We are experiencing an issue with withCheckedContinuation in our Swift project. Our implementation was working perfectly in a previous version of Xcode and continues to work in the simulator. However, it fails to work on a real device. Here’s a brief description of the problem:
• Environment:
Xcode Version: Xcode 16.0 Beta 5
Swift Version: Swift 5
OS: IOS18 beta 5
• Problem:
The code using withCheckedContinuation behaves as expected in the simulator but fails on a physical device. We are receiving a “bad access to memory” error when running on a real device.
• What We’ve Tried:
Verified that the code works in previous Xcode versions.
Tested on different simulators, where it runs without issues.
Checked for any obvious errors in memory handling or threading.
Code Example: Here’s a simplified version of the problematic code:
var body: some View {
VStack {
Text("Hello, world!")
}
.padding()
.onAppear {
Task {
await self.checkTrialOrIntroductoryDiscountEligibilityAsync()
}
}
}
func checkTrialOrIntroductoryDiscountEligibilityAsync() async {
return await withCheckedContinuation { continuation in
checkTrialOrIntroDiscountEligibility() {
continuation.resume()
}
}
}
func checkTrialOrIntroDiscountEligibility(completion: () -> Void) {
completion()
}
}
Concurrency
RSS for tagConcurrency is the notion of multiple things happening at the same time.
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I took one of my apps and have gone through the process of making it compatible with Swift 6. I started with Swift 5 and Complete Concurrency Checking and eliminated every warning. I flipped the switch to Swift 6 and I have no compile errors and no warnings. I also created a ModelActor and created async functions to fetch, delete, get an object's persistentID and save model objects. My SwiftData model has a notes property and the user can update or add onto the notes and resave the model object. The app crashes when I save the changes using an explicit save operation. However, the next time the app is launched, the changes did propagate. If I do not use the explicit save operation, the system does not auto-save and does not crash.
When I switched back to Swift 5 and tried it out, I was able to save without a crash. I did need to use an explicit save operation though.
However, I am now getting 3 warnings like the following.
Type 'ReferenceWritableKeyPath<MyModel, Optional>' does not conform to the 'Sendable' protocol; this is an error in the Swift 6 language mode
The line of code that produced that warning is
`let fetchDescriptor = FetchDescriptor(sortBy: [SortDescriptor(\MyModel.title)])
However, when I flip the switch back to Swift 6, the three warnings go away and I have zero warnings under Swift 6.
We are experiencing an issue with withCheckedContinuation in our Swift project. Our implementation was working perfectly in a previous version of Xcode and continues to work in the simulator. However, it fails to work on a real device. Here’s a brief description of the problem:
• Environment:
- Xcode Version: Xcode 16.0 Beta 5
- Swift Version: Swift 5
- OS: IOS18 beta 5
• Problem:
The code using withCheckedContinuation behaves as expected in the simulator but fails on a physical device. We are receiving a “bad access to memory” error when running on a real device.
• What We’ve Tried:
1. Verified that the code works in previous Xcode versions.
2. Tested on different simulators, where it runs without issues.
3. Checked for any obvious errors in memory handling or threading.
Code Example:
Here’s a simplified version of the problematic code:
var body: some View {
VStack {
Text("Hello, world!")
}
.padding()
.onAppear {
Task {
await self.checkTrialOrIntroductoryDiscountEligibilityAsync()
}
}
}
func checkTrialOrIntroductoryDiscountEligibilityAsync() async {
return await withCheckedContinuation { continuation in
checkTrialOrIntroDiscountEligibility() {
continuation.resume()
}
}
}
func checkTrialOrIntroDiscountEligibility(completion: () -> Void) {
completion()
}
}
Has anyone found a thread-safe pattern that can extract results from completerDidUpdateResults(MKLocalSearchCompleter) in the MKLocalSearchCompleterDelegate ?
