MessagesViewController.swift

class CompactViewController: UIViewController {
    // ...
}

class ExpandedViewController: UIViewController {
    // ...
}

class MessagesViewController: MSMessagesAppViewController {
    override func willBecomeActive(with conversation: MSConversation) {
        super.willBecomeActive(with: conversation)

        presentVC(for: conversation, with: presentationStyle)
    }

    override func willTransition(to presentationStyle: MSMessagesAppPresentationStyle) {
        guard let conversation = activeConversation else {
            fatalError("Expected the active conversation")
        }

        presentVC(for: conversation, with: presentationStyle)
    }

    private func presentVC(for conversation: MSConversation, with presentationStyle: MSMessagesAppPresentationStyle) {
        let controller: UIViewController

        if presentationStyle == .compact {
            controller = instantiateCompactVC()
        } else {
            controller = instantiateExpandedVC()
        }

        addChildViewController(controller)
        
        // ...constraints and view setup...
        
        view.addSubview(controller.view)
        controller.didMove(toParentViewController: self)
    }

    private func instantiateCompactVC() -> UIViewController {
        guard let compactVC = storyboard?.instantiateViewController(withIdentifier: "CompactVC") as? CompactViewController else {
            fatalError("Can't instantiate CompactViewController")
        }

        return compactVC
    }

    private func instantiateExpandedVC() -> UIViewController {
        guard let expandedVC = storyboard?.instantiateViewController(withIdentifier: "ExpandedVC") as? ExpandedViewController else {
            fatalError("Can't instantiate ExpandedViewController")
        }

        return expandedVC
    }
}