core/Controller.php
protected function fetch(string $tplname, string $parent = null)
    {
        $this->view()->context($this);
        if ($parent !== null) {
            return $this->view()->fetch('extends:' . $parent . '|' . $tplname);
        }
        return $this->view()->fetch($tplname);
    }
protected function fetch(string $tplname, string $parent = null)
    {
        $this->view()->context($this);
        if ($parent !== null) {
            return $this->view()->fetch('extends:' . $parent . '|' . $tplname);
        }
        return $this->view()->fetch($tplname);
    }
获取内容
@param string $tplname
@param string|null $parent
@return mixed|string|void