Swift使用代码进行约束

版本为Swift2.3

let imgCoupon = UIImageView()
imgCoupon.frame = CGRectMake(0, 0, 16, 16)
imgCoupon.loadImgByNamed("coupon")
imgCoupon.tag = 12345
cell.addSubview(imgCoupon)
imgCoupon.translatesAutoresizingMaskIntoConstraints = false
let l = NSLayoutConstraint(item: imgCoupon, attribute: .Leading, relatedBy: .Equal, 
    toItem: firstView, attribute: .Trailing, multiplier: 1, constant: 3)
l.active = true
let c = NSLayoutConstraint(item: imgCoupon, attribute: .CenterY, relatedBy: .Equal,
     toItem: lbName, attribute: .CenterY, multiplier: 1, constant: 0)
c.active = true 
let h = NSLayoutConstraint(item: imgCoupon, attribute: .Height, relatedBy: .Equal, 
    toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 16)
 h.active = true

let w = NSLayoutConstraint(item: imgCoupon, attribute: .Width, relatedBy: .Equal, 
    toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 16)
w.active = true

Swfit2.3中使用w.active = true 而Swift3.0中是addConstraint(w)，并且attribute的值也不同