likes
comments
collection
share

从Webpack编译后的代码，探讨Webpack异步加载机制

作者站长头像
站长
· 阅读数 38

从Webpack编译后的代码，探讨Webpack异步加载机制

首页加载不需要的模块，经常通过webpack的分包机制，将其独立出单独的文件。在需要的时候再加载。这样使首页加载的文件体积大大缩小，加快了加载时间。本篇探讨webpack是加载异步文件的原理以及webpack如何实现其原理的，最后在手动实现一个非常简单的demo。

原理

webpack异步加载的原理：

  1. 首先异步加载的模块，webpack在打包的时候会将独立打包成一个js文件（webpack如何将异步加载的模块独立打包成一个文件）

  2. 然后需要加载异步模块的时候： 2.1 创建script标签，src为请求该异步模块的url，并添加到document.head里，由浏览器发起请求。

    2.2 请求成功后，将异步模块添加到全局的__webpack_require__变量(该对象是用来管理全部模块)后

    2.3 请求异步加载文件的import()编译后的方法会从全局的__webpack_require__变量中找到对应的模块

    2.4 执行相应的业务代码并删除之前创建的script标签

异步加载文件里的import()里的回调方法的执行时机，通过利用promise的机制来实现的

准备工作

环境：webpack版本："5.7.0"

按一下目录结构创建文件

├── src
│   │── index.js
│   │── info.js
├── index.html
├── webpack.config.json
├── package.json
// src/index.js
function button () {
  const button = document.createElement('button')
  const text = document.createTextNode('click me')
  button.appendChild(text)
  button.onclick = e => import('./info.js').then(res => {
    console.log(res.log)
  })
  return button
}

document.body.appendChild(button())
// src/info.js
export const log = "log info"
// webpack.config.json

const path = require('path');

module.exports = {
  entry: './src/index.js',
  mode: 'development',
  output: {
    path: path.resolve(__dirname, './dist'),
    filename: 'main.js'
  }
}
// package.json
{
  "name": "import",
  "version": "1.0.0",
  "description": "",
  "main": "webpack.config.js",
  "dependencies": {
    "webpack": "^5.7.0",
    "webpack-cli": "^4.2.0"
  },
  "devDependencies": {},
  "scripts": {
    "build": "webpack --config webpack.config.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

// index.html
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script src="./dist/main.js"></script>
</body>
</html>

执行npm run build得到/dist/main.js`/dist/src_info_js.man.js`文件。这两个文件就是我们要分析webpack是如何实现异步加载的入口。

webpack如何实现的？

1.初始化(执行加载文件代码之前)

  • 根据当前script获取当前地址

根据当前执行js文件的地址，截取公共地址，并赋值带全局变量中。

  scriptUrl = document.currentScript.src
  scriptUrl = scriptUrl.replace(/#.*$/, "").replace(/\?.*$/, "").replace(/\/[^\/]+$/, "/"); // 1. 过滤hash 2.过滤参数 3. 过滤当前文件名
  __webpack_require__.p = scriptUrl;
  • 重写webpackChunkimport数组的push方法(webpackJsonpCallback)
  self["webpackChunkimport"].push = webpackJsonpCallback

2.执行中

  • 创建加载模块的promise对象， 缓存要加载模块的promise.resolve, promise.reject以及promise自身。

import()编译成__webpack_require__.e方法

__webpack_require__.e = (chunkId) => {
  return Promise.all(Object.keys(__webpack_require__.f).reduce((promises, key) => {
    __webpack_require__.f[key](chunkId, promises);
    return promises;
  }, []));
};

__webpack_required__f.j = (chunkId, promises) => {
  var promise = new Promise((resolve, reject) => {
    installedChunkData = installedChunks[chunkId] = [resolve, reject];
  });
  promises.push(installedChunkData[2] = promise);
  var url = __webpack_require__.p + __webpack_require__.u(chunkId);
  loadingEnded = (event) => {
    // ...
  }
    __webpack_require__.l(url, loadingEnded, "chunk-" + chunkId);
}

var webpackJsonpCallback = (data) => {
  var [chunkIds, moreModules, runtime] = data;
  var moduleId, chunkId, i = 0,
    resolves = [];
  for (; i < chunkIds.length; i++) {
    chunkId = chunkIds[i];
    if (__webpack_require__.o(installedChunks, chunkId) && installedChunks[chunkId]) {
      resolves.push(installedChunks[chunkId][0]);
    }
    installedChunks[chunkId] = 0;
  }
  for (moduleId in moreModules) {
    if (__webpack_require__.o(moreModules, moduleId)) {
      __webpack_require__.m[moduleId] = moreModules[moduleId];
    }
  }
  parentChunkLoadingFunction(data);
  while (resolves.length) {
    resolves.shift()();
  }
}

