promise96319

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buildModule ​

_buildModule ​

_buildModule最终调用的是module.build方法，这里以NormalModule为例，找到webpack/lib/NormalModule.js文件中的build方法：

javascript
// 源码
this._source = null;
// 源码的 ast
this._ast = null;
// 打包信息
this.buildInfo = {
  cacheable: false,
  parsed: true,
  // 依赖文件
  fileDependencies: undefined,
  contextDependencies: undefined,
  // 未找到的依赖
  missingDependencies: undefined,
  // loader 依赖
  buildDependencies: undefined,
  // 变量依赖
  valueDependencies: undefined,
  // hash 值
  hash: undefined,
  assets: undefined,
  // 文件信息
  assetsInfo: undefined
};
// 源码
this._source = null;
// 源码的 ast
this._ast = null;
// 打包信息
this.buildInfo = {
  cacheable: false,
  parsed: true,
  // 依赖文件
  fileDependencies: undefined,
  contextDependencies: undefined,
  // 未找到的依赖
  missingDependencies: undefined,
  // loader 依赖
  buildDependencies: undefined,
  // 变量依赖
  valueDependencies: undefined,
  // hash 值
  hash: undefined,
  assets: undefined,
  // 文件信息
  assetsInfo: undefined
};

首先会定义一些参数，最后调用doBuild方法开始正式打包。

loaderContext ​

doBuild方法首先会创建一个loader上下文loaderContext：

javascript
const loaderContext = this.createLoaderContext(
  resolver,
  options,
  compilation,
  fs
);
const loaderContext = this.createLoaderContext(
  resolver,
  options,
  compilation,
  fs
);

loaderContext的用于在执行loader时传入作为上下文，这样我们在编写loader的时候就可以通过this访问这个上下文里的一些方法属性了。

runLoaders ​

doBuild第二个任务是执行runLoaders函数，精简后的代码如下：

javascript
runLoaders(
  {
    resource: this.resource,
    loaders: this.loaders,
    context: loaderContext,
    processResource: (loaderContext, resource, callback) => {
      loaderContext.addDependency(resource);
      fs.readFile(resource, callback);
    }
  },
  (err, result) => {
  }
);
runLoaders(
  {
    resource: this.resource,
    loaders: this.loaders,
    context: loaderContext,
    processResource: (loaderContext, resource, callback) => {
      loaderContext.addDependency(resource);
      fs.readFile(resource, callback);
    }
  },
  (err, result) => {
  }
);

该过程会通过readFile读取文件源码，并执行匹配到的loader对读取到的代码进行处理。runLoaders函数是在node_modules/loader-runner/lib/LoaderRunner.js文件中定义：

javascript
exports.runLoaders = function runLoaders(options, callback) {
  var loaders = options.loaders || [];
  loaders = loaders.map(createLoaderObject);

  // loaderContext 的一些属性添加...

  iteratePitchingLoaders(processOptions, loaderContext, function (err, result) {});
}
exports.runLoaders = function runLoaders(options, callback) {
  var loaders = options.loaders || [];
  loaders = loaders.map(createLoaderObject);

  // loaderContext 的一些属性添加...

  iteratePitchingLoaders(processOptions, loaderContext, function (err, result) {});
}

首先会在loaderContext上添加一系列的属性方法，其中也包括loaders，通过createLoaderObject方法将路径形式的loader装换为对象形式。最后调用iteratePitchingLoaders执行loaders。

iteratePitchingLoaders ​

iteratePitchingLoaders代码如下：

javascript
function iteratePitchingLoaders(options, loaderContext, callback) {
 // 1. loaders 执行完了开始处理资源
 if (loaderContext.loaderIndex >= loaderContext.loaders.length)
  return processResource(options, loaderContext, callback);

  // 2. 获取 loader
 var currentLoaderObject = loaderContext.loaders[loaderContext.loaderIndex];

 // 3. 判断 loader 是否 pitch，如果 pitch 执行过，那么执行下一个 loader 的 pitch
 if (currentLoaderObject.pitchExecuted) {
  loaderContext.loaderIndex++;
  return iteratePitchingLoaders(options, loaderContext, callback);
 }

