/**
      * @return string|boolean the view file currently being rendered. False if no view file is being rendered.
      */
     public function getViewFile()
     {
         return end($this->_viewFiles);//返回[_viewFiles]中的最后一个view文件，即默认被渲染的文件
     }

     /**
      * This method is invoked right before [[renderFile()]] renders a view file.
      * Render的前置事件，在执行[renderFile()]方法时被调用，默认触发[[EVENT_BEFORE_RENDER]]事件
      * The default implementation will trigger the [[EVENT_BEFORE_RENDER]] event.
      * If you override this method, make sure you call the parent implementation first.
      * 如果要重写该方法，要确保首先调用父类的同名方法
      * @param string $viewFile the view file to be rendered.
      * @param array $params the parameter array passed to the [[render()]] method.
      * @return boolean whether to continue rendering the view file.
      */
     public function beforeRender($viewFile, $params)
     {
         //实例化ViewEvent
         $event = new ViewEvent([
             'viewFile' => $viewFile,
             'params' => $params,
         ]);
         //触发[EVENT_BEFORE_RENDER]事件
         $this->trigger(self::EVENT_BEFORE_RENDER, $event);

         return $event->isValid;//返回值可以判断是否继续渲染文件
     }

     /**
      * This method is invoked right after [[renderFile()]] renders a view file.
      * Render的后置事件，在执行[renderFile()]方法后被调用，默认触发[[EVENT_AFTER_RENDER]]事件
      * The default implementation will trigger the [[EVENT_AFTER_RENDER]] event.
      * If you override this method, make sure you call the parent implementation first.
      * 如果要重写该方法，要确保首先调用父类的同名方法
      * @param string $viewFile the view file being rendered.
      * @param array $params the parameter array passed to the [[render()]] method.
      * @param string $output the rendering result of the view file. Updates to this parameter
      * will be passed back and returned by [[renderFile()]].
      */
     public function afterRender($viewFile, $params, &$output)
     {
         if ($this->hasEventHandlers(self::EVENT_AFTER_RENDER)) {//判断[EVENT_AFTER_RENDER]事件是否有处理函数
            //实例化ViewEvent
             $event = new ViewEvent([
                 'viewFile' => $viewFile,
                 'params' => $params,
                 'output' => $output,
             ]);
              //触发[EVENT_AFTER_RENDER]事件
             $this->trigger(self::EVENT_AFTER_RENDER, $event);
             $output = $event->output;//执行后置事件后的输出结果
         }
     }

     /**
      * Renders a view file as a PHP script.
      * 将一个view文件当作PHP脚本渲染
      * This method treats the view file as a PHP script and includes the file.
      * It extracts the given parameters and makes them available in the view file.
      * The method captures the output of the included view file and returns it as a string.
      * 将传入的参数转换为变量，包含并执行view文件，返回执行结果
      * This method should mainly be called by view renderer or [[renderFile()]].
      *
      * @param string $_file_ the view file.
      * @param array $_params_ the parameters (name-value pairs) that will be extracted and made available in the view file.
      * @return string the rendering result
      */
     public function renderPhpFile($_file_, $_params_ = [])
     {
         //ob_start() — 打开输出控制缓冲
         ob_start();
         // ob_implicit_flush ()  — 默认为关闭缓冲区，打开绝对输出后，每个脚本输出都直接发送到浏览器，不再需要调用 flush()
         ob_implicit_flush(false);
         extract($_params_, EXTR_OVERWRITE);//extract() - 用于将一个数组转换为变量使用，键名为变量名，键值为对应的变量值
         require($_file_);
         //ob_get_clean() — 得到当前缓冲区的内容并删除当前输出缓
         return ob_get_clean();
     }

     /**
      * Renders dynamic content returned by the given PHP statements.
      * 渲染动态内容
      * This method is mainly used together with content caching (fragment caching and page caching)
      * 该方法主要用来聚合缓存的内容（片段缓存和页面缓存）
      * when some portions of the content (called *dynamic content*) should not be cached.
      * The dynamic content must be returned by some PHP statements.
      * 用来渲染某些被PHP语句返回的动态内容
      * @param string $statements the PHP statements for generating the dynamic content.
      * @return string the placeholder of the dynamic content, or the dynamic content if there is no
      * active content cache currently.
      */
     public function renderDynamic($statements)
     {
         if (!empty($this->cacheStack)) {//动态内容的栈列表不为空
             $n = count($this->dynamicPlaceholders);//计算动态内容条数
             $placeholder = "<![CDATA[YII-DYNAMIC-$n]]>";//生成占位符--动态内容前缀--起标记作用
             $this->addDynamicPlaceholder($placeholder, $statements);//添加动态内容占位符

             return $placeholder;
         } else {
             return $this->evaluateDynamicContent($statements);//动态内容的栈列表为空，值行传入的PHP语句，返回执行结果
         }
     }

     /**
      * Adds a placeholder for dynamic content.
      * 给dynamic content添加一个占位符
      * This method is internally used.
      * 该方法是内部使用的
      * @param string $placeholder the placeholder name
      * @param string $statements the PHP statements for generating the dynamic content
      */
     public function addDynamicPlaceholder($placeholder, $statements)
     {
         foreach ($this->cacheStack as $cache) {
             $cache->dynamicPlaceholders[$placeholder] = $statements;//给widget中的[FragmentCache]添加占位符
         }
         $this->dynamicPlaceholders[$placeholder] = $statements;//给当前视图添加动态内容占位符
     }

     /**
      * Evaluates the given PHP statements.
      * 求给定的PHP语句的值
      * This method is mainly used internally to implement dynamic content feature.
      * 该方法是内部使用实现动态内容功能
      * @param string $statements the PHP statements to be evaluated.
      * @return mixed the return value of the PHP statements.
      */
     public function evaluateDynamicContent($statements)
     {
         return eval($statements);//eval() 函数用于执行文本方式输入的php语句
     }