GameViewController.swift

import Cocoa
import MetalKit

// Our macOS specific view controller
class GameViewController: NSViewController {

    var renderer: Renderer!
    var mtkView: MTKView!

    var displayLink: CVDisplayLink!

    override func viewDidLoad() {
        super.viewDidLoad()

        guard let mtkView = self.view as? MTKView else {
            print("View attached to GameViewController is not an MTKView")
            return
        }
        self.mtkView = mtkView

        // Select the device to render with.  We choose the default device
        guard let defaultDevice = MTLCreateSystemDefaultDevice() else {
            print("Metal is not supported on this device")
            return
        }

        mtkView.device = defaultDevice

        guard let newRenderer = Renderer(metalKitView: mtkView) else {
            print("Renderer cannot be initialized")
            return
        }

        guard CVDisplayLinkCreateWithActiveCGDisplays(&displayLink) == kCVReturnSuccess,
              let displayLink = displayLink
        else {
            print("unable to create display link")
            return
        }

        guard CVDisplayLinkSetOutputCallback(displayLink, GameViewController._outputCallback, Unmanaged.passUnretained(self).toOpaque()) == kCVReturnSuccess else {
            fatalError("unable to configure output callback")
        }

        renderer = newRenderer

        renderer.mtkView(mtkView, drawableSizeWillChange: mtkView.drawableSize)

        mtkView.delegate = renderer
        mtkView.isPaused = true
    }

    // handle display link callback at 60fps
    static let _outputCallback: CVDisplayLinkOutputCallback = { (displayLink, inNow, inOutputTime, flagsIn, flagsOut, context) -> CVReturn in
      // convert opaque context pointer back into a reference to our view
      let vc = Unmanaged<GameViewController>.fromOpaque(context!).takeUnretainedValue()

        vc.mtkView.draw()

      return kCVReturnSuccess
    }

    override func viewWillAppear() {
        guard CVDisplayLinkStart(displayLink) == kCVReturnSuccess else {
            fatalError("unable to start display link")
        }
    }

    override func viewWillDisappear() {
        if let displayLink = displayLink {
          CVDisplayLinkStop(displayLink)
        }
    }
}