/**
      * Retrieves the child module of the specified ID.
      * 取出指定模块的子模块
      * This method supports retrieving both child modules and grand child modules.
      * 该方法支持检索子模块和子模块的子模块
      * @param string $id module ID (case-sensitive). To retrieve grand child modules,
      * use ID path relative to this module (e.g. `admin/content`).
      * @param boolean $load whether to load the module if it is not yet loaded.
      * @return Module|null the module instance, null if the module does not exist.
      * @see hasModule()
      */
     public function getModule($id, $load = true)
     {
         if (($pos = strpos($id, '/')) !== false) {//判断模块id格式是否为 `admin/content`
             // sub-module
             $module = $this->getModule(substr($id, 0, $pos));//取子模块

             return $module === null ? null : $module->getModule(substr($id, $pos + 1), $load);//子模块不为空，返回子模块的子模块，否则返回空
         }

         if (isset($this->_modules[$id])) {
             if ($this->_modules[$id] instanceof Module) {
                 return $this->_modules[$id];//如果_modules数组中有该模块，直接返回该模块
             } elseif ($load) {//否则，$load为真，先实例化后返回
                 Yii::trace("Loading module: $id", __METHOD__);
                 /* @var $module Module */
                 $module = Yii::createObject($this->_modules[$id], [$id, $this]);
                 $module->setInstance($module);
                 return $this->_modules[$id] = $module;
             }
         }
         //不存在，返回空
         return null;
     }

     /**
      * Adds a sub-module to this module.
      * 为当前模块添加子模块
      * @param string $id module ID
      * @param Module|array|null $module the sub-module to be added to this module. This can
      * be one of the following:
      *
      * - a [[Module]] object
      *   一个模块对象
      * - a configuration array: when [[getModule()]] is called initially, the array
      *   will be used to instantiate the sub-module
      *   模块参数为配置数组，用于实例化模块
      * - null: the named sub-module will be removed from this module
      *   为空，则表示删除
      */
     public function setModule($id, $module)
     {
         if ($module === null) {
             unset($this->_modules[$id]);
         } else {
             $this->_modules[$id] = $module;
         }
     }

     /**
      * Returns the sub-modules in this module.
      * 返回该模块的子模块
      * @param boolean $loadedOnly whether to return the loaded sub-modules only. If this is set false,
      * then all sub-modules registered in this module will be returned, whether they are loaded or not.
      * Loaded modules will be returned as objects, while unloaded modules as configuration arrays.
      * @param boolean $loadedOnly 可选参数，定义是否只返回已加载的子模块
      * @return array the modules (indexed by their IDs)
      */
     public function getModules($loadedOnly = false)
     {
         if ($loadedOnly) {
             $modules = [];
             foreach ($this->_modules as $module) {
                 if ($module instanceof Module) {
                     $modules[] = $module;
                 }
             }

             return $modules;
         } else {
             return $this->_modules;
         }
     }

     /**
      * Registers sub-modules in the current module.
      * 注册子模块到当前模块
      *
      * Each sub-module should be specified as a name-value pair, where
      * name refers to the ID of the module and value the module or a configuration
      * array that can be used to create the module. In the latter case, [[Yii::createObject()]]
      * will be used to create the module.
      * 子模块以键值对的方式指定，键名为模块ID，键值为模块对象或者用于创建模块对象的配置数组
      *
      * If a new sub-module has the same ID as an existing one, the existing one will be overwritten silently.
      * 如果ID相同，后者会覆盖前置
      *
      * The following is an example for registering two sub-modules:
      *
      * ```php
      * [
      *     'comment' => [
      *         'class' => 'app\modules\comment\CommentModule',
      *         'db' => 'db',
      *     ],
      *     'booking' => ['class' => 'app\modules\booking\BookingModule'],
      * ]
      * ```
      *
      * @param array $modules modules (id => module configuration or instances)
      */
     public function setModules($modules)
     {
         foreach ($modules as $id => $module) {
             $this->_modules[$id] = $module;
         }
     }

