以编程方式使用布局锚创建子视图

Question

我使用layout anchors 以编程方式创建了一个UIView。现在我想在这个视图中添加一个UILabel。到目前为止，这是我的代码：

let centerView = UIView()
centerView.translatesAutoresizingMaskIntoConstraints = false
centerView.backgroundColor = UIColor.white
view.addSubview(centerView)
centerView.leftAnchor.constraint(equalTo: view.leftAnchor, constraint: 20).isActive = true
centerView.rightAnchor.constraint(equalTo: view.rightAnchor, constraint: -20).isActive = true

let label = UILabel()
label.translatesAutoresizingMaskIntoConstraints = false
label.text = "Testing" 
label.textColor = UIColor.black
centerView.addSubview(label)
label.leftAnchor.constraint(equalTo: centerView.leftAnchor).isActive = true

我认为这个标签会参考centerView 来显示，但实际上它是参考UIWindow 来显示的。这是当前的视图层次结构：

UIWindow --> UIView (centerView) --> UILabel (label)

我需要在centerView 中添加多个标签，据我了解，这条链会变长，而我希望多个标签都在centerView 下

         UIWindow

            |

     UIView (centerView)

     /      |      \
  Label 1  Label 2  Label 3

我怎样才能实现这种类型的层次结构？

Answer 1

你做得对，只是你没有提供足够的约束。我在 Swift Playground 中尝试了您的代码并添加了一些额外的约束，它表明标签是相对于 centerView 放置的：

let view = UIView(frame: CGRect(x: 0, y: 0, width: 300, height: 500))

let centerView = UIView()
centerView.translatesAutoresizingMaskIntoConstraints = false
centerView.backgroundColor = UIColor.white
view.addSubview(centerView)
centerView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20).isActive = true
centerView.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -20).isActive = true
centerView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20).isActive = true
centerView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -20).isActive = true

let label = UILabel()
label.translatesAutoresizingMaskIntoConstraints = false
label.text = "Testing"
label.textColor = UIColor.black
label.backgroundColor = UIColor.yellow
centerView.addSubview(label)
label.leftAnchor.constraint(equalTo: centerView.leftAnchor).isActive = true
label.topAnchor.constraint(equalTo: centerView.topAnchor).isActive = true

view.layoutIfNeeded()

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这是在操场上跑步：