How, exactly, do I render Metal on a background thread? - swift

Question

This problem is caused by user interface interactions such as showing the titlebar while in fullsreen. That question's answer provides a solution, but not how to implement that solution.
The solution is to render on a background thread. The issue is, the code provided in Apple's is made to cover a lot of content so most of it will extraneous code, so even if I could understand it, it isn't feasible to use Apple's code. And I can't understand it so it just plain isn't an option. How would I make a simple Swift Metal game use a background thread being as concise as possible?
Take this, for example:
class ViewController: NSViewController {
var MetalView: MTKView {
return view as! MTKView
}
var Device: MTLDevice = MTLCreateSystemDefaultDevice()!
override func viewDidLoad() {
super.viewDidLoad()
MetalView.delegate = self
MetalView.device = Device
MetalView.colorPixelFormat = .bgra8Unorm_srgb
Device = MetalView.device
//setup code
}
}
extension ViewController: MTKViewDelegate {
func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) {
}
func draw(in view: MTKView) {
//drawing code
}
}
That is the start of a basic Metal game. What would that code look like, if it were rendering on a background thread?
To fix that bug when showing the titlebar in Metal, I need to render it on a background thread. Well, how do I render it on a background thread?
I've noticed this answer suggests to manually redraw it 60 times a second. Presumably using a loop that is on a background thread? But that seems... not a clean way to fix it. Is there a cleaner way?

Answer 1

The main trick in getting this to work seems to be setting up the CVDisplayLink. This is awkward in Swift, but doable. After some work I was able to modify the "Game" template in Xcode to use a custom view backed by CAMetalLayer instead of MTKView, and a CVDisplayLink to render in the background, as suggested in the sample code you linked — see below.
Edit Oct 22:
The approach mentioned in this thread seems to work just fine: still using an MTKView, but drawing it manually from the display link callback. Specifically I was able to follow these steps:
Create a new macOS Game project in Xcode.
Modify GameViewController to add a CVDisplayLink, similar to below (see this question for more on using CVDisplayLink from Swift). Start the display link in viewWillAppear and stop it in viewWillDisappear.
Set mtkView.isPaused = true in viewDidLoad to disable automatic rendering, and instead explicitly call mtkView.draw() from the display link callback.
The full content of my modified GameViewController.swift is available here.
I didn't review the Renderer class for thread safety, so I can't be sure no more changes are required, but this should get you up and running.
Older implementation with CAMetalLayer instead of MTKView:
This is just a proof of concept and I can't guarantee it's the best way to do everything. You might find these articles helpful too:
I didn't try this idea, but given how much convenience MTKView generally provides over CAMetalLayer, it might be worth giving it a shot:
https://developer.apple.com/forums/thread/89241?answerId=268384022#268384022
Is drawing to an MTKView or CAMetalLayer required to take place on the main thread? and https://developer.apple.com/documentation/quartzcore/cametallayer/1478157-presentswithtransaction
class MyMetalView: NSView {
var displayLink: CVDisplayLink?
var metalLayer: CAMetalLayer!
override init(frame frameRect: NSRect) {
super.init(frame: frameRect)
setupMetalLayer()
}
required init?(coder: NSCoder) {
super.init(coder: coder)
setupMetalLayer()
}
override func makeBackingLayer() -> CALayer {
return CAMetalLayer()
}
func setupMetalLayer() {
wantsLayer = true
metalLayer = layer as! CAMetalLayer?
metalLayer.device = MTLCreateSystemDefaultDevice()!
// ...other configuration of the metalLayer...
}
// 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 view = Unmanaged<MyMetalView>.fromOpaque(context!).takeUnretainedValue()
/*** render something into view.metalLayer here! ***/
return kCVReturnSuccess
}
override func viewDidMoveToWindow() {
super.viewDidMoveToWindow()
guard CVDisplayLinkCreateWithActiveCGDisplays(&displayLink) == kCVReturnSuccess,
let displayLink = displayLink
else {
fatalError("unable to create display link")
}
// pass a reference to this view as an opaque pointer
guard CVDisplayLinkSetOutputCallback(displayLink, MyMetalView._outputCallback, Unmanaged<MyMetalView>.passUnretained(self).toOpaque()) == kCVReturnSuccess else {
fatalError("unable to configure output callback")
}
guard CVDisplayLinkStart(displayLink) == kCVReturnSuccess else {
fatalError("unable to start display link")
}
}
deinit {
if let displayLink = displayLink {
CVDisplayLinkStop(displayLink)
}
}
}