使用swift在代码中的约束 - 无法激活

Question

我正在尝试了解自动布局限制。遵循Ray W教程挑战（没有解决方案或讨论），布局应如下所示：

在IB中完成它很容易 - 创建一个宽度和高度的黄色视图，垂直居中并水平固定到边缘;然后在该视图中创建具有简单约束的标签，字段和按钮。

我的第一个问题是：如果标签具有内在的内容大小，而其他所有内容都固定在黄色视图上，那么为什么我需要定义固定的宽度和高度？为什么不从其子视图的内在内容推断宽度和高度？

继续并尝试在代码中重新创建此布局给了我一个错误：

由于未捕获的异常'NSGenericException'终止应用程序，原因：'无法激活项目<UILabel: 0x7a961d60; frame = (0 0; 0 0); text = 'Password:'; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x7a961e40>>和<UIView: 0x7a96b6f0; frame = (0 0; 0 0); layer = <CALayer: 0x7a96b880>>的约束，因为它们没有共同的祖先。约束是否引用不同视图层次结构中的项目？这是非法的。

第二个问题：这些图层是什么，我的所有视图都在层次结构中 - superview包含黄色视图，包含文本标签和字段。

经过很多痛苦，我试图完全重新创建IB中的约束，但这只添加了以下错误：无法同时满足约束。

(
    "<NSLayoutConstraint:0x7ac57370 H:[UIView:0x7a96b6f0(0)]>",
    "<NSLayoutConstraint:0x7ac57400 H:|-(8)-[UILabel:0x7a96bb50'Username:']   (Names: '|':UIView:0x7a96b6f0 )>",
    "<NSLayoutConstraint:0x7ac57430 UILabel:0x7a96bb50'Username:'.trailing == UITextField:0x7a961020.leading + 8>",
    "<NSLayoutConstraint:0x7ac57520 UITextField:0x7a961020.trailing == UIView:0x7a96b6f0.trailing - 8>" )

最后的问题：我怎么知道视图是什么视图0x7aetc？我的代码中的约束在哪里？其余的看起来不错（？）。

我必须在某个基本层面做一些非常错误的事情。

这是我的代码：

import UIKit

class ViewController: UIViewController {

    let centerView = UIView()
    let usernameLabel = UILabel()
    let passwordLabel = UILabel()
    let usernameField = UITextField()
    let passwordField = UITextField()
    let submitButton = UIButton()

    override func viewDidLoad() {
        super.viewDidLoad()

        centerView.backgroundColor = UIColor.yellowColor()
        usernameLabel.text = "Username:"
        passwordLabel.text = "Password:"
        usernameField.borderStyle = .RoundedRect
        passwordField.borderStyle = .RoundedRect
        submitButton.setTitle("Submit!", forState: .Normal)
        submitButton.setTitleColor(UIColor.blackColor(), forState: .Normal)
        submitButton.setTitleColor(UIColor.blueColor(), forState: .Highlighted)

        view.addSubview(centerView)
        self.centerView.addSubview(usernameField)
        self.centerView.addSubview(passwordField)
        self.centerView.addSubview(usernameLabel)
        self.centerView.addSubview(submitButton)

        let constraintCenterViewHeight = NSLayoutConstraint(item: centerView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 0)
        let constraintCenterViewWidth = NSLayoutConstraint(item: centerView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 0)

        let constraintCenterViewCenterX = NSLayoutConstraint(item: centerView, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1.0, constant: 0)
        let constraintCenterViewCenterY = NSLayoutConstraint(item: centerView, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1.0, constant: 0)

        let constraintUsernameLabelHLeading = NSLayoutConstraint(item: usernameLabel, attribute: .Leading, relatedBy: .Equal, toItem: centerView, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintUsernameLabelHTrailing = NSLayoutConstraint(item: usernameLabel, attribute: .Trailing, relatedBy: .Equal, toItem: usernameField, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintUsernameLabelAlignBottom = NSLayoutConstraint(item: usernameLabel, attribute: .Bottom, relatedBy: .Equal, toItem: usernameField, attribute: .Bottom, multiplier: 1.0, constant: 0)

        let constraintUsernameFieldVTop = NSLayoutConstraint(item: usernameField, attribute: .Top, relatedBy: .Equal, toItem: centerView, attribute: .Top, multiplier: 1.0, constant: 8)
        let constraintUsernameFieldHTrailing = NSLayoutConstraint(item: usernameField, attribute: .Trailing, relatedBy: .Equal, toItem: centerView, attribute: .Trailing, multiplier: 1.0, constant: -8)
        let constraintUsernameFieldVBottom = NSLayoutConstraint(item: usernameField, attribute: .Bottom, relatedBy: .Equal, toItem: passwordField, attribute: .Top, multiplier: 1.0, constant: 8)

        let constraintPasswordLabelHLeading = NSLayoutConstraint(item: passwordLabel, attribute: .Leading, relatedBy: .Equal, toItem: centerView, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintPasswordLabelHTrailing = NSLayoutConstraint(item: passwordLabel, attribute: .Trailing, relatedBy: .Equal, toItem: passwordField, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintPasswordLabelAlignBottom = NSLayoutConstraint(item: passwordLabel, attribute: .Bottom, relatedBy: .Equal, toItem: passwordField, attribute: .Bottom, multiplier: 1.0, constant: 0)

        let constraintPasswordFieldHTrailing = NSLayoutConstraint(item: passwordField, attribute: .Trailing, relatedBy: .Equal, toItem: centerView, attribute: .Trailing, multiplier: 1.0, constant: -8)

        centerView.setTranslatesAutoresizingMaskIntoConstraints(false)
        usernameLabel.setTranslatesAutoresizingMaskIntoConstraints(false)
        usernameField.setTranslatesAutoresizingMaskIntoConstraints(false)
        passwordField.setTranslatesAutoresizingMaskIntoConstraints(false)
        passwordLabel.setTranslatesAutoresizingMaskIntoConstraints(false)
       // submitButton.setTranslatesAutoresizingMaskIntoConstraints(false)

        NSLayoutConstraint.activateConstraints([constraintCenterViewHeight, constraintCenterViewWidth, constraintCenterViewCenterX, constraintCenterViewCenterY, constraintUsernameLabelHLeading,
            constraintUsernameLabelHTrailing, constraintUsernameLabelAlignBottom, constraintUsernameFieldVTop, constraintUsernameFieldHTrailing, constraintUsernameFieldVBottom, constraintPasswordLabelHLeading, constraintPasswordLabelHTrailing, constraintPasswordLabelAlignBottom, constraintPasswordFieldHTrailing])
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

Answer 1

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