public function flash($message, $level = null)
    {
        if (null === $level) {
            $level = static::DEFAULT_LEVEL;
        }

        $messages = $this->core->session()->get(static::SESSION_FLASH_KEY, []);

        $messages[] = [
            'message' => $message,
            'level'   => $level,
        ];

        $this->core->session()->put(static::SESSION_FLASH_KEY, $messages);

        return $this;
    }