handled will
// terminate the Node.js process with a non-zero exit code.
process.on('unhandledRejection', err => {
  throw err;
});
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確保其餘的環境變量配置也讀進進程了,因此這裏會經過../config/env腳本進行初始化:

// Ensure environment variables are read.
require('../config/env');
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還有一些預檢查,這部分是做爲eject以前對項目目錄的檢查,這裏由於eject不在咱們範圍,直接跳過。而後進入到了咱們主腳本的依賴列表:

const fs = require('fs');
const chalk = require('react-dev-utils/chalk');
const webpack = require('webpack');
const WebpackDevServer = require('webpack-dev-server');
const clearConsole = require('react-dev-utils/clearConsole');
const checkRequiredFiles = require('react-dev-utils/checkRequiredFiles');
const {
  choosePort,
  createCompiler,
  prepareProxy,
  prepareUrls,
} = require('react-dev-utils/WebpackDevServerUtils');
const openBrowser = require('react-dev-utils/openBrowser');
const paths = require('../config/paths');
const configFactory = require('../config/webpack.config');
const createDevServerConfig = require('../config/webpackDevServer.config');

const useYarn = fs.existsSync(paths.yarnLockFile);
const isInteractive = process.stdout.isTTY;
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能夠看到,主要的依賴仍是webpack,WDS,以及自定義的一些devServer的configuration以及webpack的configuration,能夠大膽猜測原理和咱們平時使用webpack並無什麼不一樣。

由於create-react-app my-app以後經過模版生成的項目中入口腳本被放置在src/index.js,而入口html被放置在public/index.html,因此須要對這兩個文件進行檢查:

// Warn and crash if required files are missing
if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
  process.exit(1);
}
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下面這部分是涉及C9雲部署時的環境變量檢查,不在咱們考究範圍,也直接跳過。react-dev-utils/browsersHelper是一個瀏覽器支持的幫助utils,由於在react-scripts v2以後必需要提供一個browser list支持列表,不過咱們能夠在package.json中看到,模版項目中已經爲咱們生成了:

"browserslist": {
  "production": [
    ">0.2%",
    "not dead",
    "not op_mini all"
  ],
  "development": [
    "last 1 chrome version",
    "last 1 firefox version",
    "last 1 safari version"
  ]
}
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檢查完devServer端口後,進入咱們核心邏輯執行,這裏的主線仍是和咱們使用webpack方式幾乎沒什麼區別,首先會經過configFactory建立出一個webpack的configuration object,而後經過createDevServerConfig建立出一個devServer的configuration object,而後傳遞webpack config實例化一個webpack compiler實例,傳遞devServer的configuration實例化一個WDS實例開始監聽指定的端口,最後經過openBrowser調用咱們的瀏覽器,打開咱們的SPA。

其實,整個流程咱們看到這裏,已經結束了,咱們知道WDS和webpack配合,能夠進行熱更,file changes watching等功能,咱們開發時,經過修改源代碼,或者樣式文件,會被實時監聽,而後webpack中的HWR會實時刷新瀏覽器頁面,能夠很方便的進行實時調試開發。

const config = configFactory('development');
const protocol = process.env.HTTPS === 'true' ? 'https' : 'http';
const appName = require(paths.appPackageJson).name;
const useTypeScript = fs.existsSync(paths.appTsConfig);
const urls = prepareUrls(protocol, HOST, port);
const devSocket = {
  warnings: warnings =>
    devServer.sockWrite(devServer.sockets, 'warnings', warnings),
  errors: errors =>
    devServer.sockWrite(devServer.sockets, 'errors', errors),
};
// Create a webpack compiler that is configured with custom messages.
const compiler = createCompiler({
  appName,
  config,
  devSocket,
  urls,
  useYarn,
  useTypeScript,
  webpack,
});
// Load proxy config
const proxySetting = require(paths.appPackageJson).proxy;
const proxyConfig = prepareProxy(proxySetting, paths.appPublic);
// Serve webpack assets generated by the compiler over a web server.
const serverConfig = createDevServerConfig(
  proxyConfig,
  urls.lanUrlForConfig
);
const devServer = new WebpackDevServer(compiler, serverConfig);
// Launch WebpackDevServer.
devServer.listen(port, HOST, err => {
  if (err) {
    return console.log(err);
  }
  if (isInteractive) {
    clearConsole();
  }

  // We used to support resolving modules according to `NODE_PATH`.
  // This now has been deprecated in favor of jsconfig/tsconfig.json
  // This lets you use absolute paths in imports inside large monorepos:
  if (process.env.NODE_PATH) {
    console.log(
      chalk.yellow(
        'Setting NODE_PATH to resolve modules absolutely has been deprecated in favor of setting baseUrl in jsconfig.json (or tsconfig.json if you are using TypeScript) and will be removed in a future major release of create-react-app.'
      )
    );
    console.log();
  }

  console.log(chalk.cyan('Starting the development server...\n'));
  openBrowser(urls.localUrlForBrowser);
});

['SIGINT', 'SIGTERM'].forEach(function(sig) {
  process.on(sig, function() {
    devServer.close();
    process.exit();
  });
});
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經過start命令的追蹤,咱們知道CRA最終仍是經過WDS和webpack進行開發監聽的,其實build會比start更簡單,只是在webpack configuration中會進行優化。CRA作到了能夠0配置,就能進行react項目的開發,調試,打包。

實際上是由於CRA把複雜的webpack config配置封裝起來了,把babel plugins預設好了,把開發時會經常使用到的一個環境檢查,polyfill兼容都給開發者作了,因此使用起來會比咱們直接使用webpack,本身進行重複的配置信息設置要來的簡單不少。

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