如何并排添加两个 UIButton？

Question

每当我模拟我的应用程序时，UIButtons 都不会出现。我收到一条错误消息

Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with anchors and because they have no common ancestor. Does the constraint or its anchors reference items in different view hierarchies? That's illegal.'

我该如何解决这个问题？

 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。

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