用xib写UITableViewCell，绑定的变量值为nil，为什么？

[dongjize]

我用xib加载了一个UITableViewCell，然而调用bind()方法时发现，xib绑定的几个变量都是nil。请问是哪里出了问题？
代码：

class ArticleTableCell: UITableViewCell {

    @IBOutlet weak var avatar: UIImageView!
    @IBOutlet weak var titleLabel: UILabel!
    @IBOutlet weak var subtitleLabel: UILabel!
    
    var articleModel: ArticleModel?
    
    override func awakeFromNib() {
        super.awakeFromNib()
    }

    override func setSelected(selected: Bool, animated: Bool) {
        super.setSelected(selected, animated: animated)

    }
    
    func bind(model: ArticleModel) {
        self.articleModel = model 
        self.titleLabel.text = model.title! //nil
        self.subtitleLabel.text = model.subtitle! //nil
        self.avatar.sd_setImageWithURL(NSURL.init(string: model.avatar!), placeholderImage: UIImage.init(named: "placeholder.png")) //nil
        
    }
}

xib样式如图：