I have the following var in an @Observable class: var displayResult: String { if let currentResult = currentResult, let decimalResult = Decimal(string: currentResult) { let result = decimalResult.formatForDisplay() UIAccessibility.post(notification: .announcement, argument: "Current result \(result)") return result } else { return "0" } } The UIAccessiblity.post gives me this warning: Reference to static property 'announcement' is not concurrency-safe because it involves shared mutable state; this is an error in Swift 6 How can I avoid this?

I got crash report for my mobile application private var _timedEvents: SynchronizedBarrier<[String: TimeInterval]> private var timedEvents: [String: TimeInterval] { get { _timedEvents.value } set { _timedEvents.value { $0 = newValue } } } func time(event: String) { let startTime = Date.now.timeIntervalSince1970 trackingQueue.async { [weak self, startTime, event] in guard let self else { return } var timedEvents = self.timedEvents timedEvents[event] = startTime self.timedEvents = timedEvents } } From the report, the crash is happening at _timedEvents.value { $0 = newValue } struct ReadWriteLock { private let concurentQueue: DispatchQueue init(label: String, qos: DispatchQoS = .utility) { let queue = DispatchQueue(label: label, qos: qos, attributes: .concurrent) self.init(queue: queue) } init(queue: DispatchQueue) { self.concurentQueue = queue } func read<T>(closure: () -> T) -> T { concurentQueue.sync { closure() } } func write<T>(closure: () throws -> T) rethrows -> T { try concurentQueue.sync(flags: .barrier) { try closure() } } } struct SynchronizedBarrier<Value> { private let lock: ReadWriteLock private var _value: Value init(_ value: Value, lock: ReadWriteLock = ReadWriteLock(queue: DispatchQueue(label: "com.example.SynchronizedBarrier", attributes: .concurrent))) { self.lock = lock self._value = value } var value: Value { lock.read { _value } } mutating func value<T>(execute task: (inout Value) throws -> T) rethrows -> T { try lock.write { try task(&_value) } } } What could be the reason for the crash? I have attached the crash report. Masked.crash

I've been studying the AVCam example and notice that everything pertaining to state transitions for the capture session is performed on a dedicated DispatchQueue. My question is this: Can I use an actor instead?

I'm developing iOS framework with Objective C. I create a dispatch_queue_t by using dispatch_queue_create. And call CFRunLoopRun() for run the Runloop in the queue. But, It looks like the dispatch_queue_t has share the RunLoop. Some classes has add an invalid timer, and when I call the CFRunLoopRun(), It crashed on my side. Sample code: - (void)viewDidLoad { [super viewDidLoad]; self.queue1 = dispatch_queue_create("com.queue1", DISPATCH_QUEUE_CONCURRENT); self.queue2 = dispatch_queue_create("org.queue2", DISPATCH_QUEUE_CONCURRENT); } - (IBAction)btnButtonAction:(id)sender { dispatch_async(self.queue1, ^{ NSString *runloop = [NSString stringWithFormat:@"%@", CFRunLoopGetCurrent()]; runloop = [runloop substringWithRange:NSMakeRange(0, 22)]; NSLog(@"Queue1 %p run: %@", self.queue1, runloop); //NSTimer *timer = [[NSTimer alloc] initWithFireDate:[NSDate date] interval:1 target:self selector:@selector(wrongSeletor:) userInfo:nil repeats:NO]; //[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes]; }); dispatch_async(self.queue2, ^{ NSString *runloop = [NSString stringWithFormat:@"%@", CFRunLoopGetCurrent()]; runloop = [runloop substringWithRange:NSMakeRange(0, 22)]; NSLog(@"Queue2 %p run: %@", self.queue2, runloop); CFRunLoopRun(); }); } Some time they take same RunLoop: https://i.stack.imgur.com/wGcv3.png ===== You can see the crash by uncomment the code of NSTimer. The NSTimer has been added in queue1, but it still running when call CFRunLoopRun() in queue2. I have read some description like: https://stackoverflow.com/questions/38000727/need-some-clarifications-about-dispatch-queue-thread-and-nsrunloop They told that: "system creates a run loop for the thread". But, in my check, they are sharing the RunLoop. This is sad for me, because I facing that crashes happen when calling CFRunLoopRun() on production. Can someone take a look at this.

