According to the official Metal Feature Set Tables, the Apple9 family introduces full 64-bit atomics such as atomic_compare_exchange_weak_explicit() and others. However when trying to use it the compiler throws an error "no matching function call...". Example: kernel void test_64bit_cas(device atomic_ulong* atomic_var [[buffer(0)]]) { ulong expected = 0; ulong desired = 1; // This fails to compile despite Apple9 hardware supporting 64-bit buffer atomics atomic_compare_exchange_weak_explicit(atomic_var, &expected, desired, memory_order_relaxed, memory_order_relaxed); } Am i doing something wrong ?

I have noticed that the performance drop on SpriteKit-based projects running on iOS 26 is still ongoing With iOS 26 back in Sep 2025 a framerate problem was introduced. My app was always running smoothly with 60fps even on very old devices suddenly started to stutter with 40fps - and lower on a rather normal iPhone 13. This problem continued with BETA 26.1 The problem was fixed in 26.2. But 26.3 brought the problem back and its still ongoing with 26.4 of yesterday This is easily reproducible with a very simple example // // BareboneSpriteKitApp.swift // BareboneSpriteKit // // Created by Bernd Beyreuther on 24.02.26. // import SwiftUI import SpriteKit @main struct BareboneSpriteKitApp: App { var body: some Scene { WindowGroup { BareboneSceneView() } } } final class BareboneScene: SKScene { override func didMove(to view: SKView) { size = view.bounds.size scaleMode = .resizeFill anchorPoint = CGPoint(x: 0.5, y: 0.5) backgroundColor = .darkGray let s = SKSpriteNode(color: .cyan, size: CGSize(width: 64, height: 64)) addChild(s) let action = SKAction.rotate(byAngle: .pi, duration: 2) s.run(.repeatForever(action)) let t = SKLabelNode(text: deviceInfoString()) t.fontSize = 15 t.position.y = -100 addChild(t) } } struct BareboneSceneView: View { var body: some View { SpriteView( scene: BareboneScene(), debugOptions: [.showsFPS] ) .ignoresSafeArea() } } func deviceInfoString() -> String { let os = ProcessInfo.processInfo.operatingSystemVersion let osString = "iOS \(os.majorVersion).\(os.minorVersion).\(os.patchVersion)" let model = UIDevice.current.model // "iPhone", "iPad" let machine = { var sysinfo = utsname() uname(&sysinfo) return withUnsafePointer(to: &sysinfo.machine) { ptr -> String in ptr.withMemoryRebound(to: CChar.self, capacity: 1) { cptr in String(cString: cptr) } } }() // z.B. "iPhone15,2" return "Model Identifier: \(model) (\(machine)), \(osString)" } I file a bugreport via Feedback Assistant FB22038921 The problem is no around for such a long time ! This is deeply concerning, because it questions if it is really feasable to continue to develop using Spritekit ?

Through log analysis, I found that the SQLite files generated after signing in to Game Center are located in: ~/Library/Group Containers/group.com.apple.gamecenter/Library/Caches com.apple.deleted automatically cleans up cache files. This causes the following problem: after I have been signed in for some time, when I check Game Center again, I find that the profile name has changed to Unknown. When I launch Apple Arcade games, I am always asked to sign in to Game Center, and “Show Profile” also shows the same symptoms as described in this post: https://discussions.apple.com/thread/256053912 I believe they are caused by the same issue. I have to log out and then log back in to Game Center to temporarily resolve the problem, but once the cache is cleaned again, I have to repeat the same process. I also think that having less remaining free storage space may make system cache cleanup more likely to be triggered, although this is only my guess. https://www.reddit.com/r/macgaming/comments/1l0f8i4/the_avatar_and_name_in_game_center_are_displayed My system is macOS 15.7.9. I have already submitted a complete Feedback Report. FB ID: FB24975495

