Swift中的willSet与didSet(重写属性的Set方法)

var rating: CGFloat? {
        willSet {
            println("将要设置新值\(newValue)")
        } didSet {
            println("rating filed changed form \(oldValue) to \(rating)")
            println("rating old(\(oldValue)) new(\(rating)))")
            //告诉系统将要布局
            self.setNeedsLayout()
            /*
            if rating < 10
            {
                nickName = "Little"
            }else
            {
                nickName = "Big"
            }
            */
            /*
            let rect = CGRectMake(foregroundImageView.frame.origin.x,foregroundImageView.frame.origin.y,backgroundImageView.frame.size.width * (rating!/10),foregroundImageView.frame.size.height)
            foregroundImageView.frame = rect
            */
        }
        
    }

在layoutSubViews()填充数据和布局视图（注意：方法是否会被多次调用）

override func layoutSubviews() {
        super.layoutSubviews()
        //填充数据（布局视图）
        let rect = CGRectMake(foregroundImageView.frame.origin.x,backgroundImageView.frame.width * (rating! / 10),foregroundImageView.frame.height)
        foregroundImageView.frame = rect
        
    }