 // 4. 加载 loader
 loadLoader(currentLoaderObject, function (err) {});
}
function iteratePitchingLoaders(options, loaderContext, callback) {
 // 1. loaders 执行完了开始处理资源
 if (loaderContext.loaderIndex >= loaderContext.loaders.length)
  return processResource(options, loaderContext, callback);

  // 2. 获取 loader
 var currentLoaderObject = loaderContext.loaders[loaderContext.loaderIndex];

 // 3. 判断 loader 是否 pitch，如果 pitch 执行过，那么执行下一个 loader 的 pitch
 if (currentLoaderObject.pitchExecuted) {
  loaderContext.loaderIndex++;
  return iteratePitchingLoaders(options, loaderContext, callback);
 }

 // 4. 加载 loader
 loadLoader(currentLoaderObject, function (err) {});
}

loader的执行过程与事件冒泡有点类似，它包括pitch阶段，代码读取，loader执行三个阶段（具体可以参考这篇文章）。pitch阶段会通过iteratePitchingLoaders方法遍历loaders，执行loader的pitch函数。待执行完成后会读取文件代码，然后通过iterateNormalLoaders方法反向遍历loaders执行loader。

iteratePitchingLoaders中通过pitchExecuted属性标识loader是否被pitch。如果为true，那么会执行下一个loader的pitch函数。否则的话会通过loadLoader函数加载loader。

loadLoader ​

loader-runner/lib/loadLoader.js文件中loadLoader代码精简如下：

javascript
module.exports = function loadLoader(loader, callback) {
  var module = require(loader.path);
  return handleResult(loader, module, callback);
};

function handleResult(loader, module, callback) {
  loader.normal = typeof module === "function" ? module : module.default;
  loader.pitch = module.pitch;
  loader.raw = module.raw;
  callback();
}
module.exports = function loadLoader(loader, callback) {
  var module = require(loader.path);
  return handleResult(loader, module, callback);
};

function handleResult(loader, module, callback) {
  loader.normal = typeof module === "function" ? module : module.default;
  loader.pitch = module.pitch;
  loader.raw = module.raw;
  callback();
}

加载loader模块后将加载的loader模块、pitch函数等赋值到loader对象上。最后执行loadLoader回调，这里讨论的是iteratePitchingLoaders中loadLoader的回调：

javascript
var fn = currentLoaderObject.pitch;
// 标识 pitch
currentLoaderObject.pitchExecuted = true;
// 如果 pitch 函数不存在，那么开始执行下一个 loader 的 pitch
if (!fn) return iteratePitchingLoaders(options, loaderContext, callback);

runSyncOrAsync(
  fn,
  loaderContext, [loaderContext.remainingRequest, loaderContext.previousRequest, currentLoaderObject.data = {}],
  function (err) {
    var args = Array.prototype.slice.call(arguments, 1);
    var hasArg = args.some(function (value) {
      return value !== undefined;
    });
    if (hasArg) {
      loaderContext.loaderIndex--;
      iterateNormalLoaders(options, loaderContext, args, callback);
    } else {
      iteratePitchingLoaders(options, loaderContext, callback);
    }
  }
);
var fn = currentLoaderObject.pitch;
// 标识 pitch
currentLoaderObject.pitchExecuted = true;
// 如果 pitch 函数不存在，那么开始执行下一个 loader 的 pitch
if (!fn) return iteratePitchingLoaders(options, loaderContext, callback);

runSyncOrAsync(
  fn,
  loaderContext, [loaderContext.remainingRequest, loaderContext.previousRequest, currentLoaderObject.data = {}],
  function (err) {
    var args = Array.prototype.slice.call(arguments, 1);
    var hasArg = args.some(function (value) {
      return value !== undefined;
    });
    if (hasArg) {
      loaderContext.loaderIndex--;
      iterateNormalLoaders(options, loaderContext, args, callback);
    } else {
      iteratePitchingLoaders(options, loaderContext, callback);
    }
  }
);