I've downloaded the code sample from Interacting with nearby points of interest and notice the conformance throws multiple errors in Xcode 16 Beta 5 with Swift 6:
extension SearchDataSource: MKLocalSearchCompleterDelegate {
nonisolated func completerDidUpdateResults(_ completer: MKLocalSearchCompleter) {
Task {
let suggestedCompletions = completer.results
await resultStreamContinuation?.yield(suggestedCompletions)
}
}
Error: Task-isolated value of type '() async -> ()' passed as a strongly transferred parameter; later accesses could race
and
Error: Sending 'suggestedCompletions' risks causing data races
Is there another technique I can use to share state of suggestedCompletions outside of the delegate in the code sample?
This post discusses a subtlety in Swift concurrency, and specifically how it relates to SwiftUI, that I regularly see confusing folks. I decided to write it up here so that I can link to it rather than explain it repeatedly.
If you have a question or a comment, start a new thread and I’ll respond there. Put it in the App & System Services > Processes & Concurrency topic area and tag it with both Swift and Concurrency.
Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"
Task Isolation Inheritance
By default, tasks inherit their actor isolation from the surrounding code. This is a common source of confusion. My goal here is to explain why it happens, why it can cause problems, and how to resolve those problems.
Imagine you have a main actor class like this:
@MainActor
class MyClass {
var counter: Int = 0
func start() {
Task {
print("will sleep")
doSomeCPUIntensiveWork()
print("did sleep")
}
}
}
In this example the class is a model object of some form, but it could be an @Observable type, a SwiftUI view, a UIKit view controller, and so on. The key thing is that the type itself is isolated to the main actor.
Remember that Swift code inherits its isolation from the surrounding code (in compiler author speak this is called the lexical context). So the fact that MyClass is annotated with @MainActor means that both counter and start() are isolated to the main actor.
IMPORTANT This model is what allows the compiler to detect concurrency problems at compile time. I’ve found that, whenever I’m confused by Swift concurrency, it helps to ask myself “What does the compiler know?”
Folks look at this code and think “But I’ve added a Task, and that means that doSomeCPUIntensiveWork() will run on a secondary thread!” That is not true. There are a couple of easy ways to prove this to yourself:
Actually run the code. If you put this code into an app, you’ll find that your app’s UI is unresponsive for the duration of the doSomeCPUIntensiveWork(). Indeed, you can test this example for yourself, as explained below in Example Context.
Access a value that’s isolated to the main actor. For example, insert this doSomeCPUIntensiveWork():
self.counter += 1
doSomeCPUIntensiveWork()
The compiler doesn’t complain about this access to counter — a main-actor-isolated value — from this context, which tell you that this code will run on the main thread.
So, what’s going on? The task is running on the main actor because of a form of isolation inheritance. The mechanics of that are complex, something I’ll explained in the The Gory Details section below. For the moment, however, the key thing to note is that starting a task in this way — using Task.init(…) — causes the task to inherit actor isolation from the surrounding code. In this case the surrounding code is the start() method, which is isolated to the main actor because it’s part of MyClass, and thus the code ends up calling doSomeCPUIntensiveWork() on the main thread.
So, how do you prevent this? There are many different ways, but the two most obvious are:
Replace Task.init(…) with Task.detached(…):
func start() {
Task.detached() {
print("will sleep")
doSomeCPUIntensiveWork()
print("did sleep")
}
}
And how does that work? Again, see the The Gory Details section below.
Move the code to a non-isolated method:
func start() {
Task {
print("will sleep")
await self.myDoSomeCPUIntensiveWork()
print("did sleep")
}
}
nonisolated func myDoSomeCPUIntensiveWork() async {
doSomeCPUIntensiveWork()
}
In both cases you can prove to yourself that this has done the right thing: Add code to access counter from the non-isolated context and observe the complaints from the compiler.
SwiftUI
While my “What does the compiler know?” thought experiment is super helpful, sometimes it’s not easy understand that. Folks are often caught out by the way that the SwiftUI View protocol works. We’ve fixed this problem in Xcode 16, but that change has brought more confusion.
