使用特征集合更改更新自动布局

使应用程序自适应 - 即通过更改布局来响应大小类更改 - 通常需要来自 Auto Layout 系统的大量帮助。应用程序变为自适应的主要方式之一是在视图的大小类更改时更新活动的自动布局约束。

例如，考虑一个使用 UIStackView 来排列两个 UILabel 的应用程序。我们可能希望这些标签在水平紧凑的环境中叠加在一起，但是当我们在水平常规环境中有更多空间时，它们彼此相邻。

class ViewController: UIViewController {
    var stackView: UIStackView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        stackView = UIStackView()
        for text in ["foo", "bar"] {
            let label = UILabel()
            label.translatesAutoresizingMaskIntoConstraints = false
            label.text = text
            stackView.addArrangedSubview(label)
        }
        
        view.addSubview(stackView)
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        stackView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        updateAxis(forTraitCollection: traitCollection)
    }
    
    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        updateAxis(forTraitCollection: traitCollection)
    }
    
    private func updateAxis(forTraitCollection traitCollection: UITraitCollection) {
        switch traitCollection.horizontalSizeClass {
        case .regular:
            stackView.axis = .horizontal
        case .compact:
            stackView.axis = .vertical
        case .unspecified:
            print("Unspecified size class!")
            stackView.axis = .horizontal
        }
    }
}