【前端必会】tapable、hook，webpack的灵魂

背景

1. 什么是tapable、hook，平时做vue开发时的webpack 配置一直都没弄懂，你也有这种情况吗？
2. 还是看源码，闲来无聊又看一下webpack的源码，看看能否找到一些宝藏
3. tapable和webpack没有特定关系，可以先看下这篇文章，了解下这个小型库
   
   https://webpack.docschina.org/api/plugins/#tapable
   
   https://blog.csdn.net/mafan121/article/details/113120081
   
   4.下面记录下寻宝过程

开始

执行一次webpack经历了什么，先看一下代码

我们分析一下4点

1. 引用了webpack
2. 我们使用的配置文件
3. 调用webpack函数，传入配置，返回一个compiler（编译器）
4. 执行编译器的run方法

分析

引用webpack，先把这个函数找出来

https://github.com/webpack/webpack/blob/main/package.json

"main": "lib/index.js",

https://github.com/webpack/webpack/blob/main/lib/index.js

module.exports = mergeExports(fn, {
	get webpack() {
		return require("./webpack");
	},

https://github.com/webpack/webpack/blob/main/lib/webpack.js

const webpack = /** @type {WebpackFunctionSingle & WebpackFunctionMulti} */ (
	/**
	 * @param {WebpackOptions | (ReadonlyArray<WebpackOptions> & MultiCompilerOptions)} options options
	 * @param {Callback<Stats> & Callback<MultiStats>=} callback callback
	 * @returns {Compiler | MultiCompiler}
	 */
	(options, callback) => {
		const create = () => {
			if (!asArray(options).every(webpackOptionsSchemaCheck)) {
				getValidateSchema()(webpackOptionsSchema, options);
				util.deprecate(
					() => {},
					"webpack bug: Pre-compiled schema reports error while real schema is happy. This has performance drawbacks.",
					"DEP_WEBPACK_PRE_COMPILED_SCHEMA_INVALID"
				)();
			}
			/** @type {MultiCompiler|Compiler} */
			let compiler;
			let watch = false;
			/** @type {WatchOptions|WatchOptions[]} */
			let watchOptions;
			if (Array.isArray(options)) {
				/** @type {MultiCompiler} */
				compiler = createMultiCompiler(
					options,
					/** @type {MultiCompilerOptions} */ (options)
				);
				watch = options.some(options => options.watch);
				watchOptions = options.map(options => options.watchOptions || {});
			} else {
				const webpackOptions = /** @type {WebpackOptions} */ (options);
				/** @type {Compiler} */
				compiler = createCompiler(webpackOptions);
				watch = webpackOptions.watch;
				watchOptions = webpackOptions.watchOptions || {};
			}
			return { compiler, watch, watchOptions };
		};
		if (callback) {
			try {
				const { compiler, watch, watchOptions } = create();
				if (watch) {
					compiler.watch(watchOptions, callback);
				} else {
					compiler.run((err, stats) => {
						compiler.close(err2 => {
							callback(err || err2, stats);
						});
					});
				}
				return compiler;
			} catch (err) {
				process.nextTick(() => callback(err));
				return null;
			}
		} else {
			const { compiler, watch } = create();
			if (watch) {
				util.deprecate(
					() => {},
					"A 'callback' argument needs to be provided to the 'webpack(options, callback)' function when the 'watch' option is set. There is no way to handle the 'watch' option without a callback.",
					"DEP_WEBPACK_WATCH_WITHOUT_CALLBACK"
				)();
			}
			return compiler;
		}
	}
);

module.exports = webpack;

这里主要就是调用create创建一个Compiler（先不理watch）

在看一下create，这里是调用的createCompiler或者createMultiCompiler

在看一下createCompiler,这里主要就是new一个Compiler。这个时候已经开始了webpack编译的生命周期。

/**
 * @param {WebpackOptions} rawOptions options object
 * @returns {Compiler} a compiler
 */
const createCompiler = rawOptions => {
	const options = getNormalizedWebpackOptions(rawOptions);
	applyWebpackOptionsBaseDefaults(options);
	const compiler = new Compiler(options.context, options);
	new NodeEnvironmentPlugin({
		infrastructureLogging: options.infrastructureLogging
	}).apply(compiler);
	if (Array.isArray(options.plugins)) {
		for (const plugin of options.plugins) {
			if (typeof plugin === "function") {
				plugin.call(compiler, compiler);
			} else {
				plugin.apply(compiler);
			}
		}
	}
	applyWebpackOptionsDefaults(options);
	compiler.hooks.environment.call();
	compiler.hooks.afterEnvironment.call();
	new WebpackOptionsApply().process(options, compiler);
	compiler.hooks.initialize.call();
	return compiler;
};

我们简单看一下Compiler类的一些hooks

const {
	SyncHook,
	SyncBailHook,
	AsyncParallelHook,
	AsyncSeriesHook
} = require("tapable");

......

class Compiler {
	/**
	 * @param {string} context the compilation path
	 * @param {WebpackOptions} options options
	 */
	constructor(context, options = /** @type {WebpackOptions} */ ({})) {
		this.hooks = Object.freeze({
			/** @type {SyncHook<[]>} */
			initialize: new SyncHook([]),

