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,toItem: lbName,constant: 0)
c.active = true 
let h = NSLayoutConstraint(item: imgCoupon,attribute: .Height,toItem: nil,attribute: .NotAnAttribute,constant: 16)
 h.active = true

let w = NSLayoutConstraint(item: imgCoupon,attribute: .Width,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