一文快速弄懂webpack动态import原理

2024-04-02 19:55

短信预约 -IT技能 免费直播动态提醒

前言

在vue中我们经常用到动态导入页面组件，那么它是如何实现的呢，本文将通过简单的案例，快速了解实现原理

例子

// index.js
import('./test').then(fn => {
  console.log(fn.default());
})

// test.js
export default function func() {
  return 1
}

打包后的代码包含两个文件 bundle.js 和 0.js

点击展开bundle.js

 (function(modules) { // webpackBootstrap
 	// install a JSONP callback for chunk loading
 	function webpackJsonpCallback(data) {
 		var chunkIds = data[0];
 		var moreModules = data[1];


 		// add "moreModules" to the modules object,
 		// then flag all "chunkIds" as loaded and fire callback
 		var moduleId, chunkId, i = 0, resolves = [];
 		for(;i < chunkIds.length; i++) {
 			chunkId = chunkIds[i];
 			if(Object.prototype.hasOwnProperty.call(installedChunks, chunkId) && 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(data);

 		while(resolves.length) {
 			resolves.shift()();
 		}

 	};


 	// The module cache
 	var installedModules = {};

 	// object to store loaded and loading chunks
 	// undefined = chunk not loaded, null = chunk preloaded/prefetched
 	// Promise = chunk loading, 0 = chunk loaded
 	var installedChunks = {
 		"main": 0
 	};



 	// script path function
 	function jsonpScriptclass="lazy" data-src(chunkId) {
 		return __webpack_require__.p + "" + chunkId + ".bundle.js"
 	}

 	// 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 promises = [];


 		// JSONP chunk loading for javascript

 		var installedChunkData = installedChunks[chunkId];
 		if(installedChunkData !== 0) { // 0 means "already installed".

 			// a Promise means "currently loading".
 			if(installedChunkData) {
 				promises.push(installedChunkData[2]);
 			} else {
 				// setup Promise in chunk cache
 				var promise = new Promise(function(resolve, reject) {
 					installedChunkData = installedChunks[chunkId] = [resolve, reject];
 				});
 				promises.push(installedChunkData[2] = promise);

 				// start chunk loading
 				var script = document.createElement('script');
 				var onScriptComplete;

 				script.charset = 'utf-8';
 				script.timeout = 120;
 				if (__webpack_require__.nc) {
 					script.setAttribute("nonce", __webpack_require__.nc);
 				}
 				script.class="lazy" data-src = jsonpScriptclass="lazy" data-src(chunkId);

 				// create error before stack unwound to get useful stacktrace later
 				var error = new Error();
 				onScriptComplete = function (event) {
 					// avoid mem leaks in IE.
 					script.onerror = script.onload = null;
 					clearTimeout(timeout);
 					var chunk = installedChunks[chunkId];
 					if(chunk !== 0) {
 						if(chunk) {
 							var errorType = event && (event.type === 'load' ? 'missing' : event.type);
 							var realclass="lazy" data-src = event && event.target && event.target.class="lazy" data-src;
 							error.message = 'Loading chunk ' + chunkId + ' failed.\n(' + errorType + ': ' + realclass="lazy" data-src + ')';
 							error.name = 'ChunkLoadError';
 							error.type = errorType;
 							error.request = realclass="lazy" data-src;
 							chunk[1](error);
 						}
 						installedChunks[chunkId] = undefined;
 					}
 				};
 				var timeout = setTimeout(function(){
 					onScriptComplete({ type: 'timeout', target: script });
 				}, 120000);
 				script.onerror = script.onload = onScriptComplete;
 				document.head.appendChild(script);
 			}
 		}
 		return Promise.all(promises);
 	};

 	// 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, { enumerable: true, get: getter });
 		}
 	};

 	// define __esModule on exports
 	__webpack_require__.r = function(exports) {
 		if(typeof Symbol !== 'undefined' && Symbol.toStringTag) {
 			Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });
 		}
 		Object.defineProperty(exports, '__esModule', { value: true });
 	};