I maintain mps-pointops, an open-source library implementing farthest-point sampling (FPS), k-nearest-neighbor search, and radius / Ball Query operations with custom Metal kernels for PyTorch MPS tensors. The intended use is 3D point-cloud pipelines whose existing extensions depend on CUDA. I would like to prepare a contribution proposal at the appropriate layer. Apple’s PyTorch/MPS page links to the PyTorch MPS backend contribution guide and directs backend issues to PyTorch’s GitHub tracker with the module: mps label. The current implementation has numerical-contract tests for selection order, ties, radius boundaries, and padding, plus benchmark records from physical M1 and M5 Pro Macs. The repository contains the kernels, Python interfaces, tests, and measurement conditions: https://github.com/gamzerA/mps-pointops My questions are: For specialized operators such as FPS and radius search, is the recommended contribution structure to keep the operator/API in a domain library (for example, a point-cloud or graph library), while proposing only reusable backend primitives to PyTorch MPS? Is there a public Apple-maintained contribution or technical-review channel relevant to this work, beyond the PyTorch route linked from Apple’s page? For an external library that dispatches custom Metal work alongside PyTorch MPS operations, which documented extension/interoperability path should be the long-term target for command-stream ordering and buffer lifetime? I would like to minimize reliance on backend internals when preparing an upstream proposal. Pointers to the appropriate public interface, maintainers, or contribution requirements would help me scope a small, reviewable proposal. I understand that inclusion in an upstream project is subject to its maintainers’ review. Apple documentation referenced: https://developer.apple.com/metal/pytorch/

As part of the WWDC25 Keynote, a technology was announced that can present 2D images as 3D spatial scenes. This announcement is supported by a Press Release. ...developers can use the Spatial Scene API to make their app experience even more immersive. Zillow is taking advantage of the API for their Zillow Immersive app, allowing users to see images of homes and apartments with the rich depth and dimension that spatial scenes offer. The feature also appears in the Photos App on iOS 26 Developer Beta 1. Tapping "Spatial Scene" on any photo opens a view of that photo with a parallax effect. I've searched the WWDC sessions and new documentation and have come up short. Reaching out here for help. Is there any documentation for Spatial Scene API? Or any guidance on how to implement the spatial scene in iOS?

Hi, I understand Scenekit is deprecated in ios27, but expectation is that it should continue working in ios27 and not anymore maintained. With ios27 , when we attempt to set self.delegate in an ARSCNViewDelegate we receive a crash. Is there any workaround to have that fixed until we have time to move away from Scenekit? Can this be part of a fix in ios27 considering we should have sometime to migrate out production apps or that won't go through? Thank you.

I filed Feedback Assistant report FB24967734 for this issue. When two USD scenes contain explicit polygon meshes at the same prim path and are compiled together with xcrun realitytool, one scene's mesh loads with zero bounds. Renaming the mesh prims so their paths are unique makes both meshes load correctly. Reproduce Download and expand the source ZIP from https://github.com/mesqueeb/RealitykitUsdMeshDisappearanceOnMatchingPaths/archive/refs/heads/main.zip, then run these commands from the extracted directory: xcrun realitytool compile --output-reality Fixtures/Compiled/inline-same-path.bundle/Contents/Resources/RealityKitContent.reality --platform macosx --deployment-target 27.0 Fixtures/Source/inline-same-path.rkassets xcrun realitytool compile --output-reality Fixtures/Compiled/inline-unique-path.bundle/Contents/Resources/RealityKitContent.reality --platform macosx --deployment-target 27.0 Fixtures/Source/inline-unique-path.rkassets swift run The same-path case has a mesh at /Root/MeshCube in both scenes. The control changes only the mesh paths to /Root/MeshCube_A and /Root/MeshCube_B. Current output inline-same-path: SceneA.usda / MeshCube: bounds=SIMD3<Float>(0.19999999, 0.2, 0.2) SceneB.usda / MeshCube: bounds=SIMD3<Float>(0.0, 0.0, 0.0) inline-unique-path: SceneA.usda / MeshCube_A: bounds=SIMD3<Float>(0.19999999, 0.2, 0.2) SceneB.usda / MeshCube_B: bounds=SIMD3<Float>(0.19999999, 0.2, 0.2) Expected output Both meshes in the matching-path case should have nonzero bounds: inline-same-path: SceneA.usda / MeshCube: bounds=SIMD3<Float>(0.19999999, 0.2, 0.2) SceneB.usda / MeshCube: bounds=SIMD3<Float>(0.19999999, 0.2, 0.2) inline-unique-path: SceneA.usda / MeshCube_A: bounds=SIMD3<Float>(0.19999999, 0.2, 0.2) SceneB.usda / MeshCube_B: bounds=SIMD3<Float>(0.19999999, 0.2, 0.2) Reproduced on macOS 27.0 (26A428), Xcode 27.0 (27A266a). The repo includes the USDA inputs, compiled RealityKitContent assets, and the Swift code that prints these bounds. Source and downloadable ZIP: https://github.com/mesqueeb/RealitykitUsdMeshDisappearanceOnMatchingPaths

