webpack模块化原理-Code Splitting

webpack的模块化不仅支持commonjs和es module,还能通过code splitting实现模块的动态加载。根据wepack官方文档,实现动态加载的方式有两种:importrequire.ensure

那么,这篇文档就来分析一下,webpack是如何实现code splitting的。

PS:如果你对webpack如何实现commonjs和es module感兴趣,可以查看我的前两篇文章:webpack模块化原理-commonjswebpack模块化原理-ES module

准备

首先我们依然创建一个简单入口模块index.js和两个依赖模块foo.jsbar.js

// index.js
'use strict';
import(/* webpackChunkName: "foo" */ './foo').then(foo => {
    console.log(foo());
})
import(/* webpackChunkName: "bar" */ './bar').then(bar => {
    console.log(bar());
})
// foo.js
'use strict';
exports.foo = function () {
    return 2;
}
// bar.js
'use strict';
exports.bar = function () {
    return 1;
}

webpack配置如下:

var path = require("path");

module.exports = {
    entry: path.join(__dirname, 'index.js'),
    output: {
        path: path.join(__dirname, 'outs'),
        filename: 'index.js',
        chunkFilename: '[name].bundle.js'
    },
};

这是一个最简单的配置,指定了模块入口和打包文件输出路径,值得注意的是,这次还指定了分离模块的文件名[name].bundle.js(不指定会有默认文件名)。

在根目录下执行webpack,得到经过webpack打包的代码如下(去掉了不必要的注释):

(function(modules) { // webpackBootstrap
    // install a JSONP callback for chunk loading
    var parentJsonpFunction = window["webpackJsonp"];
    window["webpackJsonp"] = function webpackJsonpCallback(chunkIds, moreModules, executeModules) {
        // add "moreModules" to the modules object,
        // then flag all "chunkIds" as loaded and fire callback
        var moduleId, chunkId, i = 0, resolves = [], result;
        for(;i < chunkIds.length; i++) {
            chunkId = chunkIds[i];
            if(installedChunks[chunkId]) {
                resolves.push(installedChunks[chunkId][0]);
            }
            installedChunks[chunkId] = 0;
        }
        for(moduleId in moreModules) {
            if(Object.prototype.hasOwnProperty.call(moreModules, moduleId)) {
                modules[moduleId] = moreModules[moduleId];
            }
        }
        if(parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules, executeModules);
        while(resolves.length) {
            resolves.shift()();
        }
    };
    // The module cache
    var installedModules = {};
    // objects to store loaded and loading chunks
    var installedChunks = {
        2: 0
    };
    // The require function
    function __webpack_require__(moduleId) {
        // Check if module is in cache
        if(installedModules[moduleId]) {
            return installedModules[moduleId].exports;
        }
        // Create a new module (and put it into the cache)
        var module = installedModules[moduleId] = {
            i: moduleId,
            l: false,
            exports: {}
        };
        // Execute the module function
        modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);
        // Flag the module as loaded
        module.l = true;
        // Return the exports of the module
        return module.exports;
    }
    // This file contains only the entry chunk.
    // The chunk loading function for additional chunks
    __webpack_require__.e = function requireEnsure(chunkId) {
        var installedChunkData = installedChunks[chunkId];
        if(installedChunkData === 0) {
            return new Promise(function(resolve) { resolve(); });
        }
        // a Promise means "currently loading".
        if(installedChunkData) {
            return installedChunkData[2];
        }
        // setup Promise in chunk cache
        var promise = new Promise(function(resolve, reject) {
            installedChunkData = installedChunks[chunkId] = [resolve, reject];
        });
        installedChunkData[2] = promise;
        // start chunk loading
        var head = document.getElementsByTagName('head')[0];
        var script = document.createElement('script');
        script.type = 'text/javascript';
        script.charset = 'utf-8';
        script.async = true;
        script.timeout = 120000;
        if (__webpack_require__.nc) {
            script.setAttribute("nonce", __webpack_require__.nc);
        }
        script.src = __webpack_require__.p + "" + ({"0":"foo","1":"bar"}[chunkId]||chunkId) + ".bundle.js";
        var timeout = setTimeout(onScriptComplete, 120000);
        script.onerror = script.onload = onScriptComplete;
        function onScriptComplete() {
            // avoid mem leaks in IE.
            script.onerror = script.onload = null;
            clearTimeout(timeout);
            var chunk = installedChunks[chunkId];
            if(chunk !== 0) {
                if(chunk) {
                    chunk[1](new Error('Loading chunk ' + chunkId + ' failed.'));
                }
                installedChunks[chunkId] = undefined;
            }
        };
        head.appendChild(script);
        return promise;
    };
    // expose the modules object (__webpack_modules__)
    __webpack_require__.m = modules;
    // expose the module cache
    __webpack_require__.c = installedModules;
    // define getter function for harmony exports
    __webpack_require__.d = function(exports, name, getter) {
        if(!__webpack_require__.o(exports, name)) {
            Object.defineProperty(exports, name, {
                configurable: false,
                enumerable: true,
                get: getter
            });
        }
    };
    // getDefaultExport function for compatibility with non-harmony modules
    __webpack_require__.n = function(module) {
        var getter = module && module.__esModule ?
            function getDefault() { return module['default']; } :
            function getModuleExports() { return module; };
        __webpack_require__.d(getter, 'a', getter);
        return getter;
    };
    // Object.prototype.hasOwnProperty.call
    __webpack_require__.o = function(object, property) { return Object.prototype.hasOwnProperty.call(object, property); };
    // __webpack_public_path__
    __webpack_require__.p = "";
    // on error function for async loading
    __webpack_require__.oe = function(err) { console.error(err); throw err; };
    // Load entry module and return exports
    return __webpack_require__(__webpack_require__.s = 0);
})
([
(function(module, exports, __webpack_require__) {
    "use strict";
    __webpack_require__.e/* import() */(0).then(__webpack_require__.bind(null, 1)).then(foo => {
        console.log(foo());
    })
    __webpack_require__.e/* import() */(1).then(__webpack_require__.bind(null, 2)).then(bar => {
        console.log(bar());
    })
})
]);

