自动布局纵横比约束

Question

I want to set a subview constraint as follows. If the interface is landscape (view.bounds.width > view.bounds.height),set aspect ratio of subview to be 4:3. In portrait mode, it should be 3:4. While I can always change the constraints programmatically on auto rotation, wondering if there is a clever way of designing constraints that is only one time.

Answer 1

You'll have to listen for the UIDevice.orientationDidChangeNotification notification like in this example:

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])
    }
}