In Xcode 15 and earlier the View protocol was defined like this:
public protocol View {
…
@ViewBuilder @MainActor var body: Self.Body { get }
}
Only the body property is annotated with @MainActor. The view as a whole is not. Consider this view:
struct CounterViewOK: View {
@State var counter: Int = 0
var body: some View {
VStack {
Text("\(counter)")
Button("Increment") {
Task {
counter += 1
}
}
}
}
}
This compiles because the task inherits the main actor isolation from body. But if you make a seemingly trivial change, the compiler complains:
struct CounterViewNG: View {
@State var counter: Int = 0
var body: some View {
VStack {
Text("\(counter)")
Button("Increment") {
increment()
}
}
}
func increment() {
Task {
counter += 1
// ^ Capture of 'self' with non-sendable type 'CounterViewNG' in a `@Sendable` closure
}
}
}
That’s because the increment() method is not isolated to the main actor, and thus neither is the task. The compiler thinks you’re trying to pass an instance of the view between contexts, and rightly complains.
In contrast, in Xcode 16 (currently in beta) the View protocol looks ilke this:
@MainActor @preconcurrency public protocol View {
…
@ViewBuilder @MainActor @preconcurrency var body: Self.Body { get }
}
The entire View is now isolated to the main actor. This makes everything easier to understand. Both of the examples above work. Specifically, CounterViewNG works because the task inherits main actor isolation via the increment() > CounterViewNG > View chain.
Of course, everything is a trade-off. More of your views are now running on the main actor, which can trigger the CPU intensive work issue that I described above.
Other Options
When I crafted the doSomeCPUIntensiveWork() example above, I avoided mentioning SwiftUI. There was a specific reason for that: When working with a UI framework, it’s best to avoid doing significant work in your UI types. This is true in SwiftUI, but it’s also true in UIKit and AppKit. Indeed, doing all your app’s work in your view controllers is called the massive view controller anti-pattern.
So, if you’re find yourself doing significant work in your UI types, consider some alternatives. You have lots of options:
The simplest option is to move the code into an async function.
But you might also want to add an abstraction layer. Swift has lots of good options for that (structs, enums, classes, actors).
You can also define a new global actor.
The best option depends on your specific situation. If you’re looking for further advice, there’s no shortage of it out there on the ’net (-:
The Gory Details
To understand the difference between Task.init(…) and Task.detached(…), you have to look at their declarations. This is easy to do from Xcode — just command-click on the init or the detached — but that’s misleading. The difference is keyed off a underscore-prefixed attribute and, for better or worse, Xcode won’t show you those.
To see the actual difference you have have to open the Swift interface file. Within any given SDK the relevant file is usr/lib/swift/_Concurrency.swiftmodule/arm64e-apple-macos.swiftinterface. Here’s what you’ll see in the macOS SDK within Xcode 16.0b4:
@discardableResult
@_alwaysEmitIntoClient
public init(
priority: TaskPriority? = nil,
@_inheritActorContext @_implicitSelfCapture operation: __owned @escaping @isolated(any) @Sendable () async -> Success
) {…}
@discardableResult
@_alwaysEmitIntoClient
public static func detached(
priority: TaskPriority? = nil,
operation: __owned @escaping @isolated(any) @Sendable () async -> Success
) -> Task<Success, Failure> {…}
Note I’ve edited this significantly to make things easier to read.
The critical difference is the use of @_inheritActorContext in the Task.init(…) case. This tells the compiler that the closure argument should inherit its isolation from the surrounding code. This attribute is underscored, and thus there’s no Swift Evolution proposal for it, but there is some limited documentation.