I’m investigating GameKit Saved Games synchronization between an iPhone and an iOS app running on an Apple-silicon Mac through TestFlight—not a native macOS or Mac Catalyst app. Environment: • iPhone 11, iOS 26.6.2 • Apple-silicon Mac, macOS 27.0 (26A428) • Built with Xcode 27 (27A266a), minimum iOS 16 • Same iCloud and Game Center accounts, confirmed manually • iCloud Drive enabled, including the app’s per-app syncing setting on Mac Expected: The Mac can discover and load the existing iPhone save. Observed: The iPhone’s fetchSavedGames returns two entries, one matching our expected save name, and loadData successfully reads that matching save. On the Mac, fetchSavedGames completes without an error but returns an empty array. This is the raw result before our filename filtering. Game Center authentication succeeds. Our app-level identity checks also resolve to the same game/environment/profile scope on both devices. We understand this does not independently prove the iCloud accounts match. The installed Mac app’s signed entitlements include: • Game Center • iCloud environment Production • CloudDocuments • The expected matching iCloud and ubiquity container identifiers The unusual system observation is that brctl status for this container reports: client:blocked-app-uninstalled SYNC DISABLED (app not installed) This persists while the app is installed and running. The container reports foreground when the app is open. Completed checks: Verified installed build and signed entitlements. Confirmed LaunchServices registration of the app wrapper and inner iOS bundle. Targeted re-registration, normal Mac restart, and reopening did not clear the state. In a separately triggered diagnostic inside the signed app, called FileManager.url(forUbiquityContainerIdentifier:) off the main thread with the exact entitled container identifier. That call returned a nonnil URL, but the immediate GameKit fetch and a subsequent delayed check still returned zero saves. The system block remained unchanged. The diagnostic did not write a save, restore progress, or directly read/write container files. We have not deleted app data, reset iCloud, or rewritten the source save to force synchronization. I found the authentication-timing discussion at: https://developer.apple.com/forums/thread/718541 However, our result persists across subsequent fetches and restarting the Mac, rather than only the first fetch near authentication. Questions: • Is there an additional supported configuration or initialization requirement for GameKit Saved Games in an iOS TestFlight app running on Apple silicon? • What targeted diagnostics would help explain why the container remains marked app-uninstalled despite the app being installed and running? • How should an app distinguish a genuinely empty Saved Games result from synchronization that is not yet available? We have not established that this is an Apple defect. We currently have an instrumented application, but not a separate minimal project reproducing the issue.

FB24091347 On macOS 27.0 beta (26A5388g), MacBook Pro M4 Pro, the built-in ProMotion display never settles on a stable refresh cadence. Scrolling in SwiftUI judders constantly. The same app binary was smooth on macOS 26, and is smooth on a 120 Hz ProMotion iPad. I captured two 60-second Instruments traces — same app, same scene, same scrolling, no external display — changing only the display's refresh-rate setting. On ProMotion the vsync interval standard deviation is 4.093 ms across six different cadences, mostly flip-flopping between 120 Hz and 60 Hz. Forced to a fixed 60 Hz it drops to 0.391 ms with a single cadence. The app presented an identical 59 fps median in both runs — frame production is perfectly steady, the display just holds each frame for an unpredictable length of time. That's what makes this nasty: it's invisible to every frame-rate metric, so it looks like the app got slow when nothing about the app changed. I spent most of a day profiling my own code before realising the app was never the problem. Workaround: force the built-in display to 60 Hz. Worth noting, because it complicates the picture: attaching a 60 Hz Studio Display makes the built-in smooth, but the Studio itself then judders — despite its own vsync cadence measuring perfectly stable. So refresh rate alone isn't the whole story, and there may be a second mechanism. The clean, reproducible, single-variable result is the ProMotion vs forced-60 Hz comparison on the built-in panel. If you can reproduce this on an M-series MacBook Pro on 27 beta, please file a duplicate referencing FB24091347.

In duo because of this design, most games like Call of Duty or Minecraft didn’t support this

Hi, in RealityKit, AudioPlaybackController exposes duration, gain, speed, play/pause/stop, and a completion handler, but I can't find a way to read the current playhead position while a resource is playing. I need this to trigger animations and other timed events in sync with the audio. Is there a supported way to read the current playback position on AudioPlaybackController? If not, is this planned any time soon? Also, is there a sample-accurate way to start/keep audio playback in sync across multiple entities in RealityKit? Appreciate any guidance, thanks.

