dmytro-anokhin

enum DisplayMode {
    // Both view controllers are displayed at the same time, side-by-side
    case sideBySide
    // One view controller at a time is displayed
    case one
}

private(set) var displayMode: DisplayMode = .sideBySide

dmytro-anokhin

/// Suggests display mode based on trait collection
private func suggestDisplayMode(for traitCollection: UITraitCollection) -> DisplayMode {
    return traitCollection.horizontalSizeClass == .regular ? .sideBySide : .one
}

dmytro-anokhin

/// Defines visible view controller when one at a time displayed
private enum DisplayFocus {
    /// When .displayMode set to .one, left view is visible
    case left
    /// When .displayMode set to .one, right view is visible
    case right
}

private var displayFocus: DisplayFocus = .left

dmytro-anokhin

/// Width of the left view in side-by-side display mode
private var leftColumnWidth: CGFloat!

/// Suggests width of the left column in side-by-side display mode
private func suggestLeftColumnWidth(for availableSize: CGSize) -> CGFloat {
    return availableSize.width > availableSize.height ? 440.0 : 320.0
}

dmytro-anokhin

private class ContainerView: UIView {
    var contentView: UIView? {

        willSet {
            contentView?.removeFromSuperview()
        }

        didSet {
            guard let contentView = contentView else { return }
            addSubview(contentView)

dmytro-anokhin

private func fullFrame(for availableSize: CGSize) -> CGRect
private func fullLeftFrame(for availableSize: CGSize) -> CGRect
private func fullRightFrame(for availableSize: CGSize) -> CGRect
private func leftColumnFrame(for availableSize: CGSize) -> CGRect
private func rightColumnFrame(for availableSize: CGSize) -> CGRect

dmytro-anokhin

private func leftColumnFrame(for availableSize: CGSize) -> CGRect {
    switch traitCollection.layoutDirection {
        case .unspecified, .leftToRight:
            return CGRect(x: 0.0, y: 0.0, width: leftColumnWidth, height: size.height)
        case .rightToLeft:
            return CGRect(x: size.width — leftColumnWidth, y: 0.0, width: leftColumnWidth, height: size.height)
    }
}

dmytro-anokhin

var leftViewController: UIViewController? {
    willSet {
        guard let child = leftViewController else { return }
        child.willMove(toParentViewController: nil)
        child.view.removeFromSuperview()
        child.removeFromParentViewController()
    }

    didSet {
        guard let child = leftViewController else { return }

dmytro-anokhin

override func viewWillLayoutSubviews() {
    super.viewWillLayoutSubviews()

    // Update display state
    displayMode = suggestDisplayMode(for: traitCollection)
    
    let size = view.bounds.size

    switch displayMode {
        case .sideBySide:

dmytro-anokhin

override func willTransition(to newCollection: UITraitCollection, with coordinator: UIViewControllerTransitionCoordinator) {
    super.willTransition(to: newCollection, with: coordinator)
    
    coordinator.animate(alongsideTransition: nil) { _ in
        switch displayMode {
            case .sideBySide:
                navigationItem.rightBarButtonItem = nil
                break
            case .one:
                navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Flip", style: .plain, target: self, action: #selector(flipAction))