/**
      * Finds the view file based on the given view name.
      * 通过view文件名查找view文件
      * @param string $view the view name or the path alias of the view file. Please refer to [[render()]]
      * on how to specify this parameter.
      * @param string $view 视图文件名
      * @param object $context the context to be assigned to the view and can later be accessed via [[context]]
      * in the view. If the context implements [[ViewContextInterface]], it may also be used to locate
      * the view file corresponding to a relative view name.
      * @param object $context 情景
      * @return string the view file path. Note that the file may not exist.
      * @return string view文件路径
      * @throws InvalidCallException if a relative view name is given while there is no active context to
      * determine the corresponding view file.
      */
     protected function findViewFile($view, $context = null)
     {
         if (strncmp($view, '@', 1) === 0) {//判断$view是否是别名路径
             // e.g. "@app/views/main"
             $file = Yii::getAlias($view);//调用getAlias()方法获取真实路径
         } elseif (strncmp($view, '//', 2) === 0) {//判断$view是否以'//'开始
             // e.g. "//layouts/main"
             //调用getViewPath()方法在当前模块下查找视图文件目录路径，然后拼接文件名合成视图文件路径
             $file = Yii::$app->getViewPath() . DIRECTORY_SEPARATOR . ltrim($view, '/');
         } elseif (strncmp($view, '/', 1) === 0) {//判断$view是否以 '/'开始
             // e.g. "/site/index"
             if (Yii::$app->controller !== null) {//且控制器存在
                 //则调用getViewPath()查找当前控制器对应的视图文件目录，然后拼接文件名合成视图文件路径
                 $file = Yii::$app->controller->module->getViewPath() . DIRECTORY_SEPARATOR . ltrim($view, '/');
             } else {//否则，抛出异常
                 throw new InvalidCallException("Unable to locate view file for view '$view': no active controller.");
             }
         } elseif ($context instanceof ViewContextInterface) {
             $file = $context->getViewPath() . DIRECTORY_SEPARATOR . $view;
         } elseif (($currentViewFile = $this->getViewFile()) !== false) {
             $file = dirname($currentViewFile) . DIRECTORY_SEPARATOR . $view;
         } else {
             throw new InvalidCallException("Unable to resolve view file for view '$view': no active view context.");
         }

         if (pathinfo($file, PATHINFO_EXTENSION) !== '') {//如果视图文件的扩展名不为空，直接返回扩展名
             return $file;
         }
         $path = $file . '.' . $this->defaultExtension;//给视图文件添加扩展名
         if ($this->defaultExtension !== 'php' && !is_file($path)) {//这里应该是做二次校验，添加扩展名
             $path = $file . '.php';
         }

         return $path;
     }

     /**
      * Renders a view file.
      * 渲染一个view文件
      *
      * If [[theme]] is enabled (not null), it will try to render the themed version of the view file as long
      * as it is available.
      * 如果[theme]可用（即非空），该方法将视图渲染themed version的视图文件直到[theme]不可用
      * The method will call [[FileHelper::localize()]] to localize the view file.
      * 该方法将调用[FileHelper::localize()]方法本地化视图文件
      *
      * If [[renderers|renderer]] is enabled (not null), the method will use it to render the view file.
      * Otherwise, it will simply include the view file as a normal PHP file, capture its output and
      * return it as a string.
      * 如果[renderers|renderer]可用（即非空），该方法将渲染视图文件，否则将把视图文件作为常规的php文件包含进来
      * 捕捉它的输出并将其返回为字符串
      *
      * @param string $viewFile the view file. This can be either an absolute file path or an alias of it.
      * @param array $params the parameters (name-value pairs) that will be extracted and made available in the view file.
      * @param object $context the context that the view should use for rendering the view. If null,
      * existing [[context]] will be used.
      * @return string the rendering result
      * @throws InvalidParamException if the view file does not exist
      */
     public function renderFile($viewFile, $params = [], $context = null)
     {
         $viewFile = Yii::getAlias($viewFile);//调用getAlias()方法处理输入的视图文件名

         if ($this->theme !== null) {
             $viewFile = $this->theme->applyTo($viewFile);//如果theme非空，怎将其应用到视图文件
         }
         if (is_file($viewFile)) {
             $viewFile = FileHelper::localize($viewFile);//调用[FileHelper::localize()]方法本地化视图文件
         } else {//$viewFile不是文件，抛出异常“视图文件不存在”
             throw new InvalidParamException("The view file does not exist: $viewFile");
         }

         $oldContext = $this->context;
         if ($context !== null) {
             $this->context = $context;
         }
         $output = '';
         $this->_viewFiles[] = $viewFile;

         if ($this->beforeRender($viewFile, $params)) {//如果renderFile的前置事件执行成功
             Yii::trace("Rendering view file: $viewFile", __METHOD__);//记录trace信息
             $ext = pathinfo($viewFile, PATHINFO_EXTENSION);//获取视图文件扩展名
             if (isset($this->renderers[$ext])) {//判断视图文件的扩展名是否支持
                 if (is_array($this->renderers[$ext]) || is_string($this->renderers[$ext])) {
                     $this->renderers[$ext] = Yii::createObject($this->renderers[$ext]);
                 }
                 /* @var $renderer ViewRenderer */
                 $renderer = $this->renderers[$ext];//如果支持的，将view渲染器对象赋值给$renderer
                 $output = $renderer->render($this, $viewFile, $params);//渲染视图文件
             } else {//如果视图文件不是支持的类型，则以普通php文件处理
                 $output = $this->renderPhpFile($viewFile, $params);
             }
             $this->afterRender($viewFile, $params, $output);//调用renderFile的后置方法
         }

         array_pop($this->_viewFiles);//删除viewFiles中最后面的一个元素
         $this->context = $oldContext;

         return $output;
     }