Thank you for your response. Following your suggestion, I started with the Game template with Metal. After making the following changes to trigger draw calls on touch interactions, indicated by the git diff below, I had the same failure case as above. The first draw call functions correctly. The second call gives [CAMetalLayerDrawable texture] should not be called after already presenting this drawable. Get a nextDrawable instead. and Each CAMetalLayerDrawable can only be presented once!. So the logic to correctly iterate to the next drawable from the MTKView is missing somewhere. @@ -17,10 +17,11 @@ class GameViewController: UIViewController { override func viewDidLoad() { super.viewDidLoad() - guard let mtkView = view as? MTKView else { + guard let newMtkView = view as? MTKView else { print("View of Gameview controller is not an MTKView") return } + mtkView = newMtkView // allow us to refer to view stored in the implicitly unwrapped optional // Select the device to render with. We choose the default device guard let defaultDevice = MTLCreateSystemDefaultDevice() else { @@ -31,6 +32,9 @@ class GameViewController: UIViewController { mtkView.device = defaultDevice mtkView.backgroundColor = UIColor.black + mtkView.isPaused = true + mtkView.enableSetNeedsDisplay = false + guard let newRenderer = Renderer(metalKitView: mtkView) else { print("Renderer cannot be initialized") return @@ -42,4 +46,10 @@ class GameViewController: UIViewController { mtkView.delegate = renderer } + + override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) { + autoreleasepool { + renderer.draw(in: mtkView) + } + } }

I did hack together a minimal Metal app using SwiftUI with the MTKView wrapped in a UIRepresentable. Tapping the button changes the colour of the MTKView. I am quite new to iOS so beware of my likely less than optimal example. The problem is solved by grabbing the CAMetalLayer through the current drawable and using it to call nextDrawable(). Now the MTLRenderPassDescriptor must be created manually using the new drawable. This solution works but feels very uncomfortable so I can't believe it is the intended approach - grabbing so indirectly at the lower levels from MTKView (I also have yet to handle the depth and stencil render pass attachments). I would be very grateful if anyone can point me to a better solution of how to handle manual draw calls to an MTKView Best regards, App.swift: import SwiftUI import MetalKit @main struct ManualRenderingTestApp: App { var body: some Scene { WindowGroup { ContentView() } } } struct ContentView: View { let view = ColorView() var body: some View { VStack { view Button("Change color") { view.changeColor() } } } } class ColorViewCoordinator : NSObject, MTKViewDelegate { let view: MTKView var commandQueue: MTLCommandQueue! var pipelineState: MTLRenderPipelineState! let inFlightSemaphore = DispatchSemaphore(value: 3) let vertexData: [Float] = [-1.0, -1.0, 1.0, -1.0, -1.0, 1.0, 1.0, 1.0] var colors = [simd_float4(1.0, 0.0, 0.0, 1.0), simd_float4(0.0, 1.0, 0.0, 1.0), simd_float4(0.0, 0.0, 1.0, 1.0), simd_float4(1.0, 1.0, 0.0, 1.0)] var colorIndex = 0 init(mtkView: MTKView) { self.view = mtkView let metalDevice = MTLCreateSystemDefaultDevice()! view.device = metalDevice super.init() self.commandQueue = metalDevice.makeCommandQueue()! let library = metalDevice.makeDefaultLibrary()! let vertexDescriptor = MTLVertexDescriptor() vertexDescriptor.attributes[0].format = .float2 vertexDescriptor.attributes[0].offset = 0 vertexDescriptor.attributes[0].bufferIndex = 0 vertexDescriptor.layouts[0].stride = MemoryLayout<SIMD2<Float>>.stride vertexDescriptor.layouts[0].stepRate = 1 vertexDescriptor.layouts[0].stepFunction = .perVertex let pipelineDescriptor = MTLRenderPipelineDescriptor() pipelineDescriptor.colorAttachments[0].pixelFormat = .bgra8Unorm pipelineDescriptor.vertexFunction = library.makeFunction(name: "planeVertexShader") pipelineDescriptor.fragmentFunction = library.makeFunction(name: "constantColorFragmentShader") pipelineDescriptor.vertexDescriptor = vertexDescriptor do { pipelineState = try metalDevice.makeRenderPipelineState(descriptor: pipelineDescriptor) } catch { fatalError("Failed to create visualization render pipeline.") } } func incrementColor() { colorIndex = (colorIndex + 1) % colors.count } func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) {} func getCALayer(view: MTKView) -> CAMetalLayer? { if let drawable = view.currentDrawable { return drawable.layer }; return nil } func createRenderPassDescriptor(view: MTKView, drawable: CAMetalDrawable) -> MTLRenderPassDescriptor { let renderPassDescriptor = MTLRenderPassDescriptor() renderPassDescriptor.colorAttachments[0].texture = drawable.texture renderPassDescriptor.colorAttachments[0].loadAction = .clear renderPassDescriptor.colorAttachments[0].storeAction = .store // depth and stencil attachments have to be set manually here return renderPassDescriptor } func draw(in view: MTKView) { _ = inFlightSemaphore.wait(timeout: DispatchTime.distantFuture) guard let commandBuffer = commandQueue.makeCommandBuffer() else { return } let semaphore = inFlightSemaphore commandBuffer.addCompletedHandler { (_ commandBuffer)-> Swift.Void in semaphore.signal() } guard let layer = getCALayer(view: view) else { return } guard let drawable = layer.nextDrawable() else { return } let renderPassDescriptor = createRenderPassDescriptor(view: view, drawable: drawable) guard let renderEncoder = commandBuffer.makeRenderCommandEncoder(descriptor: renderPassDescriptor) else { return } renderEncoder.setVertexBytes(vertexData, length: vertexData.count * MemoryLayout<Float>.stride, index: 0) renderEncoder.setFragmentBytes(&colors[colorIndex], length: MemoryLayout<SIMD4<Float>>.stride, index: 0) // set color from user input renderEncoder.setRenderPipelineState(pipelineState) renderEncoder.drawPrimitives(type: .triangleStrip, vertexStart: 0, vertexCount: 4) renderEncoder.endEncoding() commandBuffer.present(drawable) commandBuffer.commit() } } struct ColorView: UIViewRepresentable { var mtkView: MTKView = MTKView() func makeCoordinator() -> ColorViewCoordinator { return ColorViewCoordinator(mtkView: mtkView) } func makeUIView(context: UIViewRepresentableContext<ColorView>) -> MTKView { mtkView.delegate = context.coordinator mtkView.isPaused = true // only push data mtkView.enableSetNeedsDisplay = false // only push data mtkView.framebufferOnly = true // we don't render to anything but the screen mtkView.colorPixelFormat = .bgra8Unorm mtkView.drawableSize = mtkView.frame.size return mtkView } func updateUIView(_ uiView: MTKView, context: UIViewRepresentableContext<ColorView>) {} func changeColor() { let coordinator = mtkView.delegate as? ColorViewCoordinator coordinator!.incrementColor() autoreleasepool { mtkView.delegate!.draw(in: mtkView) } } } Shaders.Metal: #include <metal_stdlib> using namespace metal; typedef struct { float2 position [[ attribute(0) ]]; } VertexIn; typedef struct { float4 position [[position]]; } FragIn; vertex FragIn planeVertexShader(VertexIn in [[stage_in]]) { FragIn out; out.position = float4(in.position, 0.0, 1.0); return out; } fragment float4 constantColorFragmentShader( FragIn in [[stage_in]], constant float4& color ) { return color; }