如果pitch函数存在，那么会通过runSyncOrAsync执行pitch函数：

javascript
function runSyncOrAsync(fn, context, args, callback) {
  var result = (function LOADER_EXECUTION() {
    return fn.apply(context, args);
  }());

  if (isSync) {
    if (result === undefined)
      return callback();
    if (result && typeof result === "object" && typeof result.then === "function") {
      return result.then(function (r) {
        callback(null, r);
      }, callback);
    }
    return callback(null, result);
  }
}
function runSyncOrAsync(fn, context, args, callback) {
  var result = (function LOADER_EXECUTION() {
    return fn.apply(context, args);
  }());

  if (isSync) {
    if (result === undefined)
      return callback();
    if (result && typeof result === "object" && typeof result.then === "function") {
      return result.then(function (r) {
        callback(null, r);
      }, callback);
    }
    return callback(null, result);
  }
}

最后会将结果result传入给回调函数callback。再来看下callback：

javascript
var args = Array.prototype.slice.call(arguments, 1);
var hasArg = args.some(function (value) {
  return value !== undefined;
});
var args = Array.prototype.slice.call(arguments, 1);
var hasArg = args.some(function (value) {
  return value !== undefined;
});

首先会用hasArg判断pitch函数返回的结果是否存在:

javascript
if (hasArg) {
  loaderContext.loaderIndex--;
  iterateNormalLoaders(options, loaderContext, args, callback);
} else {
  iteratePitchingLoaders(options, loaderContext, callback);
}
if (hasArg) {
  loaderContext.loaderIndex--;
  iterateNormalLoaders(options, loaderContext, args, callback);
} else {
  iteratePitchingLoaders(options, loaderContext, callback);
}

如果存在，那么就会开始调用iterateNormalLoaders函数，终止了pitch过程，此时没有processResource过程。否则会调用iteratePitchingLoaders继续下一个loader的pitch。

processResource ​

如果是pitch函数全部正常执行且没有返回值，那么最终会执行processResource方法，也就是runLoaders调用的参数：

javascript
processResource: (loaderContext, resource, callback) => {
  loaderContext.addDependency(resource);
  fs.readFile(resource, callback);
}
processResource: (loaderContext, resource, callback) => {
  loaderContext.addDependency(resource);
  fs.readFile(resource, callback);
}

首先会将文件路径作为loaderContext的dependency添加。其次会读取资源，最终执行回调中的iterateNormalLoaders：

javascript
options.resourceBuffer = buffer;
iterateNormalLoaders(options, loaderContext, [buffer], callback);
options.resourceBuffer = buffer;
iterateNormalLoaders(options, loaderContext, [buffer], callback);

iterateNormalLoaders ​

iterateNormalLoaders的执行过程与iteratePitchingLoaders类似。唯一需要注意的是它传入的参数是文件资源读取的内容，每一次loader后都会将结果返回作为上一个loader的参数传入。

javascript
iterateNormalLoaders(options, loaderContext, [buffer], callback);
iterateNormalLoaders(options, loaderContext, [buffer], callback);

pitch打断过程实际上相当于提前返回了文件资源内容。

等所有的loader执行完成后，开始执行runLoaders的回调：

javascript
(err, result) => {
  this.buildInfo.fileDependencies.addAll(result.fileDependencies);
  this.buildInfo.contextDependencies.addAll(result.contextDependencies);
  this.buildInfo.missingDependencies.addAll(result.missingDependencies);
  for (const loader of this.loaders) {
    this.buildInfo.buildDependencies.add(loader.loader);
  }
  this.buildInfo.cacheable = this.buildInfo.cacheable && result.cacheable;
  processResult(err, result.result);
}
(err, result) => {
  this.buildInfo.fileDependencies.addAll(result.fileDependencies);
  this.buildInfo.contextDependencies.addAll(result.contextDependencies);
  this.buildInfo.missingDependencies.addAll(result.missingDependencies);
  for (const loader of this.loaders) {
    this.buildInfo.buildDependencies.add(loader.loader);
  }
  this.buildInfo.cacheable = this.buildInfo.cacheable && result.cacheable;
  processResult(err, result.result);
}