 	// create a fake namespace object
 	// mode & 1: value is a module id, require it
 	// mode & 2: merge all properties of value into the ns
 	// mode & 4: return value when already ns object
 	// mode & 8|1: behave like require
 	__webpack_require__.t = function(value, mode) {
 		if(mode & 1) value = __webpack_require__(value);
 		if(mode & 8) return value;
 		if((mode & 4) && typeof value === 'object' && value && value.__esModule) return value;
 		var ns = Object.create(null);
 		__webpack_require__.r(ns);
 		Object.defineProperty(ns, 'default', { enumerable: true, value: value });
 		if(mode & 2 && typeof value != 'string') for(var key in value) __webpack_require__.d(ns, key, function(key) { return value[key]; }.bind(null, key));
 		return ns;
 	};

 	// 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; };

 	var jsonpArray = window["webpackJsonp"] = window["webpackJsonp"] || [];
 	var oldJsonpFunction = jsonpArray.push.bind(jsonpArray);
 	jsonpArray.push = webpackJsonpCallback;
 	jsonpArray = jsonpArray.slice();
 	for(var i = 0; i < jsonpArray.length; i++) webpackJsonpCallback(jsonpArray[i]);
 	var parentJsonpFunction = oldJsonpFunction;


 	// Load entry module and return exports
 	return __webpack_require__(__webpack_require__.s = "./class="lazy" data-src/index.js");
 })

 ({

 "./class="lazy" data-src/index.js":


 (function(module, exports, __webpack_require__) {

eval("__webpack_require__.e( 0).then(__webpack_require__.bind(null,  \"./class="lazy" data-src/test.js\")).then(function (fn) {\n  console.log(fn[\"default\"]());\n});\n\n//# sourceURL=webpack:///./class="lazy" data-src/index.js?");

 })

 });

点击展开0.js

(window["webpackJsonp"] = window["webpackJsonp"] || []).push([[0],{

 "./class="lazy" data-src/test.js":


 (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
eval("__webpack_require__.r(__webpack_exports__);\n __webpack_require__.d(__webpack_exports__, \"default\", function() { return func; });\nfunction func() {\n  return 1;\n}\n\n//# sourceURL=webpack:///./class="lazy" data-src/test.js?");

 })

}]);

1. 模块加载

webpack通过__webpack_require__加载模块代码

// bundle.js
function __webpack_require__(moduleId)
        // 如果模块已经加载，直接返回模块导出
	if(installedModules[moduleId]) {
		return installedModules[moduleId].exports;
	}

        // 模块导出和模块信息
	var module = installedModules[moduleId] = {
		i: moduleId,
		l: false,
		exports: {}
	}
        // 执行模块代码
	modules[moduleId].call(module.exports, module, module.exports, __webpack_require__)
	module.l = true // 标记模块已经加载完成
	return module.exports;
}

__webpack_require__('index.js')

然后执行index.js编译后的代码，如下。

Promise.all(
  [
    __webpack_require__.e(0)
  ]
).then(
  __webpack_require__.bind(null, "./class="lazy" data-src/test.js")
).then(function (fn) {
  console.log(fn.default());
}));

2. jsonp动态加载script

先一步步来，看下__webpack_require__.e这个方法，它是最先调用的。

// bundle.js

__webpack_require__.e = function requireEnsure(chunkId) {
	var promises = []
	var installedChunkData = installedChunks[chunkId];
        // 如果这个chunk已经加载过了 就不需要加载了
	if(installedChunkData !== 0) { // 0 means "already installed"
		if(installedChunkData) {
			promises.push(installedChunkData[2]);
		} else {

                        // 为这个chunk创建一个promise
			var promise = new Promise(function(resolve, reject) {
                               // 记录这个chunk对应promise的resolve和reject方法
				installedChunkData = installedChunks[chunkId] = [resolve, reject];
			});

                         // promises数组里添加这个chunk对应的promise
			promises.push(installedChunkData[2] = promise)

                        // ============== 动态创建script =================
			var script = document.createElement('script');
			var onScriptComplete
			script.charset = 'utf-8';
			script.timeout = 120;
			if (__webpack_require__.nc) {
				script.setAttribute("nonce", __webpack_require__.nc);
			}
			script.class="lazy" data-src = jsonpScriptclass="lazy" data-src(chunkId)
			// create error before stack unwound to get useful stacktrace later
			var error = new Error();
                        // =================================================

