NSLayoutConstraint和safeAreaLayoutGuide -兼容于预iOS 11的编码

Question

如果我想要一个兼容于前置iOS 11设备的应用程序，为了遵守self.view，我是否需要为将视图的某些属性链接到safeAreaLayoutGuide的每一个约束都编写这段代码？

if #available(iOS 11.0, *) {
     NSLayoutConstraint.activate([
          theImage.heightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.heightAnchor, multiplier: 0.5)
          theImage.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -20),
          theImage.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -view.frame.width/8)
     ])
} else {
     NSLayoutConstraint.activate([
          theImage.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 0.5)
          theImage.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -20),
          theImage.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -view.frame.width/8),
     ])
}

Answer 1

那密码看上去是对的。如果你担心到处都是重复，就有办法巩固它。你可以这样做：

extension UIViewController {

    var correctLayoutGuide: UILayoutGuide {
        if #available(iOS 11.0, *) {
            return view.safeAreaLayoutGuide
        }
        else {
            return view.layoutMarginsGuide
        }
    }

}

那么，您的代码片段可能就是：

NSLayoutConstraint.activate([
      theImage.heightAnchor.constraint(equalTo: correctLayoutGuide.heightAnchor, multiplier: 0.5)
      theImage.bottomAnchor.constraint(equalTo: correctLayoutGuide.bottomAnchor, constant: -20),
      theImage.trailingAnchor.constraint(equalTo: correctLayoutGuide.trailingAnchor, constant: 20)
 ])