I’m trying to use MTL4FXTemporalDenoisedScaler, and I’m seeing a crash during initialization even with a very simple sample app. I created a minimal sample here: https://github.com/tatsuya-ogawa/MetalFXInitExample The exception is: NSException: "-[AGXG16XFamilyHeap baseObject]: unrecognized selector sent to instance ..." What I found is: • This works: descriptor.makeTemporalDenoisedScaler(device: device) • This crashes: descriptor.makeTemporalDenoisedScaler(device: device, compiler: metal4Compiler) So the issue seems to happen only with the Metal4FX version. For testing, I’m using an iPhone 15 Pro. According to the Metal Feature Set Tables, MetalFX denoised upscaling should be supported on Apple9 and later, so I believe the device itself should meet the requirements. Reference: https://developer.apple.com/metal/Metal-Feature-Set-Tables.pdf Has anyone seen this before, or knows what might be causing it? I’d appreciate any advice. Thanks.

Metal already provides a strong precise floating-point contract. With fast math disabled (mathMode = .safe and mathFloatingPointFunctions = .precise, or -fno-fast-math), the Metal Shading Language specification requires correctly rounded FP32 add, subtract, multiply, reciprocal, divide, sqrt, rsqrt, and fma. I am looking for clarification and, if necessary, API support for the two remaining pieces needed for portable bit-exact numerical shaders: Arithmetic rounding mode MSL §8.2 says either round-to-nearest-ties-to-even or round-toward-zero may be supported for floating-point operations. I cannot find a way to select or query the arithmetic rounding mode. The newer MTLCompileOptions.floatingPointConversionRoundingMode appears to apply only to narrowing float-to-float conversions, not arithmetic operations. Do all currently supported Apple GPU families use round-to-nearest-ties-to-even for precise FP32 add/subtract/multiply/divide/sqrt? If so, could that be made a documented guarantee? Otherwise, could Metal expose an arithmetic rounding-mode compile option and a corresponding MTLDevice capability query? Denormal behavior MSL §8.1 and §8.5 permit denormalized FP32 operands and results to be flushed to zero, including with fast math disabled. I cannot find a control or capability query for preserving denormal inputs and results. Do current Apple GPU families support denormal-preserving FP32 arithmetic? Could Metal expose a preserve/flush mode and a MTLDevice query? A convenient end state would be a queryable strict FP32 configuration combining: safe math; precise FP32 functions; contraction disabled when separate rounding points are required; round-to-nearest-ties-to-even arithmetic; preserved FP32 denormal inputs and results; defined signed-zero, infinity, and NaN behavior. My use case is deterministic GPU numerical simulation. A small compute-shader probe can identify effective rounding and denormal behavior on one GPU/OS/compiler combination, but it cannot provide the portable or future-proof contract needed by applications and higher-level APIs such as WebGPU. Related cross-API work: SPIR-V/Vulkan: https://github.com/KhronosGroup/SPIRV-Registry/issues/448 HLSL/DXIL/D3D12: https://github.com/microsoft/hlsl-specs/issues/926 WebGPU/WGSL umbrella issue: https://github.com/gpuweb/gpuweb/issues/2259 Relevant Metal documentation: Metal Shading Language Specification, §§1.6.3 and 8.1–8.5: https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf MTLCompileOptions.mathMode: https://developer.apple.com/documentation/metal/mtlcompileoptions/mathmode MTLCompileOptions.floatingPointConversionRoundingMode: https://developer.apple.com/documentation/metal/mtlcompileoptions/floatingpointconversionroundingmode