返回的结果result包含了文件加载后并通过了loader处理的代码。并且用到的loader都会添加到buildInfo.buildDependencies当中。最后再通过processResult处理源码。

processResult ​

processResult主要作用是将源码内容封装成RawSource对象。

javascript
this._source = this.createSource(
  options.context,
  this.binary ? asBuffer(source) : asString(source),
  sourceMap,
  compilation.compiler.root
);
this._ast =
  typeof extraInfo === "object" &&
  extraInfo !== null &&
  extraInfo.webpackAST !== undefined
  ? extraInfo.webpackAST
: null;
callback()
this._source = this.createSource(
  options.context,
  this.binary ? asBuffer(source) : asString(source),
  sourceMap,
  compilation.compiler.root
);
this._ast =
  typeof extraInfo === "object" &&
  extraInfo !== null &&
  extraInfo.webpackAST !== undefined
  ? extraInfo.webpackAST
: null;
callback()

随后会执行callback，也就是_doBuild方法的回调函数，其核心代码如下：

javascript
// 1. 获取源码
const source = this._source.source();
// 2. 解析源码
result = this.parser.parse(this._ast || source, {
  source,
  current: this,
  module: this,
  compilation: compilation,
  options: options
});
} catch (e) {
  handleParseError(e);
  return;
}
// 3. 处理解析后的源码
handleParseResult(result);
// 1. 获取源码
const source = this._source.source();
// 2. 解析源码
result = this.parser.parse(this._ast || source, {
  source,
  current: this,
  module: this,
  compilation: compilation,
  options: options
});
} catch (e) {
  handleParseError(e);
  return;
}
// 3. 处理解析后的源码
handleParseResult(result);

parse ​

解析代码的parse方法在webpack/lib/javascript/JavascriptParser.js文件中定义：

javascript
parse(source, state) {
  let ast;

  // 1. 解析成 ast 树
  ast = JavascriptParser._parse(source, {
    sourceType: this.sourceType,
    onComment: comments,
    onInsertedSemicolon: pos => semicolons.add(pos)
  });

  // ...
 
  // 2. 转换
  if (this.hooks.program.call(ast, comments) === undefined) {
    this.detectMode(ast.body);
    this.preWalkStatements(ast.body);
    this.prevStatement = undefined;
    this.blockPreWalkStatements(ast.body);
    this.prevStatement = undefined;
    this.walkStatements(ast.body);
  }
  return state;
}
parse(source, state) {
  let ast;

  // 1. 解析成 ast 树
  ast = JavascriptParser._parse(source, {
    sourceType: this.sourceType,
    onComment: comments,
    onInsertedSemicolon: pos => semicolons.add(pos)
  });

  // ...
 
  // 2. 转换
  if (this.hooks.program.call(ast, comments) === undefined) {
    this.detectMode(ast.body);
    this.preWalkStatements(ast.body);
    this.prevStatement = undefined;
    this.blockPreWalkStatements(ast.body);
    this.prevStatement = undefined;
    this.walkStatements(ast.body);
  }
  return state;
}

其中JavascriptParser._parse方法使用的是acorn库，解析过程可以在这里进行在线调试。

hooks.program ​

hooks.program的调用触发了四个回调函数。

CompatibilityPlugin ​

处理第一行开头为#!开头的注释，创建一个ConstDependency添加到module.presentationalDependencies中:

javascript
const dep = new ConstDependency("//", 0);
dep.loc = c.loc;
parser.state.module.addPresentationalDependency(dep);
const dep = new ConstDependency("//", 0);
dep.loc = c.loc;
parser.state.module.addPresentationalDependency(dep);

