Can't access MTKView

Question

I'm new to UIKit and I'm trying to write a Metal app to run on an iPhone 12. Here's the code in AppDelegate.swift:

class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        window = UIWindow(frame: UIScreen.main.bounds)
        window?.rootViewController = ViewController()
        window?.makeKeyAndVisible()
        return true
    }
}

Here's the code in ViewController.swift:

class ViewController: UIViewController, MTKViewDelegate {
    override func viewDidLoad() {
        super.viewDidLoad()

        // Access the Metal view
        if let view = self.view! as? MTKView {  // Isn't working
            view.device = MTLCreateSystemDefaultDevice()
            view.delegate = self
            print("Obtained MTKView")
        } else {
            print("Failed to obtain MTKView")   // The result
        }
    }
    func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) {}
    func draw(in view: MTKView) {}
}

This code compiles and runs, but it doesn't access the MTKView. It prints the Failed to obtain MTKView message. Any ideas? Thanks.

EDIT: I've tried self.view! and self.view, but neither work.

Answer 1

If you aren't using the storyboard, you can create a new MTKView like this:

class ViewController: UIViewController, MTKViewDelegate {

    var metalView: MTKView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        /// Make a MetalKit view
        let metalView = MTKView()
            
        /// add positioning constraints
        metalView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            metalView.topAnchor.constraint(equalTo: view.topAnchor),
            metalView.rightAnchor.constraint(equalTo: view.rightAnchor),
            metalView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
            metalView.leftAnchor.constraint(equalTo: view.leftAnchor)
        ])
        
        metalView.device = MTLCreateSystemDefaultDevice()
        metalView.delegate = self
        
        self.metalView = metalView /// assign to the property, so you can access this later
    }

    func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) {}
    func draw(in view: MTKView) {}
}