基本的なところはここを見ながら進める。
https://qiita.com/tkyaji/items/7dbd56b41b6ac3e72635

SwiftでUnityクラスを作る。上の記事ならこんなかんじ。

class Unity: NSObject, UnityFrameworkListener, NativeCallsProtocol {
    static let shared = Unity()
    private let unityFramework: UnityFramework
    
    override init() {
        let bundlePath = Bundle.main.bundlePath
        let frameworkPath = bundlePath + "/Frameworks/UnityFramework.framework"
        let bundle = Bundle(path: frameworkPath)!
        if !bundle.isLoaded {
            bundle.load()
        }
        let frameworkClass = bundle.principalClass as! UnityFramework.Type
        let framework = frameworkClass.getInstance()!
        if framework.appController() == nil {
            var header = _mh_execute_header
            framework.setExecuteHeader(&header)
        }
        unityFramework = framework
        super.init()
    }
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) {
        unityFramework.register(self)
        FrameworkLibAPI.registerAPIforNativeCalls(self)
        unityFramework.setDataBundleId("com.unity3d.framework")
        unityFramework.runEmbedded(withArgc: CommandLine.argc, argv: CommandLine.unsafeArgv, appLaunchOpts: launchOptions)
    }
    
    var view: UIView {
        unityFramework.appController()!.rootView!
    }
    
    func showHostMainWindow(_ color: String!) {
        
    }
}

ViewControllerを作る。


class LaunchViewController: UIViewController {}

class ViewController: UIViewController {
    private let unityView = Unity.shared.view
    
    override func loadView() {
        super.loadView()
        view.addSubview(unityView)
        NSLayoutConstraint.activate([
            unityView.topAnchor.constraint(equalTo: view.topAnchor),
            unityView.leftAnchor.constraint(equalTo: view.leftAnchor),
            unityView.rightAnchor.constraint(equalTo: view.rightAnchor),
            unityView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
    }
}

後述するが、LaunchViewControllerなるものを作っておく。

AppDelegate.swift

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    lazy var window: UIWindow? = .init(frame: UIScreen.main.bounds)
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        window?.rootViewController = LaunchViewController()
        window?.makeKeyAndVisible()
        Unity.shared.application(application, didFinishLaunchingWithOptions: launchOptions)
        window?.rootViewController = ViewController()
        
        return true
    }
}

起動時にUnityクラスを初期化しておく。
runEmbeddedはwindowが無いと上手く動かないので、先ほど作ったLaunchViewControllerでwindowを作り、そのあとでメインのViewControllerに差し替える。
起動直後にUnityを出さないのであれば、適当にwindowが出来てから呼べばOK