构建 react应用程序 （二）（react-scripts实现原理）
在前面讲到了使用create-react-app来创建项目，这节我们来分析下原理。
react-scripts有以下支持，都帮你配置好了：
React, JSX, ES6, and Flow syntax support.
Language extras beyond ES6 like the object spread operator.
Import CSS and image files directly from JavaScript.
Autoprefixed CSS, so you don’t need -webkit or other prefixes.
A build script to bundle JS, CSS, and images for production, with sourcemaps.
A dev server that lints for common errors.翻译就不翻译了，大概就是说es6、css依赖啊 图片依赖之类的都已经通过react-scripts配置好了。
其实babel-core，webpack，等等这些 你都没下载，配置。
这些活，react-scripts 都帮你做了。
在命令窗口输入的是npm start，而start调用的是
"start": "react-scripts start",找到node_modules里面的react-scripts 插件。点开bin\react-scripts.js文件内容
'use strict';
process.on('unhandledRejection', err => {
throw err;
});
const spawn = require('react-dev-utils/crossSpawn');
const args = process.argv.slice(2);
const scriptIndex = args.findIndex(
x => x === 'build' || x === 'eject' || x === 'start' || x === 'test'
);
const script = scriptIndex === -1 ? args[0] : args[scriptIndex];
const nodeArgs = scriptIndex > 0 ? args.slice(0, scriptIndex) : [];
switch (script) {
case 'build':
case 'eject':
case 'start':
case 'test': {
const result = spawn.sync(
'node',
nodeArgs
.concat(require.resolve('../scripts/' + script))
//require.resolve函数来查询某个模块文件的带有完整绝对路径的文件名
.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;
} .concat(require.resolve('../scripts/' + script))从上面这话可以看出调用start时需要调用scripts/start.js，点开该文件：
// @remove-on-eject-begin
/**
* Copyright (c) 2015-present, Facebook, Inc.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*/
// @remove-on-eject-end
'use strict';
// 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';
// 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;
});
// Ensure environment variables are read.
require('../config/env');
// @remove-on-eject-begin
// Do the preflight check (only happens before eject).
const verifyPackageTree = require('./utils/verifyPackageTree');
if (process.env.SKIP_PREFLIGHT_CHECK !== 'true') {
verifyPackageTree();
}
const verifyTypeScriptSetup = require('./utils/verifyTypeScriptSetup');
verifyTypeScriptSetup();
// @remove-on-eject-end
const fs = require('fs');
const chalk = require('react-dev-utils/chalk');
const webpack = require('webpack');//这里引入了webpack
const WebpackDevServer = require('webpack-dev-server');//这里引入了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');//文件处理的配置
const configFactory = require('../config/webpack.config');
const createDevServerConfig = require('../config/webpackDevServer.config');
const useYarn = fs.existsSync(paths.yarnLockFile);
const isInteractive = process.stdout.isTTY;
// Warn and crash if required files are missing
if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
process.exit(1);
}
// Tools like Cloud9 rely on this.
const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
//端口设置 这就是为什么 端口号 不是8080 而是 3000 的原因，在这里可以改成8080，重新npm run start 生效
const HOST = process.env.HOST || '0.0.0.0';
if (process.env.HOST) {
console.log(
chalk.cyan(
`Attempting to bind to HOST environment variable: ${chalk.yellow(
chalk.bold(process.env.HOST)
)}`
)
);
console.log(
`If this was unintentional, check that you haven't mistakenly set it in your shell.`
);
console.log(
`Learn more here: ${chalk.yellow('https://bit.ly/CRA-advanced-config')}`
);
console.log();
}
// We require that you explictly set browsers and do not fall back to
// browserslist defaults.
const { checkBrowsers } = require('react-dev-utils/browsersHelper');
checkBrowsers(paths.appPath, isInteractive)
.then(() => {
// We attempt to use the default port but if it is busy, we offer the user to
// run on a different port. `choosePort()` Promise resolves to the next free port.
return choosePort(HOST, DEFAULT_PORT);
})
.then(port => {
if (port == null) {
// We have not found a port.
return;
}
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();
}
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();
});
});
})
.catch(err => {
if (err && err.message) {
console.log(err.message);
}
process.exit(1);
});
打开config/paths.js，发现其实里面好多默认的配置都是写在该文件，可以通过修改改文件来实现自己文件的存放配置
'use strict';
const path = require('path');
const fs = require('fs');
const url = require('url');
// Make sure any symlinks in the project folder are resolved:
// https://github.com/facebook/create-react-app/issues/637
const appDirectory = fs.realpathSync(process.cwd());
const resolveApp = relativePath => path.resolve(appDirectory, relativePath);
const envPublicUrl = process.env.PUBLIC_URL;
function ensureSlash(inputPath, needsSlash) {
const hasSlash = inputPath.endsWith('/');
if (hasSlash && !needsSlash) {
return inputPath.substr(0, inputPath.length - 1);
} else if (!hasSlash && needsSlash) {
return `${inputPath}/`;
} else {
return inputPath;
}
}
const getPublicUrl = appPackageJson =>
envPublicUrl || require(appPackageJson).homepage;
// We use `PUBLIC_URL` environment variable or "homepage" field to infer
// "public path" at which the app is served.
// Webpack needs to know it to put the right