			onScriptComplete = function (event) {
				// avoid mem leaks in IE.
				script.onerror = script.onload = null;
				clearTimeout(timeout);
				var chunk = installedChunks[chunkId];
				if(chunk !== 0) {
					if(chunk) {
						var errorType = event && (event.type === 'load' ? 'missing' : event.type);
						var realclass="lazy" data-src = event && event.target && event.target.class="lazy" data-src;
						error.message = 'Loading chunk ' + chunkId + ' failed.\n(' + errorType + ': ' + realclass="lazy" data-src + ')';
						error.name = 'ChunkLoadError';
						error.type = errorType;
						error.request = realclass="lazy" data-src;
						chunk[1](error);
					}
					installedChunks[chunkId] = undefined;
				}
			};
			var timeout = setTimeout(function(){
				onScriptComplete({ type: 'timeout', target: script });
			}, 120000);
			script.onerror = script.onload = onScriptComplete;
			document.head.appendChild(script);
		}
	}
	return Promise.all(promises);
};

总结一下，上述代码做的事情

如果chunk没有被加载过，会为这个chunk创建一个promise对象
将promise对象存在promises数组中
将promise的resolve 和 reject存在installedChunks[chunkId]中

3. 执行异步脚本

经过上面的过程，会动态加载0.js的脚本代码

(window["webpackJsonp"] = window["webpackJsonp"] || []).push([[0],{
"./class="lazy" data-src/test.js":
(function(module, __webpack_exports__, __webpack_require__) {
"use strict";
eval("__webpack_require__.r(__webpack_exports__);\n __webpack_require__.d(__webpack_exports__, \"default\", function() { return func; });\nfunction func() {\n  return 1;\n}\n\n//# sourceURL=webpack:///./class="lazy" data-src/test.js?");
 })
}]);

可以看到window上有一个webpackJsonp数组，那么这个东西是从哪里来的呢？，我们来看下面的代码。

// bundle.js
var jsonpArray = window["webpackJsonp"] = window["webpackJsonp"] || [];
var oldJsonpFunction = jsonpArray.push.bind(jsonpArray);
jsonpArray.push = webpackJsonpCallback;
jsonpArray = jsonpArray.slice();
for(var i = 0; i < jsonpArray.length; i++) webpackJsonpCallback(jsonpArray[i]);
var parentJsonpFunction = oldJsonpFunction;

其实一开始初始化时已经覆盖实现了webpackJsonp.push方法

(window["webpackJsonp"] = window["webpackJsonp"] || []).push([[0],{
  // test.js引入的模块代码
})
// 等价于
webpackJsonpCallback([[0],{
  // test.js引入的模块代码
})

下面再看看webpackJsonpCallback代码的实现

4. webpackJsonpCallback

// bundle.js
function webpackJsonpCallback(data) {
        // chunkid
	var chunkIds = data[0];
       // chunkid对应的模块
	var moreModules = data[1]
	var moduleId, chunkId, i = 0, resolves = [];

	for(;i < chunkIds.length; i++) {
		chunkId = chunkIds[i];
		if(Object.prototype.hasOwnProperty.call(installedChunks, chunkId) && installedChunks[chunkId]) {
                        // 收集chunk对应的resolve方法
			resolves.push(installedChunks[chunkId][0]);
		}
                // 标记该chunk已经加载
		installedChunks[chunkId] = 0;
	}
	for(moduleId in moreModules) {
		if(Object.prototype.hasOwnProperty.call(moreModules, moduleId)) {
                         // 添加chunk模块，到全局modules对象中
			modules[moduleId] = moreModules[moduleId];
		}
	}
	if(parentJsonpFunction) parentJsonpFunction(data)

         // 依次执行chunk对应promise的resolve方法
	while(resolves.length) {
		resolves.shift()();
	}
};

还是总结一下上面代码的过程

收集chunk对应的resolve方法, 前面执行__webpack_require__.e时放在了installedChunks[chunkId]里
将异步chunk下的所有模块添加到全局modules
依次执行chunk对应promise的resolve方法

5. 执行异步模块代码

回到一开始index.js的代码

Promise.all(
  [
    __webpack_require__.e(0)
  ]
).then(
  __webpack_require__.bind(null, "./class="lazy" data-src/test.js")
).then(function (fn) {
  console.log(fn.default());
}));

经过上面的步骤，此时promise已经resolve了，__webpack_require__.bind(null, "./class="lazy" data-src/test.js") 会被执行，此时异步模块的代码已经在modules上了，所以可以直接加载。

最后，执行fn方法