     /**
      * Runs a controller action specified by a route.
      * 运行路由中指定的控制器方法
      * This method parses the specified route and creates the corresponding child module(s), controller and action
      * instances. It then calls [[Controller::runAction()]] to run the action with the given parameters.
      * If the route is empty, the method will use [[defaultRoute]].
      * 解析指定的路由，创建对应的子模块、控制器、方法实例，然后调用[[Controller::runAction()]]用给定的参数运行控制器中的方法
      * @param string $route the route that specifies the action.
      * @param array $params the parameters to be passed to the action
      * @return mixed the result of the action.
      * @throws InvalidRouteException if the requested route cannot be resolved into an action successfully
      */
     public function runAction($route, $params = [])
     {
         $parts = $this->createController($route);//根据路由创建控制器
         if (is_array($parts)) {
             /* @var $controller Controller */
             list($controller, $actionID) = $parts;//获得$actionId和$controller
             $oldController = Yii::$app->controller;
             Yii::$app->controller = $controller;
             $result = $controller->runAction($actionID, $params);//运行使用控制器加载 action方法
             Yii::$app->controller = $oldController;//将对象交给Yii::$app->controller 这里面起的作用应该是运行控制器，最后释放控制器的对象变量

             return $result;
         } else {
             $id = $this->getUniqueId();
             throw new InvalidRouteException('Unable to resolve the request "' . ($id === '' ? $route : $id . '/' . $route) . '".');
         }
     }

     /**
      * Creates a controller instance based on the given route.
      *
      * The route should be relative to this module. The method implements the following algorithm
      * to resolve the given route:
      *
      * 1. If the route is empty, use [[defaultRoute]];
      * 2. If the first segment of the route is a valid module ID as declared in [[modules]],
      *    call the module's `createController()` with the rest part of the route;
      * 3. If the first segment of the route is found in [[controllerMap]], create a controller
      *    based on the corresponding configuration found in [[controllerMap]];
      * 4. The given route is in the format of `abc/def/xyz`. Try either `abc\DefController`
      *    or `abc\def\XyzController` class within the [[controllerNamespace|controller namespace]].
      *
      * If any of the above steps resolves into a controller, it is returned together with the rest
      * part of the route which will be treated as the action ID. Otherwise, false will be returned.
      *
      * @param string $route the route consisting of module, controller and action IDs.
      * @return array|boolean If the controller is created successfully, it will be returned together
      * with the requested action ID. Otherwise false will be returned.
      * @throws InvalidConfigException if the controller class and its file do not match.
      */
     public function createController($route)
     {
         if ($route === '') {//路由为空，调用默认的路由
             $route = $this->defaultRoute;
         }

         // double slashes or leading/ending slashes may cause substr problem
         //双斜线或者开始/结束的斜线会引起函数问题，去掉两边的斜线，如果路径中包含双斜线，返回false
         $route = trim($route, '/');
         if (strpos($route, '//') !== false) {
             return false;
         }

         if (strpos($route, '/') !== false) {//路径中包含斜线
             list ($id, $route) = explode('/', $route, 2);//将路径按斜线分割为两个元素的数组,[0]为$id，[1]为$route
         } else {
             $id = $route;
             $route = '';
         }

         // module and controller map take precedence
         // 优先判断模块和控制器映射
         if (isset($this->controllerMap[$id])) {//如果$id是控制器ID，实例化控制器，返回控制器实例和后面的路径$route
             $controller = Yii::createObject($this->controllerMap[$id], [$id, $this]);
             return [$controller, $route];
         }
         $module = $this->getModule($id);//如果$id是模块ID，实例化控制器，返回控制器实例和后面的路径$route
         if ($module !== null) {
             return $module->createController($route);//调用自身实例化控制器，返回控制器实例和后面的路径$route
         }

         if (($pos = strrpos($route, '/')) !== false) {//上面两种情况都不是，则表示该模块还有子模块，构造子模块的id
             $id .= '/' . substr($route, 0, $pos);
             $route = substr($route, $pos + 1);
         }

         $controller = $this->createControllerByID($id);//调用createControllerByID()方法实例化控制器
         if ($controller === null && $route !== '') {
             $controller = $this->createControllerByID($id . '/' . $route);
             $route = '';
         }

         return $controller === null ? false : [$controller, $route];//实例化成功返回控制器实例和后面的路径$route，失败返回false
     }