Ios 如何执行激活编程约束

我编写了一个函数，该函数定义UILabel并将其添加到UIView中，但是由于标签保留在屏幕的左上角，因此没有执行约束

代码如下：

func timeAdded(screenHeight: CGFloat, screenWidth: CGFloat, viewController: UIView) {


    let readyLabel = UILabel()
    readyLabel.backgroundColor = UIColor.white
    readyLabel.textAlignment = .center
    readyLabel.text = "00:00"
    readyLabel.textColor = UIColor.blue
    //readyLabel.alpha = 0
    readyLabel.font = UIFont(name: "panl-font-4", size: 60)
    readyLabel.sizeToFit()

    viewController.addSubview(readyLabel)

    readyLabel.centerXAnchor.constraint(equalTo: viewController.centerXAnchor, constant: 0).isActive = true
    readyLabel.centerYAnchor.constraint(equalTo: viewController.centerYAnchor, constant: 0).isActive = true

}

您可能需要的任何建议，敬请谅解

readyLabel.translatesAutoresizingMaskIntoConstraints = false 
NSLayoutConstraint.activate([
    readyLabel.centerXAnchor.constraint(equalTo: viewController.centerXAnchor)
    readyLabel.centerYAnchor.constraint(equalTo: viewController.centerYAnchor)
])

所以最好是读书