SnapKit/SnapKit

import SnapKit

class MyViewController: UIViewController {

    lazy var box = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.addSubview(box)
        box.snp_makeConstraints { (make) -> Void in
           make.width.height.equalTo(50)
           make.center.equalTo(self.view)
        }
    }

}