Example Context
To run the example in context, create a new command-line tool project, rename main.swift to start.swift, and insert MyClass into this scaffolding:
import Foundation
@MainActor
class MyClass {
… code above …
}
func doSomeCPUIntensiveWork() {
sleep(5)
}
@main
struct Main {
static func main() {
Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { _ in
print("tick")
}
let m = MyClass()
m.start()
withExtendedLifetime(m) {
RunLoop.current.run()
}
}
}
In this context:
doSomeCPUIntensiveWork() uses the sleep system call to hog the current thread for 5 seconds.
The timer tick helps illustrate the unresponsive main thread.
It’s also need to ensure that the run loop continues to run indefinitely.
More Reading
There is a lot of good information available about Swift concurrency. My favourite resources include:
The Swift Programming Language > Concurrency
Migrating to Swift 6
The Avoid hangs by keeping the main thread free from non-UI work section of Improving app responsiveness
WWDC 2023 Session 10248 Analyze hangs with Instruments, especially the section starting at 31:42.
Swift Evolution proposals
SE-0431 @isolated(any) Function Types which covers another subtle issue with tasks
Matt Massicotte blog at https://www.massicotte.org
Revision History
2024-08-05 Added the Other Options section. Added some more links to the More Reading section. Made other minor editorial changes.
2024-08-01 First posted.
Hello,
I have a lot of apps and I am currently trying to port them over to Swift 6. I thought that this process should be relatively simple but I have to admit that I have a lot of trouble to understand how the Concurrency system works.
Let's start with some code that shows how I am currently working when it comes to asynchronous work in my apps:
I have a Model that is marked with @Observable.
Inside this model, a Controller is hosted.
The Controller has its own ControllerDelegate.
The Model has a search function. Inside this function a lot of IO stuff is executed. This can take a lot of time. Because of this fact, I am doing this in a separate Thread.
I all is put together, it looks a little bit like this:
@main
struct OldExampleApp : App {
@State private var model = Model()
var body: some Scene {
WindowGroup {
ContentView()
.environment(self.model)
}
}
}
struct ContentView: View {
@Environment(Model.self) private var model
var body: some View {
if self.model.isSearching {
ProgressView()
}
else {
Button("Start") {
self.model.search()
}
}
}
}
protocol ControllerDelegate : AnyObject {
func controllerDidStart()
func controllerDidEnd()
}
class Controller {
weak var delegate: ControllerDelegate?
func search() {
let thread = Thread {
DispatchQueue.main.async {
self.delegate?.controllerDidStart()
}
// Do some very complex stuff here. Let's use sleep to simulate this.
Thread.sleep(forTimeInterval: 2.0)
DispatchQueue.main.async {
self.delegate?.controllerDidEnd()
}
}
thread.start()
}
}
@Observable
class Model {
private(set) var isSearching = false
var controller = Controller()
init() {
self.controller.delegate = self
}
func search() {
self.controller.search()
}
}
extension Model : ControllerDelegate {
func controllerDidStart() {
self.isSearching = true
}
func controllerDidEnd() {
self.isSearching = false
}
}
This works perfectly fine and by that I mean:
The task is run in the background.
The main thread is not blocked. The main window can be dragged around, no beach ball cursor etc.
Now comes the Swift 6 part:
I want to merge the Model and Controller into one class (Model).
I still want the Model to be Observable.
I want to run arbitrary code in the Model. This means that the code is not necessarily a prime candidate for await like getting data from a web server etc.
The main thread should not be blocked, so the main window should still be movable while the app calculates data in the background.
I have this example:
struct ContentView: View {
@Environment(Model.self) private var model
var body: some View {
if self.model.controller.isSearching
{
ProgressView()
}
else
{
Button("Search") {
Task {
await self.model.controller.heavyWork()
}
}
}
}
}
@Observable
final class Model : Sendable
{
@MainActor var controller = AsyncController()
init()
{
}
}
@Observable
@MainActor
class AsyncController
{
private(set) var isSearching = false
public func heavyWork() async
{
self.isSearching = true
Swift.print(Date.now)
let i = self.slowFibonacci(34)
Swift.print(i)
Swift.print(Date.now)
self.isSearching = false
}
func slowFibonacci(_ n: Int) -> Int
{
if n <= 1 {
return n
}
let x = slowFibonacci(n - 1)
let y = slowFibonacci(n - 2)
return x + y
}
}
I come from a C# background and my expectation is that when I use a Task with await, the main thread is not blocked and the Code that is called inside the Task runs in the background.
