ContainerView.swift

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

        willSet {
            contentView?.removeFromSuperview()
        }

        didSet {
            guard let contentView = contentView else { return }
            addSubview(contentView)
        }
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        contentView?.frame = bounds
    }
}

private var leftContainerView: ContainerView!
private var rightContainerView: ContainerView!