namespace yii\web;

 use Yii;
 use yii\base\InvalidRouteException;

 /**
  * Application is the base class for all web application classes.
  * Application 是所有web应用的基类
  *
  * @property string $homeUrl The homepage URL.
  * @property Session $session The session component. This property is read-only.
  * @property User $user The user component. This property is read-only.
  *
  * @author Qiang Xue <qiang.xue@gmail.com>
  * @since 2.0
  */
 class Application extends \yii\base\Application
 {
     /**
      * @var string the default route of this application. Defaults to 'site'.
      * @var string 默认的路由，默认值为‘site’.
      */
     public $defaultRoute = 'site';
     /**
      * @var array the configuration specifying a controller action which should handle
      * all user requests. This is mainly used when the application is in maintenance mode
      * and needs to handle all incoming requests via a single action.
      * @var array 配置项，指定一个控制器处理所有用户的请求，主要用于维护模式
      * The configuration is an array whose first element specifies the route of the action.
      * The rest of the array elements (key-value pairs) specify the parameters to be bound
      * to the action. For example,
      * 配置项是一个第一个元素指定Action的路由，其余元素为参数的数组，例如：
      *
      * ```php
      * [
      *     'offline/notice',
      *     'param1' => 'value1',
      *     'param2' => 'value2',
      * ]
      * ```
      *
      * Defaults to null, meaning catch-all is not used.
      */
     public $catchAll;
     /**
      * @var Controller the currently active controller instance
      * @var Controller 默认的控制器实例
      */
     public $controller;

     /**
      * @inheritdoc
      */
     protected function bootstrap()
     {
         $request = $this->getRequest();//获取请求的组件
         Yii::setAlias('@webroot', dirname($request->getScriptFile()));//设置webroot别名，该别名指向正在运行的应用的入口文件 index.php 所在的目录
         Yii::setAlias('@web', $request->getBaseUrl());//定义别名web，指向当前应用的根URL，主要用于前端

         parent::bootstrap();//调用父类方法，初始化应用组件
     }

     /**
      * Handles the specified request.
      * 处理指定的请求
      * @param Request $request the request to be handled
      * @return Response the resulting response
      * @throws NotFoundHttpException if the requested route is invalid
      */
     public function handleRequest($request)
     {
         if (empty($this->catchAll)) {
             list ($route, $params) = $request->resolve();//取出路由及参数
         } else {
             $route = $this->catchAll[0];
             $params = $this->catchAll;
             unset($params[0]);
         }
         try {
             Yii::trace("Route requested: '$route'", __METHOD__);
             $this->requestedRoute = $route;
             $result = $this->runAction($route, $params);//运行控制器中的Acition
             if ($result instanceof Response) {
                 return $result;
             } else {
                 /**
                  *这个是加载yii\base\Response类，在外部可以Yii::$app->get('response')、Yii::$app->getResponse()、Yii::$app->response 等等方式来加载response类
                  *主要用来加载http状态，及头信息，如301,302，404，ajax头等等的获取
                  */
                 $response = $this->getResponse();
                 if ($result !== null) {
                     $response->data = $result;
                 }

                 return $response;
             }
         } catch (InvalidRouteException $e) {
             throw new NotFoundHttpException(Yii::t('yii', 'Page not found.'), $e->getCode(), $e);
         }
     }

     private $_homeUrl;

     /**
      * @return string the homepage URL
      * @return string 返回主页的URL
      */
     public function getHomeUrl()
     {
         if ($this->_homeUrl === null) {
             if ($this->getUrlManager()->showScriptName) {//如果请求路径中显示入口脚本
                 return $this->getRequest()->getScriptUrl();//返回入口脚本的相对路径
             } else {
                 return $this->getRequest()->getBaseUrl() . '/';//否则返回BaseUrl，即去掉入口脚本名和结束斜线的路径
             }
         } else {
             return $this->_homeUrl;
         }
     }

     /**
      * @param string $value the homepage URL
      * 设置主页URL
      */
     public function setHomeUrl($value)
     {
         $this->_homeUrl = $value;
     }

     /**
      * Returns the session component.
      * 返回session组件对象
      * @return Session the session component.
      */
     public function getSession()
     {
         return $this->get('session');
     }

     /**
      * Returns the user component.
      * 返回user组件对象
      * @return User the user component.
      */
     public function getUser()
     {
         return $this->get('user');
     }

     /**
      * @inheritdoc
      * 定义核心组件，用于程序初始化时加载
      */
     public function coreComponents()
     {
         return array_merge(parent::coreComponents(), [
             'request' => ['class' => 'yii\web\Request'],
             'response' => ['class' => 'yii\web\Response'],
             'session' => ['class' => 'yii\web\Session'],
             'user' => ['class' => 'yii\web\User'],
             'errorHandler' => ['class' => 'yii\web\ErrorHandler'],
         ]);
     }
 }