NSLayoutConstraint参数说明：

/**

 * 创建约束 NSLayoutConstraint  参数 说明：

 * item 自己

 * attribute

 * relatedBy 大于等于 小于等于 等于 ...

 * toItem 另外一个控件

 * attribute 另一个控件的属性

 * multiplier 乘以多少

 * constant : 加上多少

 * NSLayoutConstraint : 某个控件的属性值 等于 另外一个控件的属性值

                乘以多少 加上多少

 * 添加约束 addConstraint

 */

NSLayoutConstraint(item: AnyObject, attribute: NSLayoutAttribute, relatedBy: NSLayoutRelation, toItem: AnyObject?, attribute: NSLayoutAttribute, multiplier: CGFloat, constant: CGFloat)

创建子控件

let childView = UILabel()

childView.text = "正在链接服务器"

childView.backgroundColor = UIColor.yellow //背影黄色

childView.textAlignment = .center //文本居中对齐

parentView.backgroundColor = UIColor.red//设置父元素背影红色，方便查看效果

关闭autoresizing

// 关闭autoresizing，不然无效果

childView.translatesAutoresizingMaskIntoConstraints =false

创建约束:

//创建相对父view的左右间距各20

//左约束

let leftConstraint = NSLayoutConstraint(item: childView, attribute: NSLayoutAttribute.leading, relatedBy: NSLayoutRelation.equal, toItem: topStatusBarView, attribute:NSLayoutAttribute.leading, multiplier: 1.0, constant: 20)

//右约束

let rightConstraint = NSLayoutConstraint(item: childView, attribute: NSLayoutAttribute.trailing, relatedBy: NSLayoutRelation.equal, toItem: topStatusBarView, attribute:NSLayoutAttribute.trailing, multiplier: 1.0, constant: -20)

let centerConstraint = NSLayoutConstraint(item: childView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: topStatusBarView, attribute:NSLayoutAttribute.centerY, multiplier: 1.0, constant: 0)

//子控件自身高度

let heigtConstraint = NSLayoutConstraint(item: childView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute:NSLayoutAttribute.notAnAttribute, multiplier: 1.0, constant: 25)

//子控件内部自己的属性（高度，自己添加）

childView.addConstraint(heigtConstraint)

//子元素相对父亲的元素，由父添加

parentView.addConstraints([leftConstraint, rightConstraint, centerConstraint])

例:

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

PS:苹果官网API - NSLayoutConstraint