HarmonyDetectionParserPlugin ​

处理import和export语法。如果存在import/export语法，那么会创建一个HarmonyCompatibilityDependency添加到module.presentationalDependencies中：

javascript
statement.type === "ImportDeclaration" ||
  statement.type === "ExportDefaultDeclaration" ||
  statement.type === "ExportNamedDeclaration" ||
  statement.type === "ExportAllDeclaration"

const compatDep = new HarmonyCompatibilityDependency();
module.addPresentationalDependency(compatDep);
statement.type === "ImportDeclaration" ||
  statement.type === "ExportDefaultDeclaration" ||
  statement.type === "ExportNamedDeclaration" ||
  statement.type === "ExportAllDeclaration"

const compatDep = new HarmonyCompatibilityDependency();
module.addPresentationalDependency(compatDep);

UseStrictPlugin ​

如果第一句以'use strict'开头，会创建一个空字符串的ConstDependency。因为后续webpack会自动添加use strict，所以这里需要移除：

javascript
const dep = new ConstDependency("", firstNode.range);
parser.state.module.addPresentationalDependency(dep);
const dep = new ConstDependency("", firstNode.range);
parser.state.module.addPresentationalDependency(dep);

DefinePlugin ​

定义全局变量：

javascript
buildInfo.valueDependencies.set(VALUE_DEP_MAIN, mainValue);
buildInfo.valueDependencies.set(VALUE_DEP_MAIN, mainValue);

walkStatements ​

walkStatements的过程是将ast树进行解析转换，这里主要以import/export为例。

ImportDeclaration ​

ImportDeclaration语句(如import A from './a.js')会触发hooks.import，执行HarmonyImportDependencyParserPlugin的回调：

javascript
parser.hooks.import.tap(
  "HarmonyImportDependencyParserPlugin",
  (statement, source) => {
    // 1. import 的序号
    parser.state.lastHarmonyImportOrder =
      (parser.state.lastHarmonyImportOrder || 0) + 1;
    const clearDep = new ConstDependency(
      parser.isAsiPosition(statement.range[0]) ? ";" : "",
      statement.range
    );
    clearDep.loc = statement.loc;

    // 2. 替换成一个 clearDep
    parser.state.module.addPresentationalDependency(clearDep);
    parser.unsetAsiPosition(statement.range[1]);
    const assertions = getAssertions(statement);

    // 3. 添加一个 dependency
    const sideEffectDep = new HarmonyImportSideEffectDependency(
      source,
      parser.state.lastHarmonyImportOrder,
      assertions
    );
    sideEffectDep.loc = statement.loc;
    parser.state.module.addDependency(sideEffectDep);
    return true;
  }
)
parser.hooks.import.tap(
  "HarmonyImportDependencyParserPlugin",
  (statement, source) => {
    // 1. import 的序号
    parser.state.lastHarmonyImportOrder =
      (parser.state.lastHarmonyImportOrder || 0) + 1;
    const clearDep = new ConstDependency(
      parser.isAsiPosition(statement.range[0]) ? ";" : "",
      statement.range
    );
    clearDep.loc = statement.loc;

    // 2. 替换成一个 clearDep
    parser.state.module.addPresentationalDependency(clearDep);
    parser.unsetAsiPosition(statement.range[1]);
    const assertions = getAssertions(statement);

    // 3. 添加一个 dependency
    const sideEffectDep = new HarmonyImportSideEffectDependency(
      source,
      parser.state.lastHarmonyImportOrder,
      assertions
    );
    sideEffectDep.loc = statement.loc;
    parser.state.module.addDependency(sideEffectDep);
    return true;
  }
)

添加完dependency后，开始定义变量：

javascript
for (const specifier of statement.specifiers) {
  const name = specifier.local.name;
  switch (specifier.type) {
    case "ImportDefaultSpecifier":
      if (
        !this.hooks.importSpecifier.call(statement, source, "default", name)
      ) {
        this.defineVariable(name);
      }
      break;
    case "ImportSpecifier":
      if (
        !this.hooks.importSpecifier.call(
          statement,
          source,
          specifier.imported.name,
          name
        )
      ) {
        this.defineVariable(name);
      }
      break;
    case "ImportNamespaceSpecifier":
      if (!this.hooks.importSpecifier.call(statement, source, null, name)) {
        this.defineVariable(name);
      }
      break;
    default:
      this.defineVariable(name);
  }
}
for (const specifier of statement.specifiers) {
  const name = specifier.local.name;
  switch (specifier.type) {
    case "ImportDefaultSpecifier":
      if (
        !this.hooks.importSpecifier.call(statement, source, "default", name)
      ) {
        this.defineVariable(name);
      }
      break;
    case "ImportSpecifier":
      if (
        !this.hooks.importSpecifier.call(
          statement,
          source,
          specifier.imported.name,
          name
        )
      ) {
        this.defineVariable(name);
      }
      break;
    case "ImportNamespaceSpecifier":
      if (!this.hooks.importSpecifier.call(statement, source, null, name)) {
        this.defineVariable(name);
      }
      break;
    default:
      this.defineVariable(name);
  }
}