Hi all, This has been bothering me for quite a while. Basically my new iMac (bought for a few months only) started to crash randomly. I went to the genius bar and they couldn't do anything to identify the issue, I tried reinstalling the OS and even reinstalling an older version of Mac OS as well, but still seeing this issue. Today it happened twice and below are the details: Mac specs: Model Name: iMac  Model Identifier: iMac20,1  Processor Name: 10-Core Intel Core i9  Processor Speed: 3.6 GHz  Number of Processors: 1  Total Number of Cores: 10  L2 Cache (per Core): 256 KB  L3 Cache: 20 MB  Hyper-Threading Technology: Enabled  Memory: 16 GB  Boot ROM Version: 1554.100.64.0.0 (iBridge: 18.16.14556.0.0,0)  Serial Number (system): xxx  Hardware UUID: xxx  Activation Lock Status: Enabled The DiagnosticReports around the time it crashed has a lot of files with .gpuRestart, e.g.: Kernel_2021-04-27-213412_Zhuzengs-iMac.gpuRestart and file WindowServer_2021-04-27-213319_Zhuzengs-iMac.userspace_watchdog_timeout.spin in between. The details of the the first gpuRestart file Tue Apr 27 21:32:13 2021 Event: GPU Reset Date/Time: Tue Apr 27 21:32:13 2021 Application: Path: Tailspin: /Library/Logs/DiagnosticReports/gpuRestart2021-04-27-213213.tailspin GPUSubmission Trace ID: 0 OS Version: Mac OS X Version 10.15.7 (Build 19H1030) Graphics Hardware: AMD Radeon Pro 5300 Signature: 2 Report Data: GPU Log Version: 1 Restart Channel: 18 VMPT --THE STATE OF THE DRIVER AMDRadeonX6000_AMDNavi14GraphicsAccelerator state: ENABLED PCIe Device: [3:0:0], DID=0x7340, RID=0x47, SSID=0x219 TotalVideoRAMBytes: 0x00000000ff000000 (4278190080) Uptime 21:50:05.077572 [00] Channel: GFX, last reset at 0:00:00.000000 CompletedTS = 0x005be078, SubmittedTS = 0x005be079 SentTS = 0x005be078, sent at 21:49:00.896511, ScheduledTS = 0x005be079, submitted at 21:50:03.672539 Wait for Channel 18, TS 0xef924 PendingEvent: YES NumberOfPendingCB = 1, FirstPendingTS = 0x005be079, LastPendingTS = 0x005be079 FirstPendingCB: ProcessID = 225, ProcessName = WindowServer, SubmitContext = Unknown (0) GPUAddress = 0x0000000431cef000, Size = 0x000001d3, VMID = 2 ContentValidation = PASS Buffer range 0x0 .. 0x100:c0012800 80000000 80000000 c0026900 00000081 80000000 40004000 c0026900 By searching online this seems to be happening to others as well but I failed to find a common fix for this. Any help would be hugely appreciated!!!

Hello Apple engineers, I’m trying to determine whether what I’m seeing is expected behavior or a software issue with the new M5 Pro platform. System MacBook Pro (M5 Pro) Latest macOS Beta External 5K 165Hz monitor connected via DisplayPort Refresh rate correctly detected as 165Hz What I observe The display itself is clearly running at 165Hz. For example: Mouse cursor movement is extremely smooth. Dragging the desktop by holding an empty area is also perfectly smooth. However: Moving application windows feels much closer to 60Hz. Scrolling in Safari, Chrome and other applications also appears to run at a much lower frame rate than the display refresh rate. Mission Control animations sometimes show similar micro-stutters. This makes the cursor and desktop movement noticeably smoother than normal window animations. ⸻ Troubleshooting already performed Different DisplayPort cables Different timing configurations Different resolutions / HiDPI modes DSC enabled and disabled Refresh rate confirmed at 165Hz Same behavior across multiple applications The issue appears unrelated to the monitor itself because the cursor is clearly rendered at the full refresh rate. ⸻ Additional observation Interestingly, I previously used another external 4K 144Hz HDR monitor and did not notice this behavior. I also found another M5 Pro user reporting nearly the same issue: external 165Hz display smooth cursor window dragging jitter / micro-stuttering At the same time, I haven’t found similar reports from M4 Pro or the base M5 running the same monitor. ⸻ My question Could this be related to the new M5 Pro display pipeline (WindowServer, Display Engine, or DCP)? Is there any known issue regarding high-refresh-rate external displays on the M5 Pro platform? Or is there additional diagnostic logging (WindowServer, DCP, Metal, etc.) that would help identify whether frames are actually being presented at the display refresh rate? I’d be happy to provide: sysdiagnose WindowServer logs Screen recordings Display timing information IORegistry dumps if they would be helpful. Thank you!

