SplitOrFlipViewController_viewWillLayoutSubviews.swift

override func viewWillLayoutSubviews() {
    super.viewWillLayoutSubviews()

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

    switch displayMode {
        case .sideBySide:
            // Add subviews. Order is not important
            view.addSubview(leftContainerView)
            view.addSubview(rightContainerView)

            // Update left column width
            leftColumnWidth = suggestLeftColumnWidth(for: size)
            
            // Position container views
            leftContainerView.frame = leftColumnFrame(for: size)
            rightContainerView.frame = rightColumnFrame(for: size)

        case .one:
            // Add/remove subviews based on display focus
            switch displayFocus {
                case .left:
                    view.addSubview(leftContainerView)
                    leftContainerView.frame = fullFrame(for: size)
                    rightContainerView.removeFromSuperview()
                case .right:
                    view.addSubview(rightContainerView)
                    rightContainerView.frame = fullFrame(for: size)
                    leftContainerView.removeFromSuperview()
            }
    }
}