在Swift中以编程方式添加iOS约束

Question

那么，我已经不再使用Xcode中的IB，希望能够用Swift编写所有的UI。

所以我做的事情是:

• Created a new UIView to contain the elements i want to write - lets call it "TestView"
• I've added TestView to a VC as a sub view.

• In the TestView class I've added the elements like this:

  class TestView: UIView {
        var someLabel:UILabel!
        override init(frame: CGRect) {
              super.init(frame: frame)
  
              self.someLabel = UILabel(frame: CGRect(x: self.frame.midX, y: oneSixthHeight, width: 100, height: 22))
              self.someLabel.text = "test"
  
              var constraints:[NSLayoutConstraint] = []
              self.someLabel.translatesAutoresizingMaskIntoConstraints = false
              let rightsideAnchor:NSLayoutConstraint = NSLayoutConstraint(item: self.someLabel, attribute: .Trailing, relatedBy: .Equal, toItem: self, attribute: .Trailing, multiplier: 1, constant: 1)
  
               constraints.append(rightsideAnchor)
               NSLayoutConstraint.activateConstraints(constraints)
        }
  }
  

我希望这样就可以将UILabel锚定在视图的右侧。

然而，我遇到了以下错误：

未捕获异常“NSGenericException”，原因是“无法激活具有项目>和>的约束，因为它们没有共同的祖先。
是否约束引用了不同视图层次结构中的项目？ 这是非法的。'

我做错了什么？

- Simon k

---

你把 someLabel 添加到哪个视图的子视图中了吗？只有当它们已经作为子视图添加时，才能应用约束并激活它们。否则尝试会导致崩溃，就像你刚才看到的那样。 - Sandeep Bhandari

Answer 1

只有在视图添加到视图层次结构后才应添加约束。从您的代码中可以清楚地看出，您尚未将UILabel实例添加到视图中。

- msk

---

谢谢！ :) 将标签添加为子视图，一切都像魔术般地运行。 <3 - Simon k

Answer 2

已更新至Swift 3

import UIKit

class ViewController: UIViewController {

let redView: UIView = {

    let view = UIView()
    view.translatesAutoresizingMaskIntoConstraints = false
    view.backgroundColor = .red
    return view
}()

override func viewDidLoad() {
    super.viewDidLoad()

    setupViews()
    setupAutoLayout()
}

func setupViews() {

    view.backgroundColor = .white
    view.addSubview(redView)
}

func setupAutoLayout() {

    // Available from iOS 9 commonly known as Anchoring System for AutoLayout...
    redView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20).isActive = true
    redView.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -20).isActive = true

    redView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    redView.heightAnchor.constraint(equalToConstant: 300).isActive = true

    // You can also modified above last two lines as follows by commenting above & uncommenting below lines...
    // redView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20).isActive = true
    // redView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
 }
}

约束类型：

/*
// regular use
1.leftAnchor
2.rightAnchor
3.topAnchor
// intermediate use
4.widthAnchor
5.heightAnchor
6.bottomAnchor
7.centerXAnchor
8.centerYAnchor
// rare use
9.leadingAnchor
10.trailingAnchor
etc. (note: very project to project)
*/

- iAj

---

谢谢，我总是忘记“view.translatesAutoresizingMaskIntoConstraints = false”。 - Pomo-J

9和10也应经常使用，并且不应该很少出现，特别是对于标签。有时语言是从右到左的。 - CyberMew