When we use MTLBlitCommandEncoder.copy to copy from buffer to textures with specific size, we find that the texture content is corruppt. The specific rules we found are: On Apple GPUs (reproduced on Apple M5, macOS 26.3 and iPhone 13 / A15), for ASTC textures uploaded to private storage via the blit encoder, the sampler / texel-fetch mis-addresses the tail of a mip level when ALL of the following hold: that level's texel width is an exact multiple of the 16 KB page width (32 blocks) — 128 texels for ASTC 4x4, 192 for 6x6, 256 for 8x8; the texture width is not divisible by 2^level (a partial block column exists in the chain); that level is taller than one page (>32 block rows). The level's first 128 texel rows (first page row, 4x4) are always read correctly; everything beyond is mis-addressed. Reads landing on unmapped/invalid memory decode as opaque magenta (1,0,1); reads aliasing valid memory show wrong image content. This produced visible purple artifacts in a shipping game (impostor tree atlas, 514x1024 ASTC 4x4, mip2 bottom half). The minimum reproduce code is: ASTCMipTailBugRepro.swift The output is: 514x1024 blit(private) mip2: CORRUPTED (bottom of mip2 mis-addressed) 512x1024 blit(private) mip2: INTACT 514x1024 replaceRegion mip2: INTACT

I have a metal kit view and drain the draw method of its delegate like shown below. Let's say I have one or more MTLBuffers with vertex resources bound via the argument table. When is it ok to drop these buffers? As far as I know one cannot schedule a completion handler In Metal 4 and I haven't been able to find any documentation about the lifetime requirements here. Any pointers/ideas appreciated. class RenderCoordinator: NSObject, MTKViewDelegate { public func draw(in view: MTKView) { let commandAllocator: any MTL4CommandAllocator = ... let commandBuffer: any MTL4CommandBuffer = ... let commandQueue: any MTL4CommandQueue = ... guard let drawable = view.currentDrawable else { return } commandBuffer.beginCommandBuffer(allocator: commandAllocator) let state: any MTLRenderPipelineState = ... let encoder: any MTL4RenderCommandEncoder = ... let argTable: any MTL4ArgumentTable = ... encoder.setRenderPipelineState(state) encoder.setArgumentTable(argTable, stages: .vertex) commandBuffer.endCommandBuffer() commandQueue.waitForDrawable(drawable) commandQueue.commit([commandBuffer]) commandQueue.signalDrawable(drawable) drawable.present() } }

I was using Times New Roman and trying to use superscript numbers, but found out that Times New Roman only has superscript numbers 0, 1, 2, 3, ¹ (like so) and since I'm writing out transcriptions of pinyin, I also need superscript 4, 5, 6, 7. When I do that, the font changes to Lucida Grande automatically. I would like to change the default unicode font/alt font from Lucida Grande to EB Garamond (downloaded from Google) since it more closely resembles Times New Roman and unfortunately, I need to keep the font as close to Times New Roman as possible. Is there a way to change the default the computer chooses to when Times New Roman/alt font fails unicode? Alternatively, is there a font that looks exactly like Times New Roman that has the superscript numbers 0-9? 2022 laptop, M2, Tahoe 26.6.1 Move post at will if need be.

Tengo un iphone 13, estoy con ios 27 ultima beta reciente y juego efootball mobile pero anda horrible. Anda muy pesado.

Hey everyone 👋 I'm trying to initialize a part of a CALayer hierarchy on a background thread and then attach the root of that hierarchy to a CALayer that backs a UIView. The motivation is to keep the main thread responsive when constructing a complex layer hierarchy. This isn't a case where I'm creating two or three layers and then switching back to the main thread. The hierarchy can potentially contain a large number of layers, with animations being created/configured for those layers as well. My first approach was to create and configure the layers entirely on a background thread. While the output might be the expected one (not always), CoreAnimation emits an assertion along the lines of: "Modifications to the layer tree from a background thread may not be committed" (or something like this). This makes sense to me if implicit CATransaction is thread-local. In that case, the implicit transaction opened by the layer modification on the background thread would not be part of the transaction that is already open on the main thread. Therefore, committing the main-thread transaction would not commit the changes made on the background thread. My second approach was to explicitly create and commit a CATransaction on the background thread. This appears to be accepted by Core Animation's threading model, but I'm seeing unreliable results. Sometimes parts of the hierarchy are missing, and in other cases the hierarchy is present but the animations don't appear to run at all. I do understand that this is private behavior of the framework, but I wanted to know if what I am trying to achieve is possible and, if so, what the solution would be (obviously if you can share this information). Besides this, I would also like to know what behavior CATransactions have when they are created on different threads. What I mean by this is that the transactions work as a stack, and the changes are committed when the stack is empty. Does this behavior still apply when having transactions on different threads? Any weird behaviours that might appear between transactions operated on main vs background threads? Thank you! Vlad.