ExportNamedDeclaration ​

如export const a = 0，会触发hooks.export：

javascript
parser.hooks.export.tap(
  "HarmonyExportDependencyParserPlugin",
  statement => {
    const dep = new HarmonyExportHeaderDependency(
      statement.declaration && statement.declaration.range,
      statement.range
    );
    dep.loc = Object.create(statement.loc);
    dep.loc.index = -1;
    // 添加 dependency
    parser.state.module.addPresentationalDependency(dep);
    return true;
  }
);
parser.hooks.export.tap(
  "HarmonyExportDependencyParserPlugin",
  statement => {
    const dep = new HarmonyExportHeaderDependency(
      statement.declaration && statement.declaration.range,
      statement.range
    );
    dep.loc = Object.create(statement.loc);
    dep.loc.index = -1;
    // 添加 dependency
    parser.state.module.addPresentationalDependency(dep);
    return true;
  }
);

hooks.finish ​

解析源码结束后调用hooks.finish。该hook除清除一些遍历引用之外，还会将解析过程中定义的变量添加到topLevelDeclarations中：

javascript
parser.hooks.finish.tap("JavascriptMetaInfoPlugin", () => {
  let topLevelDeclarations =
      parser.state.module.buildInfo.topLevelDeclarations;
  if (topLevelDeclarations === undefined) {
    topLevelDeclarations =
      parser.state.module.buildInfo.topLevelDeclarations = new Set();
  }
  for (const name of parser.scope.definitions.asSet()) {
    const freeInfo = parser.getFreeInfoFromVariable(name);
    if (freeInfo === undefined) {
      // 添加变量名称
      topLevelDeclarations.add(name);
    }
  }
});
parser.hooks.finish.tap("JavascriptMetaInfoPlugin", () => {
  let topLevelDeclarations =
      parser.state.module.buildInfo.topLevelDeclarations;
  if (topLevelDeclarations === undefined) {
    topLevelDeclarations =
      parser.state.module.buildInfo.topLevelDeclarations = new Set();
  }
  for (const name of parser.scope.definitions.asSet()) {
    const freeInfo = parser.getFreeInfoFromVariable(name);
    if (freeInfo === undefined) {
      // 添加变量名称
      topLevelDeclarations.add(name);
    }
  }
});

handleParseResult ​

handleParseResult会为module创建hash:

javascript
_initBuildHash(compilation) {
  const hash = createHash(compilation.outputOptions.hashFunction);
  if (this._source) {
    hash.update("source");
    this._source.updateHash(hash);
  }
  hash.update("meta");
  hash.update(JSON.stringify(this.buildMeta));
  this.buildInfo.hash = /** @type {string} */ (hash.digest("hex"));
}
_initBuildHash(compilation) {
  const hash = createHash(compilation.outputOptions.hashFunction);
  if (this._source) {
    hash.update("source");
    this._source.updateHash(hash);
  }
  hash.update("meta");
  hash.update(JSON.stringify(this.buildMeta));
  this.buildInfo.hash = /** @type {string} */ (hash.digest("hex"));
}