It seems like the function is run in the background, but the UI is not updated. Because I set the isSearching flag to true, I would expect that the app would display the ProgressView - but it does not.
I changed the code to this:
public func heavyWork() async
{
self.isSearching = true
Swift.print(Date.now)
let i = await self.slowFibonacci(20)
Swift.print(i)
Swift.print(Date.now)
self.isSearching = false
}
func slowFibonacci(_ n: Int) async -> Int
{
let task = Task { () -> Int in
if n <= 1 {
return n
}
let x = await slowFibonacci(n - 1)
let y = await slowFibonacci(n - 2)
return x + y
}
return await task.value
}
This seems to work - but is this correct?
I have this pattern implemented in one of my apps and there the main thread is blocked when the code is run.
So I think it all comes down to this:
Is it possible, to run a arbitrary code block (without an await in it) in a Task, that can be awaited so the main thread is not blocked?
The class (or actor?) that contains the function that is called via await should be Observable.
Or should I simply keep my Swift 5 code and move on? :D
Regards,
Sascha
I'm currently in the process of migrating to Swift 6. A lot of my code triggers the warning from the title. Passing argument of non-sendable type 'ContentView' outside of main actor-isolated context may introduce data races. I depend on the .task/.refreshable modifiers and buttons that trigger asynchronous work that cannot be done on the Main Actor since it takes way to long.
The below code demonstrates the problem. Some comments explain my problems further. I read a lot of articles and documentations but couldn't find an answer to such a seemingly simple error
struct ContentView: View { // Marking Senable as suggested by the warning causes different warning for @State
@State private var authorizationStatus: MusicAuthorization.Status = .notDetermined // Sole purpose to trigger the errors
var body: some View {
VStack {
Text("Hello, world!")
Button("Some button") {
Task {
await doingSomeAsyncWork()
// WARNING: Passing argument of non-sendable type 'ContentView' outside of main actor-isolated context may introduce data races
}
}
}
.task { // Or refreshable I believe both behave the same
await doingSomeAsyncWork()
// WARNING: Passing argument of non-sendable type 'ContentView' outside of main actor-isolated context may introduce data races
}
}
// Marking @MainActor is not an option since some of these functions might be running for more than 10 seconds
// Tried marking func as nonisolated but that obviously had no effect
func doingSomeAsyncWork() async {
authorizationStatus = await MusicAuthorization.request() // Just to have a easy asynchronous function. Without some async code in here, the errors disappear
}
}
Thank you
When adopting Swift 6, it’s common to encounter frameworks and libraries that haven’t been audited for sendability. I get pinged about this regularly, so I decided to write up my take on it.
If you have questions or comments, put them in a new thread. Use the Programming Languages > Swift subtopic and tag it with Concurrency; that way I’ll be sure to I see it.
IMPORTANT This is covered really well in the official documentation. Specifically, look at the Under-Specified Protocol section of Migrating to Swift 6. I wrote this up most as an excuse to get it all straight in my head.
Oh, one last thing: This is all based on the Swift 6 compiler in Xcode 16.0b4. Swift concurrency is evolving rapidly, so you might see different results in newer or older compilers.
Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"
Implementing a Main Actor Protocol That’s Not @MainActor
Imagine you’re using the WaffleOMatic framework. It has a WaffleVarnisher class like this:
class WaffleVarnisher {
weak var delegate: Delegate?
protocol Delegate: AnyObject {
func varnisher(_ varnisher: WaffleVarnisher, didVarnish waffle: Waffle)
}
}
class Waffle {
var isGlossy: Bool = false
}
You are absolutely sure that the varnisher calls its delegate on the main thread, but the framework hasn’t been audited for sendability [1]. When you adopt it in a main-actor class, you hit this problem:
@MainActor
class WaffleState: WaffleVarnisher.Delegate {
var lastWaffle: Waffle? = nil
func varnisher(_ varnisher: WaffleVarnisher, didVarnish waffle: Waffle) {
// ^ Main actor-isolated instance method 'varnished(_:didVarnish:)'
// cannot be used to satisfy nonisolated protocol requirement
self.lastWaffle = waffle
}
}
That error has three fix-its:
Add 'nonisolated' to 'varnished(_:didVarnish:)' to make this instance method not isolated to the actor
Add '@preconcurrency' to the 'Delegate' conformance to defer isolation checking to run time
Mark the protocol requirement 'varnished(_:didVarnish:)' 'async' to allow actor-isolated conformances
I’ll discuss each in turn, albeit out of order.
[1] If it had, WaffleVarnisher.Delegate would be annotated with the @MainActor attribute.
Fix-it 3: Apply async
If you choose fix-it 3, Mark the protocol requirement 'varnished(_:didVarnish:)' 'async' to allow actor-isolated conformances, the compiler changes the varnished(_:didVarnish:) to be async:
class WaffleVarnisher {
…
protocol Delegate: AnyObject {
func varnisher(_ varnisher: WaffleVarnisher, didVarnish waffle: Waffle) async
}
}
This is a non-starter because one of our assumptions is that you can’t change the WaffleOMatic framework [1].
[1] If you could, you’d add the @MainActor attribute to WaffleVarnisher.Delegate and this whole problem goes away.
Fix-it 1: Apply non-isolated
If you choose fix-it 1, Add 'nonisolated' to 'varnished(_:didVarnish:)' to make this instance method not isolated to the actor, you get this:
@MainActor
class WaffleState1: WaffleVarnisher.Delegate {
var lastWaffle: Waffle? = nil
nonisolated func varnisher(_ varnisher: WaffleVarnisher, didVarnish waffle: Waffle) {
self.lastWaffle = waffle
// ^ Main actor-isolated property 'lastWaffle' can not be mutated from a non-isolated context
}
}
It’s fixed the original error but now you have a new one. The protocol method is non-isolated, so it can’t access the main-actor-only lastWaffle property.
You can work around this with assumeIsolated(…), but this yields another error:
@MainActor
class WaffleState1: WaffleVarnisher.Delegate {
var lastWaffle: Waffle? = nil
nonisolated func varnisher(_ varnisher: WaffleVarnisher, didVarnish waffle: Waffle) {
// A
MainActor.assumeIsolated {
// B
self.lastWaffle = waffle
// ^ Sending 'waffle' risks causing data races
}
}
}
You’re now passing the waffle object from a non-isolated context (A) to the main-actor-isolated context (B), and you can’t do that because that object is not sendable [1].
You can’t make Waffle sendable because you don’t own the WaffleOMatic framework. That leaves two options. The first is to extract sendable properties from waffle and pass them between the isolation contexts. For example, imagine that you only care about the isGlossy property of the last waffle. In that case, you might write code like this:
@MainActor
class WaffleState1: WaffleVarnisher.Delegate {
var wasLastWaffleGlossy: Bool? = nil
nonisolated func varnisher(_ varnisher: WaffleVarnisher, didVarnish waffle: Waffle) {
let wasGlossy = waffle.isGlossy
MainActor.assumeIsolated {
self.wasLastWaffleGlossy = wasGlossy
}
}
}
Problem solved!
The other option is to disable concurrency checking. There are a variety of ways you might do that. For example, you might apply @preconcurrency on the import, or use an @unchecked Sendable box to transport the waffle, or whatever. I’m not going to discuss these options in detail here because they run counter to the overall goal of Swift concurrency.
