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.

Given that SwiftUI and modern programming idioms promote asynchronous activity, and observing a data model and reacting to changes, I wonder why it's so cumbersome in Swift at this point. Like many, I have run up against the problem where you perform an asynchronous task (like fetching data from the network) and store the result in a published variable in an observed object. This would appear to be an extremely common scenario at this point, and indeed it's exactly the one posed in question after question you find online about this resulting error: Publishing changes from background threads is not allowed Then why is it done? Why aren't the changes simply published on the main thread automatically? Because it isn't, people suggest a bunch of workarounds, like making the enclosing object a MainActor. This just creates a cascade of errors in my application; but also (and I may not be interpreting the documentation correctly) I don't want the owning object to do everything on the main thread. So the go-to workaround appears to be wrapping every potentially problematic setting of a variable in a call to DispatchQueue.main. Talk about tedious and error-prone. Not to mention unmaintainable, since I or some future maintainer may be calling a function a level or two or three above where a published variable is actually set. And what if you decide to publish a variable that wasn't before, and now you have to run around checking every potential change to it? Is this not a mess?

I have enabled “StrictConcurrency” warnings in my project that uses SwiftUI. I have a Commands struct. It has a Button, whose action is calling an async method via Task{}. This builds without warnings within Views, but not Commands. There the compiler reports “Main actor-isolated property 'body' cannot be used to satisfy nonisolated protocol requirement”. Looking at SwiftUI: In View, body is declared @MainActor: @ViewBuilder @MainActor var body: Self.Body { get } In Commands, body is not declared @MainActor: @CommandsBuilder var body: Self.Body { get } So the common practice of making a Button action asynchronous: Button { Task { await model.load() } } label:{ Text("Async Button") } will succeed without warnings in Views, but not in Commands. Is this intentional? I've filed FB13212559. Thank you.

I have an 8th generation iPad, now updated with iPadOS 16.2 (20C65) and I have an issue that I also saw on earlier 16.* betas. Task is not executing at all. This is so frustrating because I have adopted async/await in my app, I support iOS 15+, everything was working fine but now that stuff inside Task { } is not executed my app seems to be broken. (Note: my main device is an iPhone 11, still on iOS 16.0, and it works fine there.) It is also frustrating to see no other developers are complaining about this, like it happens only with my app. I have debugged with print statements and breakpoints and I can say for sure that stuff is not executing. Does anybody have any ideas? Anything else I can try? FB11866066

I have an application that communicates with custom external hardware on the network (using UDP). I have a thread that receives and process the UDP data and then signals a waiting thread by releasing a semaphore when data is available. A have a asyncSendAndReceive and asyncReceive function that just begs to use async/await. But I cannot simply switch because of the use of the semaphore. Various forums and discussions said that semaphores should no longer be used for signalling. If not semaphores, then what else? Note that my two async functions may not always block. If data was received before they were called, then it is queued (and the semaphore is signalled).

Is it ok for an Actor type to have a Publisher as a property to let others observe changes over time? Or use the @Published property wrapper to achieve this? actor MyActor { var publisher = PassthroughSubject<Int, Never>() var data: Int { didSet { publisher.send(data) } } ... } // Usage var tasks = Set<AnyCancellable>() let actor = MyActor() Task { let publisher = await actor.publisher publisher.sink { print($0) }.store(in: &tasks) } This seems like this should be acceptable. I would expect a Publisher to be thread safe, and as long as the Output is a value type things should be fine. I have been getting random EXC_BAD_ACCESS errors when using this approach. But turning on the address sanitizer causes these crashes to go away. I know that isn't very specific but I wanted to start by seeing if this type of pattern is ok to do.

Hello, When attempting to assign the UNNotificationResponse to a Published property on the main thread inside UNUserNotificationCenterDelegate's method func userNotificationCenter(_ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse) async both Task { @MainActor in } and await MainActor.run are throwing a NSInternalInconsistencyException: 'Call must be made on main thread'. I thought both of them were essentially doing the same thing, i.e. call their closure on the main thread. So why is this exception thrown? Is my understanding of the MainActor still incorrect, or is this a bug? Thank you Note: Task { await MainActor.run { ... } } and DispatchQueue.main.async don't throw any exception.

I have enabled runtime concurrency warnings to check for future problems concerning concurrency: Build Setting / Other Swift Flags: -Xfrontend -warn-concurrency -Xfrontend -enable-actor-data-race-checks When trying to call the async form of PHPhotoLibrary.shared().performChanges{} I get the following runtime warning: warning: data race detected: @MainActor function at ... was not called on the main thread in the line containing performChanges. My sample code inside a default Xcode multi platform app template is as follows: import SwiftUI import Photos @MainActor class FotoChanger{     func addFotos() async throws{         await PHPhotoLibrary.requestAuthorization(for: .addOnly)         try! await PHPhotoLibrary.shared().performChanges{             let data = NSDataAsset(name: "Swift")!.data             let creationRequest = PHAssetCreationRequest.forAsset()             creationRequest.addResource(with: .photo, data: data, options: PHAssetResourceCreationOptions())         }     } } struct ContentView: View {     var body: some View {         ProgressView()             .task{                 try! await FotoChanger().addFotos()             }     } } You would have to have a Swift data asset inside the asset catalog to run the above code, but the error can even be recreated if the data is invalid. But what am I doing wrong? I have not found a way to run perform changes, the block or whatever causes the error on the main thread. PS: This is only test code to show the problem, don't mind the forced unwraps.

I'm trying to understand a design pattern for accessing the isolated state held in an actor type from within a SwiftUI view. Take this naive code: actor Model: ObservableObject { @Published var num: Int = 0 func updateNumber(_ newNum: Int) { self.num = newNum } } struct ContentView: View { @StateObject var model = Model() var body: some View { Text("\(model.num)") // <-- Compiler error: Actor-isolated property 'num' can not be referenced from the main actor Button("Update number") { Task.detached() { await model.updateNumber(1) } } } } Understandably I get the compiler error Actor-isolated property 'num' can not be referenced from the main actor when I try and access the isolated value. Yet I can't understand how to display this data in a view. I wonder if I need a ViewModel that observes the actor, and updates itself on the main thread, but get compile time error Actor-isolated property '$num' can not be referenced from a non-isolated context. class ViewModel: ObservableObject { let model: Model @Published var num: Int let cancellable: AnyCancellable init() { let model = Model() self.model = model self.num = 0 self.cancellable = model.$num // <-- compile time error `Actor-isolated property '$num' can not be referenced from a non-isolated context` .receive(on: DispatchQueue.main) .sink { self.num = $0 } } } Secondly, imagine if this code did compile, then I would get another error when clicking the button that the interface is not being updated on the main thread...again I'm not sure how to effect this from within the actor?