我如何并排添加两个UIButton？

Question

[每当我模拟我的应用程序时，UIButton都不会出现。我收到一个错误消息：

由于未捕获的异常'NSGenericException'而终止应用程序，原因：'无法使用锚点激活约束和因为他们没有共同的祖先。是约束还是它的约束在不同的视图层次结构中锚定参考项？那是非法。'

我该如何解决？

 func setView(){

        signInButton.addTarget(self, action: #selector(userSignIn), for: .touchUpInside)
        signInButton.heightAnchor.constraint(equalToConstant: 120).isActive = true
        signInButton.widthAnchor.constraint(equalToConstant: 120).isActive = true
        signInButton.setTitle("Sign in",for: .normal)

        createAnAccountButton.addTarget(self, action: #selector(createAnAccount), for: .touchUpInside)
        createAnAccountButton.setTitle("Create an Account", for: .normal)
        createAnAccountButton.heightAnchor.constraint(equalToConstant: 120).isActive = true
        createAnAccountButton.widthAnchor.constraint(equalToConstant: 120).isActive = true

        let buttonStack = UIStackView()

        buttonStack.axis = NSLayoutConstraint.Axis.vertical
        buttonStack.distribution = UIStackView.Distribution.equalSpacing
        buttonStack.alignment = UIStackView.Alignment.center
        buttonStack.spacing = 16

        buttonStack.addArrangedSubview(signInButton)
        buttonStack.addArrangedSubview(createAnAccountButton)
        buttonStack.translatesAutoresizingMaskIntoConstraints = false
        buttonStack.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        buttonStack.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
        buttonStack.frame = CGRect(x: 0, y: 0, width: 100, height: 60)
        self.view.addSubview(buttonStack)

//        let barButtonSignIn = UIBarButtonItem(customView: signInButton)
//        self.navigationItem.leftBarButtonItem = barButtonSignIn

    }

Answer 1

您必须在激活约束之前调用addSubview。