视图的布局文件中的资源加载 [ 求助交流 ]

在layout/main.php中看到这样一行代码 $this->head;

追踪代码看到：

protected function renderHeadHtml()
    {
        $lines = [];
        if (!empty($this->metaTags)) {
            $lines[] = implode("\n", $this->metaTags);
        }

        if (!empty($this->linkTags)) {
            $lines[] = implode("\n", $this->linkTags);
        }
        if (!empty($this->cssFiles)) {
            $lines[] = implode("\n", $this->cssFiles);
        }
        if (!empty($this->css)) {
            $lines[] = implode("\n", $this->css);
        }
        if (!empty($this->jsFiles[self::POS_HEAD])) {
            $lines[] = implode("\n", $this->jsFiles[self::POS_HEAD]);
        }
        if (!empty($this->js[self::POS_HEAD])) {
            $lines[] = Html::script(implode("\n", $this->js[self::POS_HEAD]), ['type' => 'text/javascript']);
        }

        return empty($lines) ? '' : implode("\n", $lines);
    }

请问方法中的$this->css,$this->js,$this->jsFile在什么时候加载的？