			/** @type {SyncBailHook<[Compilation], boolean>} */
			shouldEmit: new SyncBailHook(["compilation"]),
			/** @type {AsyncSeriesHook<[Stats]>} */
			done: new AsyncSeriesHook(["stats"]),
			/** @type {SyncHook<[Stats]>} */
			afterDone: new SyncHook(["stats"]),
			/** @type {AsyncSeriesHook<[]>} */
			additionalPass: new AsyncSeriesHook([]),
			/** @type {AsyncSeriesHook<[Compiler]>} */
			beforeRun: new AsyncSeriesHook(["compiler"]),
			/** @type {AsyncSeriesHook<[Compiler]>} */
			run: new AsyncSeriesHook(["compiler"]),
			/** @type {AsyncSeriesHook<[Compilation]>} */
			emit: new AsyncSeriesHook(["compilation"]),
			/** @type {AsyncSeriesHook<[string, AssetEmittedInfo]>} */
			assetEmitted: new AsyncSeriesHook(["file", "info"]),
			/** @type {AsyncSeriesHook<[Compilation]>} */
			afterEmit: new AsyncSeriesHook(["compilation"]),

每个hook都是一个 tapable包里对应后hook的实例

在回到创建编译器那里,这时创建一个插件的实例，并且执行apply方法，插件就会向自己关系的hook添加事件处理函数（其实还是一个事件监听），NodeEnvironmentPlugin代码可以自行在源码中查看

new NodeEnvironmentPlugin({
		infrastructureLogging: options.infrastructureLogging
	}).apply(compiler);

一切都准备好了之后，我们再看一下编译器的run方法

/**
	 * @param {Callback<Stats>} callback signals when the call finishes
	 * @returns {void}
	 */
	run(callback) {
		if (this.running) {
			return callback(new ConcurrentCompilationError());
		}

		let logger;

		const finalCallback = (err, stats) => {
			if (logger) logger.time("beginIdle");
			this.idle = true;
			this.cache.beginIdle();
			this.idle = true;
			if (logger) logger.timeEnd("beginIdle");
			this.running = false;
			if (err) {
				this.hooks.failed.call(err);
			}
			if (callback !== undefined) callback(err, stats);
			this.hooks.afterDone.call(stats);
		};

		const startTime = Date.now();

		this.running = true;

		const onCompiled = (err, compilation) => {
			if (err) return finalCallback(err);

			if (this.hooks.shouldEmit.call(compilation) === false) {
				compilation.startTime = startTime;
				compilation.endTime = Date.now();
				const stats = new Stats(compilation);
				this.hooks.done.callAsync(stats, err => {
					if (err) return finalCallback(err);
					return finalCallback(null, stats);
				});
				return;
			}

			process.nextTick(() => {
				logger = compilation.getLogger("webpack.Compiler");
				logger.time("emitAssets");
				this.emitAssets(compilation, err => {
					logger.timeEnd("emitAssets");
					if (err) return finalCallback(err);

					if (compilation.hooks.needAdditionalPass.call()) {
						compilation.needAdditionalPass = true;

						compilation.startTime = startTime;
						compilation.endTime = Date.now();
						logger.time("done hook");
						const stats = new Stats(compilation);
						this.hooks.done.callAsync(stats, err => {
							logger.timeEnd("done hook");
							if (err) return finalCallback(err);

							this.hooks.additionalPass.callAsync(err => {
								if (err) return finalCallback(err);
								this.compile(onCompiled);
							});
						});
						return;
					}

					logger.time("emitRecords");
					this.emitRecords(err => {
						logger.timeEnd("emitRecords");
						if (err) return finalCallback(err);

						compilation.startTime = startTime;
						compilation.endTime = Date.now();
						logger.time("done hook");
						const stats = new Stats(compilation);
						this.hooks.done.callAsync(stats, err => {
							logger.timeEnd("done hook");
							if (err) return finalCallback(err);
							this.cache.storeBuildDependencies(
								compilation.buildDependencies,
								err => {
									if (err) return finalCallback(err);
									return finalCallback(null, stats);
								}
							);
						});
					});
				});
			});
		};

		const run = () => {
			this.hooks.beforeRun.callAsync(this, err => {
				if (err) return finalCallback(err);

				this.hooks.run.callAsync(this, err => {
					if (err) return finalCallback(err);

					this.readRecords(err => {
						if (err) return finalCallback(err);

						this.compile(onCompiled);
					});
				});
			});
		};

		if (this.idle) {
			this.cache.endIdle(err => {
				if (err) return finalCallback(err);

				this.idle = false;
				run();
			});
		} else {
			run();
		}
	}

https://github.com/webpack/webpack/blob/main/lib/Compiler.js

这里简单分析一下，主要就是执行run相关生命周期，以及编译。并且编译完成后传入回调函数onCompiled

const run = () => {
			this.hooks.beforeRun.callAsync(this, err => {
				if (err) return finalCallback(err);

				this.hooks.run.callAsync(this, err => {
					if (err) return finalCallback(err);

					this.readRecords(err => {
						if (err) return finalCallback(err);

						this.compile(onCompiled);
					});
				});
			});
		};

整体逻辑不是很复杂，我们主要可以感受到webpack启动后对hook的一些使用方式。整体的逻辑差不多都是一样的。是不是很简单。

总结

1. 想了解一个框架，一定要找到入口函数，一点一点向前探索。
2. tapable 是个好东西，
3. 关于webpack生命周期，有疑问的时候，除了看文档意外，还可以结合源码去理解，去感受

你学会了吗？欢迎留下你的感受！