In the 'Discussion' section of the current documentation for Swift's DispatchQueue, it says: If the target queue is a concurrent queue, the blocks run in parallel and must therefore be reentrant-safe. However, unlike dispatch_apply (on which this API is built), this method provides no direct means of specifying a target queue, so this callout is somewhat more confusing than it ought to be. IMO, it's important to highlight the reentrancy considerations that apply in most (all?) cases, but the implicit reference to the implementation details should be removed or clarified. Feedback filed as: FB13708750

Hello, I have received 3 almost identical crash reports from the App Store. They all come from the same user, and they are spaced just ± 45 seconds apart. This is the backtrace of the crashed thread: Crashed Thread: 3 Exception Type: EXC_CRASH (SIGABRT) Exception Codes: 0x0000000000000000, 0x0000000000000000 Termination Reason: Namespace SIGNAL, Code 6 Abort trap: 6 Terminating Process: Ssssssss [46033] Thread 3 Crashed: 0 libsystem_kernel.dylib 0x00007ff81b90f196 __pthread_kill + 10 1 libsystem_pthread.dylib 0x00007ff81b946ee6 pthread_kill + 263 (pthread.c:1670) 2 libsystem_c.dylib 0x00007ff81b86dbdf __abort + 139 (abort.c:155) 3 libsystem_c.dylib 0x00007ff81b86db54 abort + 138 (abort.c:126) 4 libc++abi.dylib 0x00007ff81b901282 abort_message + 241 5 libc++abi.dylib 0x00007ff81b8f33fb demangling_terminate_handler() + 267 6 libobjc.A.dylib 0x00007ff81b5c67ca _objc_terminate() + 96 (objc-exception.mm:498) 7 libc++abi.dylib 0x00007ff81b9006db std::__terminate(void (*)()) + 6 8 libc++abi.dylib 0x00007ff81b900696 std::terminate() + 54 9 libdispatch.dylib 0x00007ff81b7a6047 _dispatch_client_callout + 28 (object.m:563) 10 libdispatch.dylib 0x00007ff81b7a87c4 _dispatch_queue_override_invoke + 800 (queue.c:4882) 11 libdispatch.dylib 0x00007ff81b7b5fa2 _dispatch_root_queue_drain + 343 (queue.c:7051) 12 libdispatch.dylib 0x00007ff81b7b6768 _dispatch_worker_thread2 + 170 (queue.c:7119) 13 libsystem_pthread.dylib 0x00007ff81b943c0f _pthread_wqthread + 257 (pthread.c:2631) 14 libsystem_pthread.dylib 0x00007ff81b942bbf start_wqthread + 15 (:-1) In the backtrace of the main thread, I can see that the error is caught by the app delegate which tries to display an alert, but obviously the message has no time to appear. Incidentally (but it's not my main question), I would like to know if it would be possible in such a case to block the background thread for the time the alert is displayed (e.g. using a dispatch queue)? ... (many other related lines) 72 SSUuuuIIIIIIIIIUUuuuu 0x000000010e8089f2 +[NSAlert(Ssssssss) fatalError:] + 32 73 Ssssssss 0x000000010dd5e75b __universalExceptionHandler_block_invoke (in Ssssssss) (SQAppDelegate.m:421) + 333659 74 libdispatch.dylib 0x00007ff81b7a4d91 _dispatch_call_block_and_release + 12 (init.c:1518) 75 libdispatch.dylib 