Creating UIViews Programmatically doesn't work in Xcode 11.1

Question

I want to create an UIView as a container view or even more programmatically with the snapkit in Xcode 11.1, but it seems there is something wrong and I think Apple has changed UIView in Xcode 11.x (iOS 13.x) because I did it very easily in the previous versions of Xcode.

In SceneDelegate.swift (Apple has moved the window variable from AppDelegate.swift to SceneDelegate.swift)

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let windowScene = (scene as? UIWindowScene) else { return }
            self.window = UIWindow(windowScene: windowScene)
            self.window?.rootViewController = ViewController()
            self.window?.makeKeyAndVisible()
        }
    }
}

ViewController.swift

  class ViewController: UIViewController {

    var containerView: UIView = {
        let view = UIView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
        view.backgroundColor = .green
        view.translatesAutoresizingMaskIntoConstraints = false
        return view
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = .blue
        setUpView()
    }
    
    func setUpView() {
        self.view.backgroundColor = .blue
        self.view.addSubview(containerView)
        containerView.snp.makeConstraints { (make) in
            make.centerX.equalTo(self.view.snp.centerX)
            make.centerY.equalTo(self.view.snp.centerY)
        }
    }
}

and the result:

Answer 1

To displaying a view using autolayout, you need to provide all W H X Y satisfy the layout engine. But you missed width and height constraints.

Try this:

func setUpView() {
    self.view.backgroundColor = .blue
    self.view.addSubview(containerView)
    containerView.snp.makeConstraints { (make) in
        make.width.height.equalTo(100)
        make.center.equalTo(self.view)
    }
}