webpack是如何执行加载异步模块的?1.这里将webpackJsonpCalback放在一起，理解起来会跟好。由webpack将import()编译成的__webpack_require__.e方法，实际上是一个由Promise.all返回的Promise对象，每加载一个异步模块都会新建一个promise对象，并将其resolve、reject以及自身保存在installedChunks变量中。2.webpackJsonpCallback是在异步加载文件中执行webpackChunkimport数组的push才会调用的，执行到webpackJsonpCallback方法时意味着异步加载的文件已经加载成功了。所以在该方法里将异步加载文件里的模块添加到__webpack_require__.m变量中(该变量维护着所有模块)。并将之前的创建的promise对象的resolve方法执行。3.

// 请求异步加载的代码(编译前的代码)
function button () {
  const button = document.createElement('button')
  const text = document.createTextNode('click me')
  button.appendChild(text)
  button.onclick = e => import('./info.js').then(res => {
    console.log(res.log)
  })
  return button
}

document.body.appendChild(button())

// 请求异步加载的代码(编译后的代码)
function button() {
const button = document.createElement('button');
const text = document.createTextNode('click me');
button.appendChild(text);
button.onclick = e =>
  __webpack_require__.e( /*! import() */ "src_info_js")
  .then(__webpack_require__.bind(__webpack_require__, "./src/info.js"))
  .then(res => {
    console.log(res.log)
  })
return button
}
document.body.appendChild(button())

观察请求异步加载的代码编译前后的不同，会发现编译后import()方法变成了__webpack_requre__.e，而且还多了个then方法。为什么多了个then方法呢?因为__webpack_require__.e执行resolve，没有返回的值，只是说明该异步文件已经加载成功了并将模块添加到了__webpack_require__.m, 而多的then方法里的代码就是从__webpack_require__.m变量里获取模块的。

  • 生成url

    var url = __webpack_require__.p + __webpack_require__.u(chunkId);

  • 创建script标签，并添加加载成功script.onload和失败的函数script.onerror

    __webpack_require__.l = (url, done, key) => {
if (inProgress[url]) {
  inProgress[url].push(done);
  return;
}
var script, needAttach;
// ...
if (!script) {
  needAttach = true;
  script = document.createElement('script');
  script.charset = 'utf-8';
  script.timeout = 120;
  if (__webpack_require__.nc) {
    script.setAttribute("nonce", __webpack_require__.nc);
  }
  script.setAttribute("data-webpack", dataWebpackPrefix + key);
  script.src = url;
}
inProgress[url] = [done];
var onScriptComplete = (prev, event) => {
  /******/ // avoid mem leaks in IE.
  script.onerror = script.onload = null;
  clearTimeout(timeout);
  var doneFns = inProgress[url];
  delete inProgress[url];
  script.parentNode && script.parentNode.removeChild(script);
  doneFns && doneFns.forEach((fn) => fn(event));
  if (prev) return prev(event);
}
;
var timeout = setTimeout(onScriptComplete.bind(null, undefined, {
  type: 'timeout',
  target: script
}), 120000);
script.onerror = onScriptComplete.bind(null, script.onerror);
script.onload = onScriptComplete.bind(null, script.onload);
needAttach && document.head.appendChild(script);
};

3.执行完成后script.onload加载时机当异步加载的文件加载完成并执行完之后，触发onload方法，将之前新增的script标签删除。

简单实现

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button class="btn">import something</button>
  <script>
    document.querySelector(".btn").addEventListener("click", () => {
      ensure("jsonp.js")
        .then(() => {
          return requireModule("jsonp.js")();
        })
        .then(res => {
          console.log(res.log);
        })
    })
    
    let modules = {};
    let handlers;
    window.jsonp = [];
    window.jsonp.push = webpackJsonpCallback;
    function requireModule (id) {
      return modules[id];
    }

    function webpackJsonpCallback (data) {
      let [id, moreModule] = data;
      modules[id] = moreModule;
      handlers.shift()();
    }

    function ensure (id, promises) {
      let promise = new Promise((resolve, reject) => {
        handlers = [resolve]
      })
      
      script = document.createElement('script');
      script.src = "jsonp.js";
      document.head.appendChild(script)

      return promise;
    }

  </script>
</body>
</html>
window.jsonp.push(["jsonp.js", () => ({
  "log": "log info"
})])

参考：你的 import 被 webpack 编译成了什么？webpack 异步加载原理