iOS在Swift中以编程方式添加约束

Question

所以,我已经完成了使用IBin Xcode并希望UI在Swift中编写所有内容.

所以我做的是:

• 创建一个新UIView的包含我想写的元素 - 让我们称之为"TestView"
• 我已添加TestView到a VC作为子视图.

• 在TestView课堂上我添加了这样的元素:

  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)
        }
  }
  

有了这个,我希望将UILabel它固定在视图的右侧.

但是,我确实收到此错误:

由于未捕获的异常'NSGenericException'而终止应用程序,原因:'无法激活带有项的约束>和>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.

我究竟做错了什么？

Answer 1

只有在将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到,您尚未添加要查看的UILabel实例.

Answer 2

为 Swift 3 更新

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)
*/