ios - 自动布局宽高比限制

Question

我想设置一个 subview 约束，如下所示。如果界面是横向的(view.bounds.width > view.bounds.height)，设置 subview 的宽高比为4:3。在纵向模式下，它应该是 3:4。虽然我总是可以通过编程方式更改自动旋转的约束，但想知道是否有一种巧妙的方法来设计仅一次的约束。

Answer 1

您必须监听 UIDevice.orientationDidChangeNotification 通知，如下例所示:

class ViewController3: UIViewController {

    let blueSubview: UIView = {
        let view = UIView()
        view.backgroundColor = .blue
        view.translatesAutoresizingMaskIntoConstraints = false
        return view
    }()

    var blueSubviewHeight = NSLayoutConstraint()
    var blueSubviewWidth = NSLayoutConstraint()

    override func viewDidLoad() {
        super.viewDidLoad()
        NotificationCenter.default.addObserver(self, selector: #selector(refreshConstraints), name: UIDevice.orientationDidChangeNotification, object: nil)

        refreshConstraints()
        setUI()
    }

    fileprivate func setUI() {
        view.backgroundColor = .white

        view.addSubview(blueSubview)
        [blueSubview.leftAnchor.constraint(equalTo: view.leftAnchor),
        blueSubview.topAnchor.constraint(equalTo: view.topAnchor),
        blueSubviewWidth,
        blueSubviewHeight].forEach({ $0.isActive = true })
    }

    @objc func refreshConstraints() {
        NSLayoutConstraint.deactivate([blueSubviewWidth, blueSubviewHeight])

        if view.frame.width > view.frame.height {
            blueSubviewWidth = blueSubview.widthAnchor.constraint(equalToConstant: view.frame.height * (4/3))
            blueSubviewHeight = blueSubview.heightAnchor.constraint(equalToConstant: view.frame.height)
        }
        else {
            blueSubviewWidth = blueSubview.widthAnchor.constraint(equalToConstant: view.frame.width)
            blueSubviewHeight = blueSubview.heightAnchor.constraint(equalToConstant: view.frame.width * (4/3))
        }

        NSLayoutConstraint.activate([blueSubviewWidth, blueSubviewHeight])
    }
}