尝试设置 UIButton 的约束时出错

Question

错误：

2018-05-13 01：32：25.577503-0400 登录屏幕[2606：70991] *** 由于未捕获的异常"NSGenericException"而终止应用，原因："无法使用锚点激活约束<NSLayoutYAxisAnchor：0x60000046acc0"UIButton：0x7fb4cad0b120'忘记密码？top">和<NSLayoutYAxisAnchor：0x60000046ad40"UIButton：0x7fb4cad0a160'Log>因为它们没有共同的祖先。 约束或其定位点是否引用不同视图层次结构中的项？ 这是违法的。

源代码：

import UIKit
class ViewController: UIViewController {

let emailTextField: UITextField = {
let e = UITextField()
let attributedPlaceHolder = NSAttributedString(string: "Email", attributes:
[NSAttributedStringKey.foregroundColor: UIColor.white])
e.attributedPlaceholder = attributedPlaceHolder
e.backgroundColor = GREEN_THEME
return e
}()
let passwordTextField: UITextField = {
let p = UITextField()
let attributedPlaceHolder = NSAttributedString(string: "Password", attributes:
[NSAttributedStringKey.foregroundColor: UIColor.white])
p.attributedPlaceholder = attributedPlaceHolder
p.backgroundColor = GREEN_THEME
return p
}()
let loginButton: UIButton = {
let l = UIButton(type: .system)
l.setTitleColor( .white, for: .normal)
l.setTitle("Log In", for: .normal)
l.backgroundColor = GREEN_THEME
return l
}()
let forgotPasswordButton: UIButton = {
let f = UIButton(type: .system)
f.setTitleColor( .white, for: .normal)
f.setTitle("Forgot Password?", for: .normal)
f.backgroundColor = GREEN_THEME
return f
}()
let haveAccountButton: UIButton = {
let color = UIColor(red: 89/255, green: 156/255, blue: 120/255, alpha: 1 )
let font = UIFont.systemFont(ofSize: 16)
let h = UIButton(type: .system)
h.backgroundColor = GREEN_THEME
let attributedTitle = NSMutableAttributedString(string:
"Don't have an accoun?", attributes:
[NSAttributedStringKey.foregroundColor: color, NSAttributedStringKey.font: font ])
attributedTitle.append(NSAttributedString(string: "Sign Up", attributes: [NSAttributedStringKey.foregroundColor: UIColor.white,
            NSAttributedStringKey.font: font ]))
h.setAttributedTitle(attributedTitle, for: .normal)
return h
}()
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = GREEN_THEME
setupTextFieldComponents()
setupLoginButton()
} 
fileprivate func setupTextFieldComponents(){
setupEmailField()
setupPasswordField()
setupHaveAccountButton()
setupForgotPasswordButton()
}
override var preferredStatusBarStyle: UIStatusBarStyle{
return .lightContent
}

fileprivate func setupEmailField(){
view.addSubview(emailTextField)
emailTextField.translatesAutoresizingMaskIntoConstraints = false
emailTextField.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 16).isActive = true
emailTextField.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 24).isActive = true
emailTextField.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -24).isActive = true
emailTextField.heightAnchor.constraint(equalToConstant: 30).isActive = true
emailTextField.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
}

fileprivate func setupPasswordField(){
view.addSubview(passwordTextField)
passwordTextField.translatesAutoresizingMaskIntoConstraints = false
passwordTextField.topAnchor.constraint(equalTo: emailTextField.bottomAnchor, constant: 8).isActive = true
passwordTextField.leftAnchor.constraint(equalTo: emailTextField.leftAnchor, constant: 0).isActive = true
passwordTextField.rightAnchor.constraint(equalTo: emailTextField.rightAnchor, constant: 0).isActive = true
passwordTextField.heightAnchor.constraint(equalToConstant: 30).isActive = true
}

fileprivate func setupLoginButton(){
view.addSubview(loginButton)
loginButton.translatesAutoresizingMaskIntoConstraints = false
loginButton.topAnchor.constraint(equalTo: passwordTextField.bottomAnchor, constant: 8).isActive = true
loginButton.leftAnchor.constraint(equalTo: passwordTextField.leftAnchor, constant: 0).isActive = true
loginButton.rightAnchor.constraint(equalTo: passwordTextField.rightAnchor, constant: 0).isActive = true
loginButton.heightAnchor.constraint(equalToConstant: 30).isActive = true
}
fileprivate func setupForgotPasswordButton(){
view.addSubview(forgotPasswordButton)

forgotPasswordButton.translatesAutoresizingMaskIntoConstraints = false
forgotPasswordButton.topAnchor.constraint(equalTo: loginButton.bottomAnchor, constant: 8).isActive = true
forgotPasswordButton.leftAnchor.constraint(equalTo: loginButton.leftAnchor, constant: 0).isActive = true
forgotPasswordButton.rightAnchor.constraint(equalTo: loginButton.rightAnchor, constant: 0).isActive = true
forgotPasswordButton.heightAnchor.constraint(equalToConstant: 30).isActive = true
}
fileprivate func setupHaveAccountButton(){
view.addSubview(haveAccountButton)
haveAccountButton.translatesAutoresizingMaskIntoConstraints = false
haveAccountButton.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -8).isActive = true
haveAccountButton.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 12).isActive = true
haveAccountButton.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -12).isActive = true
haveAccountButton.heightAnchor.constraint(equalToConstant: 30).isActive = true
}
}

Answer 1

首先，您应该将forgotPasswordButton和loginButton作为subView添加到您的view

view.addSubview(forgotPasswordButton)
view.addSubview(loginButton)

然后，constraints应该可以工作。