分析

编译后的代码,整体跟前两篇文章中使用commonjs和es6 module编写的代码编译后的结构差别不大,都是通过IFFE的方式启动代码,然后使用webpack实现的requireexports实现的模块化。

而对于code splitting的支持,区别在于这里使用__webpack_require__.e实现动态加载模块和实现基于promise的模块导入。

所以首先分析__webpack_require__.e函数的定义,这个函数实现了动态加载:

__webpack_require__.e = function requireEnsure(chunkId) {
    // 1、缓存查找
    var installedChunkData = installedChunks[chunkId];
    if(installedChunkData === 0) {
        return new Promise(function(resolve) { resolve(); });
    }
    if(installedChunkData) {
        return installedChunkData[2];
    }
    // 2、缓存模块
    var promise = new Promise(function(resolve, reject) {
        installedChunkData = installedChunks[chunkId] = [resolve, reject];
    });
    installedChunkData[2] = promise;
    // 3、加载模块
    var head = document.getElementsByTagName('head')[0];
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.charset = 'utf-8';
    script.async = true;
    script.timeout = 120000;
    if (__webpack_require__.nc) {
        script.setAttribute("nonce", __webpack_require__.nc);
    }
    script.src = __webpack_require__.p + "" + ({"0":"foo"}[chunkId]||chunkId) + ".bundle.js";
    // 4、异常处理
    var timeout = setTimeout(onScriptComplete, 120000);
    script.onerror = script.onload = onScriptComplete;
    function onScriptComplete() {
        // avoid mem leaks in IE.
        script.onerror = script.onload = null;
        clearTimeout(timeout);
        var chunk = installedChunks[chunkId];
        if(chunk !== 0) {
            if(chunk) {
                chunk[1](new Error('Loading chunk ' + chunkId + ' failed.'));
            }
            installedChunks[chunkId] = undefined;
        }
    };
    head.appendChild(script);
    // 5、返回promise
    return promise;
};

代码大致逻辑如下:

  1. 缓存查找:从缓存installedChunks中查找是否有缓存模块,如果缓存标识为0,则表示模块已加载过,直接返回promise;如果缓存为数组,表示缓存正在加载中,则返回缓存的promise对象
  2. 如果没有缓存,则创建一个promise,并将promiseresolvereject缓存在installedChunks
  3. 构建一个script标签,append到head标签中,src指向加载的模块脚本资源,实现动态加载js脚本
  4. 添加script标签onload、onerror 事件,如果超时或者模块加载失败,则会调用reject返回模块加载失败异常
  5. 如果模块加载成功,则返回当前模块promise,对应于import()

以上便是模块加载的过程,当资源加载完成,模块代码开始执行,那么我们来看一下模块代码的结构:

webpackJsonp([0],[
/* 0 */,
/* 1 */
/***/ (function(module, exports, __webpack_require__) {
"use strict";
exports.foo = function () {
    return 2;
}
/***/ })
]);

可以看到,模块代码不仅被包在一个函数中(用来模拟模块作用域),外层还被当做参数传入webpackJsonp中。那么这个webpackJsonp函数的作用是什么呢?

其实这里的webpackJsonp类似于jsonp中的callback,作用是作为模块加载和执行完成的回调,从而触发importresolve

具体细看webpackJsonp代码来分析:

window["webpackJsonp"] = function webpackJsonpCallback(chunkIds, moreModules, executeModules) {
    var moduleId, chunkId, i = 0, resolves = [], result;
    // 1、收集模块resolve
    for(;i < chunkIds.length; i++) {
        chunkId = chunkIds[i];
        if(installedChunks[chunkId]) {
            resolves.push(installedChunks[chunkId][0]);
        }
        installedChunks[chunkId] = 0;
    }
    // 2、copy模块到modules
    for(moduleId in moreModules) {
        if(Object.prototype.hasOwnProperty.call(moreModules, moduleId)) {
            modules[moduleId] = moreModules[moduleId];
        }
    }
    if(parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules, executeModules);
    // 3、resolve import
    while(resolves.length) {
        resolves.shift()();
    }
};

代码大致逻辑如下:

  1. 根据chunkIds收集对应模块的resolve,这里的chunkIds为数组是因为require.ensure是可以实现异步加载多个模块的,所以需要兼容
  2. 把动态模块添加到IFFE的modules中,提供其他CMD方案使用模块
  3. 直接调用resolve,完成整个异步加载

总结

webpack通过__webpack_require__.e函数实现了动态加载,再通过webpackJsonp函数实现异步加载回调,把模块内容以promise的方式暴露给调用方,从而实现了对code splitting的支持。

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