0x00007ff81b7a6033 _dispatch_client_callout + 8 (object.m:560) 76 libdispatch.dylib 0x00007ff81b7b2fcf _dispatch_main_queue_drain + 954 (queue.c:7794) 77 libdispatch.dylib 0x00007ff81b7b2c07 _dispatch_main_queue_callback_4CF + 31 (queue.c:7954) 78 CoreFoundation 0x00007ff81ba62195 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9 (CFRunLoop.c:1780) 79 CoreFoundation 0x00007ff81ba21ebf __CFRunLoopRun + 2452 (CFRunLoop.c:3147) 80 CoreFoundation 0x00007ff81ba20ec1 CFRunLoopRunSpecific + 560 (CFRunLoop.c:3418) 81 HIToolbox 0x00007ff8254a5f3d RunCurrentEventLoopInMode + 292 (EventLoop.c:455) 82 HIToolbox 0x00007ff8254a5d4e ReceiveNextEventCommon + 657 (EventBlocking.c:384) 83 HIToolbox 0x00007ff8254a5aa8 _BlockUntilNextEventMatchingListInModeWithFilter + 64 (EventBlocking.c:171) 84 AppKit 0x00007ff81eabfb18 _DPSNextEvent + 858 (CGDPSReplacement.m:818) 85 AppKit 0x00007ff81eabe9c2 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1214 (appEventRouting.m:407) 86 AppKit 0x00007ff81eab1037 -[NSApplication run] + 586 (NSApplication.m:3432) 87 AppKit 0x00007ff81ea85251 NSApplicationMain + 817 (NSApplication.m:9427) 88 dyld 0x00007ff81b5ec41f start + 1903 (dyldMain.cpp:1165) In all 3 reports, another thread indicates that it is sending or receiving a MIDI data (it's exactly the same backtrace in the 3 reports): Thread 1: 0 libsystem_kernel.dylib 0x00007ff81b908552 mach_msg2_trap + 10 1 libsystem_kernel.dylib 0x00007ff81b9166cd mach_msg2_internal + 78 (mach_msg.c:201) 2 libsystem_kernel.dylib 0x00007ff81b90f584 mach_msg_overwrite + 692 (mach_msg.c:0) 3 libsystem_kernel.dylib 0x00007ff81b90883a mach_msg + 19 (mach_msg.c:323) 4 CoreMIDI 0x00007ff834adfd50 XServerMachPort::ReceiveMessage(int&amp;, void*, int&amp;) + 94 (XMachPort.cpp:62) 5 CoreMIDI 0x00007ff834b118c5 MIDIProcess::MIDIInPortThread::Run() + 105 (MIDIClientLib.cpp:204) 6 CoreMIDI 0x00007ff834af9c44 CADeprecated::XThread::RunHelper(void*) + 10 (XThread.cpp:21) 7 CoreMIDI 0x00007ff834afae9f CADeprecated::CAPThread::Entry(CADeprecated::CAPThread*) + 77 (CAPThread.cpp:324) 8 libsystem_pthread.dylib 0x00007ff81b9471d3 _pthread_start + 125 (pthread.c:893) 9 libsystem_pthread.dylib 0x00007ff81b942bd3 thread_start + 15 (:-1) I wonder if this MIDI activity may be related to the crash, even if it doesn't occur in the crashed thread. I also wonder if the dispatch block starting the backtrace of the thread 3 is the same that leads to the thread 1 (to open the NSAlert), which would mean that it's after the error, or if it's a dispatch block into which the error occurs. Finally, 2024-03-25_13-04-40.6314.crash I wonder if std::terminate() could indicate a problem in C++ codes because I have some part of the application written in C++, and it would be a clue. More generally, is it something in these backtraces that can help me to find what's the problem ? Any help greatly appreciated, thanks! -dp

