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make阶段 ​

hooks.make ​

在《整体流程》篇中提到，make阶段正式的编译过程，此时会调用hooks.make钩子：

javascript
this.hooks.make.callAsync(compilation, err => {})
this.hooks.make.callAsync(compilation, err => {})

此时会触发EntryPlugin（该插件是在entryOptions钩子触发时监听make钩子的）：

javascript
compiler.hooks.make.tapAsync("EntryPlugin", (compilation, callback) => {
  const { entry, options, context } = this;
 // 创建 EntryDependency
  const dep = EntryPlugin.createDependency(entry, options);
  // 添加 entry 入口
  compilation.addEntry(context, dep, options, err => {
    callback(err);
  });
});
compiler.hooks.make.tapAsync("EntryPlugin", (compilation, callback) => {
  const { entry, options, context } = this;
 // 创建 EntryDependency
  const dep = EntryPlugin.createDependency(entry, options);
  // 添加 entry 入口
  compilation.addEntry(context, dep, options, err => {
    callback(err);
  });
});

该插件最终会调用compilation.addEntry方法开始从入口进行编译。在webpack/lib/Compilation.js文件中找到addEntry方法，会用一连串调用，调用栈如下：

javascript
addEntry => _addEntryItem => addModuleTree => handleModuleCreation => factorizeModule => ...
addEntry => _addEntryItem => addModuleTree => handleModuleCreation => factorizeModule => ...

最终主要的执行函数为factorizeModule。

javascript
factorizeModule(options, callback) {
  this.factorizeQueue.add(options, callback);
}

// 在 constructor 初始化时定义
this.factorizeQueue = new AsyncQueue({
  name: "factorize",
  parent: this.addModuleQueue,
  processor: this._factorizeModule.bind(this)
});
factorizeModule(options, callback) {
  this.factorizeQueue.add(options, callback);
}

// 在 constructor 初始化时定义
this.factorizeQueue = new AsyncQueue({
  name: "factorize",
  parent: this.addModuleQueue,
  processor: this._factorizeModule.bind(this)
});

在看具体的执行内容之前，先看一下AsyncQueue。

AsyncQueue ​

constructor ​

javascript
constructor({ name, parallelism, parent, processor, getKey }) {
  this._processor = processor;
  this._root = parent ? parent._root : this;
  if (parent) {
    if (this._root._children === undefined) {
      this._root._children = [this];
    } else {
      this._root._children.push(this);
    }
  }
}
constructor({ name, parallelism, parent, processor, getKey }) {
  this._processor = processor;
  this._root = parent ? parent._root : this;
  if (parent) {
    if (this._root._children === undefined) {
      this._root._children = [this];
    } else {
      this._root._children.push(this);
    }
  }
}

constructor函数要注意的有两点：一是_processor，用于指向真正的执行函数；二是_root，用于表示该队列的根parent。在Compilation的constructor中有如下定义：

javascript
this.processDependenciesQueue = new AsyncQueue({
  name: "processDependencies",
  parallelism: options.parallelism || 100,
  processor: this._processModuleDependencies.bind(this)
});
this.addModuleQueue = new AsyncQueue({
  name: "addModule",
  parent: this.processDependenciesQueue,
  getKey: module => module.identifier(),
  processor: this._addModule.bind(this)
});
this.factorizeQueue = new AsyncQueue({
  name: "factorize",
  parent: this.addModuleQueue,
  processor: this._factorizeModule.bind(this)
});
this.buildQueue = new AsyncQueue({
  name: "build",
  parent: this.factorizeQueue,
  processor: this._buildModule.bind(this)
});
this.processDependenciesQueue = new AsyncQueue({
  name: "processDependencies",
  parallelism: options.parallelism || 100,
  processor: this._processModuleDependencies.bind(this)
});
this.addModuleQueue = new AsyncQueue({
  name: "addModule",
  parent: this.processDependenciesQueue,
  getKey: module => module.identifier(),
  processor: this._addModule.bind(this)
});
this.factorizeQueue = new AsyncQueue({
  name: "factorize",
  parent: this.addModuleQueue,
  processor: this._factorizeModule.bind(this)
});
this.buildQueue = new AsyncQueue({
  name: "build",
  parent: this.factorizeQueue,
  processor: this._buildModule.bind(this)
});

因此processDependenciesQueue的_children为addModuleQueue、factorizeQueue、buildQueue，反之是它们三者的_root。

add方法 ​

add方法会执行root的_ensureProcessing方法：

javascript
add(item, callback) {
  const newEntry = new AsyncQueueEntry(item, callback);
  this._queued.enqueue(newEntry);
  const root = this._root;
  setImmediate(root._ensureProcessing);
}
add(item, callback) {
  const newEntry = new AsyncQueueEntry(item, callback);
  this._queued.enqueue(newEntry);
  const root = this._root;
  setImmediate(root._ensureProcessing);
}

_ensureProcessing方法 ​

_ensureProcessing精简代码如下：

javascript
_ensureProcessing() {
  // 1. 执行当前的 _queue
  while (this._activeTasks < this._parallelism) {
    const entry = this._queued.dequeue();
    this._startProcessing(entry);
  }
  // 2. 遍历执行 children 的 _queue
  for (const child of this._children) {
    while (this._activeTasks < this._parallelism) {
      const entry = child._queued.dequeue();
      child._startProcessing(entry);
    }
  }
}
_ensureProcessing() {
  // 1. 执行当前的 _queue
  while (this._activeTasks < this._parallelism) {
    const entry = this._queued.dequeue();
    this._startProcessing(entry);
  }
  // 2. 遍历执行 children 的 _queue
  for (const child of this._children) {
    while (this._activeTasks < this._parallelism) {
      const entry = child._queued.dequeue();
      child._startProcessing(entry);
    }
  }
}

该方法主要由两个任务，一是执行当前的_queue，其次是遍历_children执行child的queue。

_startProcessing方法 ​

该方法主要执行_processor方法，也就是在定义AsyncQueue的时候传入的执行函数：

javascript
_startProcessing(entry) {
  this.hooks.beforeStart.callAsync(entry.item, err => {
    this._processor(entry.item, (e, r) => {
      this._handleResult(entry, e, r);
    });
  });
}
_startProcessing(entry) {
  this.hooks.beforeStart.callAsync(entry.item, err => {
    this._processor(entry.item, (e, r) => {
      this._handleResult(entry, e, r);
    });
  });
}

总结 ​

AsyncQueue在调用add方法时，最终会执行传入的_processor方法，一些执行细节都封装在AsyncQueue当中。因此我们主要关注_processor方法的执行即可。

make流程 ​

清楚了AsyncQueue的作用后，回到compilation中的流程。factorizeModule方法最终调用的是_factorizeModule方法。执行完成后执行回调，会遇到addModule，最终调用的是_addModule方法。再次执行回调，然后执行buildModule方法，也就是调用_buildModule方法。打包完成后执行processModuleDependencies方法，又对应于_processModuleDependencies方法。

javascript
factorizeModule => _factorizeModule => 执行回调函数 
=> addModule => _addModule => 执行回调函数 
=> buildModule => _buildModule => 执行回调函数 
=> processModuleDependencies => _processModuleDependencies => 执行回调函数 
=> ...
factorizeModule => _factorizeModule => 执行回调函数 
=> addModule => _addModule => 执行回调函数 
=> buildModule => _buildModule => 执行回调函数 
=> processModuleDependencies => _processModuleDependencies => 执行回调函数 
=> ...

接下来会分别讲解这些方法做了哪些事。

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