ViewController 不显示视图

将 UILabel 添加到我的 ViewController 时，它显示正常，但是一旦我使用单独的 UIView，控制器就不会显示任何内容。ViewController 是从 TableViewController.didSelectRowAt 推送的

在 didSelectRowAt 中调用了 ViewController

let labelViewController = TempViewController()                
navigationController?.pushViewController(labelViewController, animated: true)

场景 1：仅 ViewController - 按预期显示蓝色屏幕，文本位于屏幕的 y 中心

class TempViewController: UIViewController {
    var tempView: UILabel = {
        let label = UILabel()
        label.text = "Please enter your email address"
        label.translatesAutoresizingMaskIntoConstraints = false
        label.backgroundColor = UIColor.blue
        return label
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = UIColor.red
        navigationItem.title = "Temp"
        edgesForExtendedLayout = []

        view.addSubview(tempView)
        tempView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        tempView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        tempView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        tempView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
    }

场景 2：ViewController 和 View - 显示一个没有文本也没有蓝色的红色屏幕

class TempViewController: UIViewController {

    var tempView = TempView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = UIColor.red
        navigationItem.title = "Temp"
        edgesForExtendedLayout = []

        view.addSubview(tempView)
        tempView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        tempView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        tempView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        tempView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
    }
}

class TempView: UIView {
    var tempView: UILabel = {
        let label = UILabel()
        label.text = "Please enter your email address"
        label.translatesAutoresizingMaskIntoConstraints = false
        label.backgroundColor = UIColor.blue
        return label
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        setView()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setView()
    }

    private func setView() {
        addSubviews()
        setLayout()
    }

    private func addSubviews() {
        addSubview(tempView)
    }

    private func setLayout() {
                tempView.topAnchor.constraint(equalTo: topAnchor).isActive = true
                tempView.leadingAnchor.constraint(equalTo: leadingAnchor).isActive = true
                tempView.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
                tempView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
    }
}

我希望这两种情况都能产生相同的结果？但是场景2不显示视图？

谢谢你的帮助