随后会将依赖生成snapshot，记录在buildInfo上：

javascript
compilation.fileSystemInfo.createSnapshot(
  startTime,
  this.buildInfo.fileDependencies,
  this.buildInfo.contextDependencies,
  this.buildInfo.missingDependencies,
  snapshotOptions,
  (err, snapshot) => {
    this.buildInfo.fileDependencies = undefined;
    this.buildInfo.contextDependencies = undefined;
    this.buildInfo.missingDependencies = undefined;
    this.buildInfo.snapshot = snapshot;
    return callback();
  }
);
compilation.fileSystemInfo.createSnapshot(
  startTime,
  this.buildInfo.fileDependencies,
  this.buildInfo.contextDependencies,
  this.buildInfo.missingDependencies,
  snapshotOptions,
  (err, snapshot) => {
    this.buildInfo.fileDependencies = undefined;
    this.buildInfo.contextDependencies = undefined;
    this.buildInfo.missingDependencies = undefined;
    this.buildInfo.snapshot = snapshot;
    return callback();
  }
);

最终执行callback，回到了compilation.buildModule的回调函数当中：

javascript
this.buildModule(module, err => {
  this.processModuleDependencies(module, err => {
    callback(null, module);
  });
});
this.buildModule(module, err => {
  this.processModuleDependencies(module, err => {
    callback(null, module);
  });
});

processModuleDependencies ​

processModuleDependencies对应于_processModuleDependencies方法：

javascript
if (block.dependencies) {
  currentBlock = block;
  let i = 0;
  for (const dep of block.dependencies) processDependency(dep, i++);
}
if (block.dependencies) {
  currentBlock = block;
  let i = 0;
  for (const dep of block.dependencies) processDependency(dep, i++);
}

该方法遍历打包后的module的dependencies，执行processDependency:

javascript
const processDependency = (dep, index) => {
  this.moduleGraph.setParents(dep, currentBlock, module, index);
  // ...
  processDependencyForResolving(dep);
};

const processDependencyForResolving = dep => {
  const resourceIdent = dep.getResourceIdentifier();
  if (resourceIdent !== undefined && resourceIdent !== null) {
    const category = dep.category;
    const factory = this.dependencyFactories.get(constructor);
    sortedDependencies.push({
      factory: factoryCacheKey2,
      dependencies: list,
      originModule: module
    });
  }
};
const processDependency = (dep, index) => {
  this.moduleGraph.setParents(dep, currentBlock, module, index);
  // ...
  processDependencyForResolving(dep);
};

const processDependencyForResolving = dep => {
  const resourceIdent = dep.getResourceIdentifier();
  if (resourceIdent !== undefined && resourceIdent !== null) {
    const category = dep.category;
    const factory = this.dependencyFactories.get(constructor);
    sortedDependencies.push({
      factory: factoryCacheKey2,
      dependencies: list,
      originModule: module
    });
  }
};

首先通过moduleGraph.setParents建立dependency和当前module的联系。其次根据dependency获取创建module的工厂函数，并添加到sortedDependencies中，最后会执行onDependenciesSorted方法：

javascript
for (const item of sortedDependencies) {
  this.handleModuleCreation(item, err => {
    // ...
  });
}
for (const item of sortedDependencies) {
  this.handleModuleCreation(item, err => {
    // ...
  });
}

对于每个依赖的dependency，递归调用handleModuleCreation创建module。至此，整个打包过程就完成了。

总结 ​

buildModule阶段主要任务是将生成的module进行打包，这个过程包含：

首先，创建loaderContext，形成loader的执行上下文。通过runLoaders执行该文件匹配到的loaders。

loader的执行过程又包含pitch阶段，源码读取阶段，loader执行阶段。

  • pitch阶段会调用loadLoader加载loader模块，然后调用pitch函数。可以通过pitch函数提前返回源码内容中断后续loader的调用。

  • 源码读取阶段会直接读取对应文件的源码内容。

  • loader执行阶段会依次从后向前执行loader函数，每次都会将执行结果作为下一个loader函数的参数传入。

其次，loaders执行完成后拿到加载后的源码内容，通过acorn库对源码内容进行解析，形成ast。

然后通过hooks.program和walkStatements对ast树进行分析，如变量定义、语法分析替换、模块依赖等等。

最后，根据模块的依赖（import xxx from xxx）进行遍历，递归创建子模块。

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