Laravel通知类未传递变量

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Notifications\Notification;

class ExampleNotification extends Notification
{
    use Queueable;

    protected $userName;

    /**
     * Create a new notification instance.
     *
     * @param  string  $userName
     * @return void
     */
    public function __construct($userName)
    {
        $this->userName = $userName;
    }

    /**
     * Get the notification's delivery channels.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function via($notifiable)
    {
        return ['mail'];
    }

    /**
     * Get the mail representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return \Illuminate\Notifications\Messages\MailMessage
     */
    public function toMail($notifiable)
    {
        return (new MailMessage)
            ->greeting('Hello, '.$this->userName)
            ->line('This is an example notification.')
            ->action('Notification Action', url('/'))
            ->line('Thank you for using our application!');
    }
}