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