I'm trying to reproduce a case when there are more dispatch queues than there are threads serving them. Is that a possible scenario?

Per my understanding of the DispatchQueue docs, and various WWDC videos on the matter, if one creates a queue in the following manner: let q = DisqpatchQueue( label: "my-q", qos: .utility, target: .global(qos: .userInteractive) ) then one should expect work items submitted via async() to effectively run at userInteractive QoS, as the target queue should provide a 'floor' on the effective QoS value (assuming no additional rules are in play, e.g. higher priority items have been enqueued, submitted work items enforce QoS, etc). In practice, however, this particular formulation does not appear to function that way, and the 'resolved' QoS value seems to be utility, contrary to what the potentially relevant documentation suggests. This behavior appears to be inconsistent with other permutations of queue construction, which makes it even more surprising. Here's some sample code I was experimenting with to check the behavior of queues created in various ways that I would expect to function analogously (in regards to the derived QoS value for the threads executing their work items): func test_qos_permutations() { // q1 let utilTargetingGlobalUIQ = DispatchQueue( label: "qos:util tgt:globalUI", qos: .utility, target: .global(qos: .userInitiated) ) let customUITargetQ = DispatchQueue( label: "custom tgt, qos: unspec, tgt:globalUI", target: .global(qos: .userInitiated) ) // q2 let utilTargetingCustomSerialUIQ = DispatchQueue( label: "qos:util tgt:customSerialUI", qos: .utility, target: customUITargetQ ) // q3 let utilDelayedTargetingGlobalUIQ = DispatchQueue( label: "qos:util tgt:globalUI-delayed", qos: .utility, attributes: .initiallyInactive ) utilDelayedTargetingGlobalUIQ.setTarget(queue: .global(qos: .userInitiated)) utilDelayedTargetingGlobalUIQ.activate() let queues = [ utilTargetingGlobalUIQ, utilTargetingCustomSerialUIQ, utilDelayedTargetingGlobalUIQ, ] for q in queues { q.async { Thread.current.name = q.label let threadQos = qos_class_self() print(""" q: \(q.label) orig qosClass: \(q.qos.qosClass) thread qosClass: \(DispatchQoS.QoSClass(rawValue: threadQos)!) """) } } } Running this, I get the following output: q: qos:util tgt:customSerialUI orig qosClass: utility thread qosClass: userInitiated q: qos:util tgt:globalUI-delayed orig qosClass: utility thread qosClass: userInitiated q: qos:util tgt:globalUI orig qosClass: utility thread qosClass: utility This test suggests that constructing a queue with an explicit qos parameter and targeting a global queue of nominally 'higher' QoS does not result in a queue that runs its items at the target's QoS. Perhaps most surprisingly is that if the target queue is set after the queue was initialized, you do get the expected 'QoS floor' behavior. Is this behavior expected, or possibly a bug?

Swift Concurrency Resources: DevForums tags: Concurrency The Swift Programming Language > Concurrency documentation Migrating to Swift 6 documentation WWDC 2022 Session 110351 Eliminate data races using Swift Concurrency — This ‘sailing on the sea of concurrency’ talk is a great introduction to the fundamentals. WWDC 2021 Session 10134 Explore structured concurrency in Swift — The table that starts rolling out at around 25:45 is really helpful. Swift Async Algorithms package Swift Concurrency Proposal Index DevForum post Why is flow control important? DevForums post Matt Massicotte’s blog Dispatch Resources: DevForums tags: Dispatch Dispatch documentation — Note that the Swift API and C API, while generally aligned, are different in many details. Make sure you select the right language at the top of the page. Dispatch man pages — While the standard Dispatch documentation is good, you can still find some great tidbits in the man pages. See Reading UNIX Manual Pages. Start by reading dispatch in section 3. WWDC 2015 Session 718 Building Responsive and Efficient Apps with GCD [1] WWDC 2017 Session 706 Modernizing Grand Central Dispatch Usage [1] Avoid Dispatch Global Concurrent Queues DevForums post Share and Enjoy — Quinn “The Eskimo!” @ Developer Technical Support @ Apple let myEmail = "eskimo" + "1" + "@" + "apple.com" [1] These videos may or may not be available from Apple. If not, the URL should help you locate other sources of this info.

Hi, I work on a game for iOS and the framerate decreases progressively when the debugger is attached. Running it for 2mins, it went from 30 to 1 FPS while rendering a simple static scene. I narrowed it down to a call to dispatch_async_f which takes longer to execute over time. clock_t t1 = clock(); dispatch_async_f(queue, context, function); clock_t t2 = clock(); double duration = (double)(t2 -t1)/(double)CLOCKS_PER_SEC; Dodumentation says dispatch_async_f is supposed to return immediatly. So what could explain duration to increases in debug? Am i measuring this incorrectly? The game is written in mixed C++ and ObjC. It uses Metal as graphic API and GCD for dispatching jobs. I have Xcode 13.4.1 and test on an iPhone 13 Pro with iOS 15.7. Thanks.