SnapKit自动布局的使用

        let redView : UIView = UIView.init()
        redView.backgroundColor = UIColor.red
        self.view.addSubview(redView)
        redView.snp.makeConstraints { (make) in
            make.width.height.equalTo(50) //宽度和高度设置为50
            make.center.equalTo(self.view)//在self.view上居中
        }

        let blue : UIView = UIView.init()
        blue.backgroundColor = UIColor.blue
        self.view.addSubview(blue)
        blue.snp.makeConstraints { (make) in
            make.top.equalTo(redView.snp.bottom)
            make.left.equalTo(redView.snp.left)
            make.width.equalTo((redView.snp.width))
            make.height.equalTo((redView.snp.height)).multipliedBy(0.5)
        }

       let blue : UIView = UIView.init()
        blue.backgroundColor = UIColor.blue
        self.view.addSubview(blue)
        blue.snp.makeConstraints { (make) in
            make.top.equalTo(redView.snp.bottom)
            make.left.equalTo(redView)
            make.width.equalTo((redView))
            make.height.equalTo((redView)).multipliedBy(0.5)
        }

      let blue : UIView = UIView.init()
        blue.backgroundColor = UIColor.blue
        self.view.addSubview(blue)
        blue.snp.makeConstraints { (make) in
            make.edges.equalTo(self.view).inset(UIEdgeInsets.init(top: 10, left: 10, bottom: 10, right: 10))
        }

        let lab:UILabel = UILabel.init()
        lab.textAlignment = .center
        lab.backgroundColor = UIColor.red
        lab.numberOfLines = 0
        lab.text = "我是一个UILabel"
        self.view.addSubview(lab)
        lab.snp.makeConstraints { (make) in
            make.center.equalTo(self.view)
//            make.width.greaterThanOrEqualTo(200)//最小的宽度是200
            make.width.lessThanOrEqualTo(20)//最大的宽度是20
        }