leftViewController.swift

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 }

        loadViewIfNeeded() // Make sure the view is loaded

        addChildViewController(child)
        child.view.frame = leftContainerView.bounds
        leftContainerView.contentView = child.view // Layout managed by container view
        child.didMove(toParentViewController: self)
    }
}