[1] Of course both of these contexts are the same!, that is, the main actor context. However, the Swift compiler doesn’t know that. Remember that the goal of Swift concurrency is to have your concurrency checked at compile time, so it’s critical to view errors like this from the perspective of the compiler.
Fix-it 2: Apply preconcurrency
If you choose fix-it 2, Add '@preconcurrency' to the 'Delegate' conformance to defer isolation checking to run time, you get this [1]:
@MainActor
class WaffleState3: @preconcurrency WaffleVarnisher.Delegate {
var lastWaffle: Waffle? = nil
func varnisher(_ varnisher: WaffleVarnisher, didVarnish waffle: Waffle) {
self.lastWaffle = waffle
}
}
This is the best solution to this problem IMO. In this context the @preconcurrency attribute [2] does two things:
It tells the compiler that it can assume that the WaffleVarnisher.Delegate methods are called in the appropriate isolation context for this type. In that case that means the main actor.
It inserts runtime checks to these delegate methods to verify that assumption.
The key advantage of fix-it 2 over fix-it 1 is that compiler knows that the delegate callback is isolated to the main actor, and so:
It doesn’t complain when you access main-actor-isolated constructs like lastWaffle.
It knows that you’re not smuggling waffles across state lines isolation contexts.
[1] Or it will, once we fix the fix-it (r. 132570262) (-:
[2] The @preconcurrency attribute has very different different meanings depending on the context!
Synchronous Results
The advantages of fix-it 2 increase when the delegate protocol includes methods that return a result synchronously. Imagine that the WaffleVarnisher.Delegate protocol has a second callback like this:
class WaffleVarnisher {
…
protocol Delegate: AnyObject {
func varnisher(_ varnisher: WaffleVarnisher, shouldMakeGlossy waffle: Waffle) -> Bool
…
}
}
The fix-it 2 approach lets you implement that delegate using state that’s isolated to the main actor:
@MainActor
class WaffleState: @preconcurrency WaffleVarnisher.Delegate {
var lastWaffle: Waffle? = nil
func varnisher(_ varnisher: WaffleVarnisher, shouldMakeGlossy waffle: Waffle) -> Bool {
return !(self.lastWaffle?.isGlossy ?? false)
}
…
}
In this case it’s possible to solve this problem with the fix-it 1 approach as well, but the code is uglier:
nonisolated func varnisher(_ varnisher: WaffleVarnisher, shouldMakeGlossy waffle: Waffle) -> Bool {
return MainActor.assumeIsolated {
return !(self.lastWaffle?.isGlossy ?? false)
}
}
However, that doesn’t always work. If the delegate method returns a non-sendable type, this approach will fail with a does not conform to the 'Sendable' protocol error.
I'm making a loading screen, but I can't figure out how to make the loading indicator animate smoothly while work is being performed. I've tried a variety of tactics, including creating confining the animation to a .userInitiated Task, or downgrading the loading Task to .background, and using TaskGroups. All of these resulted in hangs, freezes, or incredibly long load times.
I've noticed that standard ProgressViews work fine when under load, but the documentation doesn't indicate why this is the case. Customized ProgressViews don't share this trait (via .progressViewStyle()) also choke up. Finding out why might solve half the problem.
Note: I want to avoid async complications that come with using nonisolated functions. I've used them elsewhere, but this isn't the place for them.
I'm getting a lot of warnings of this within my app. I'm trying to migrate to Swift 6.
I'm going through the migration to Swift 6 and I am running up with a few things. I have two view controllers which conform to the CLLocationManagerDelegate protocol. Both methods of the delegate have the same issue in my code. Below is an example of the warning received.
Main actor-isolated instance method 'locationManagerDidChangeAuthorization' cannot be used to satisfy nonisolated protocol requirement; this is an error in the Swift 6 language mode
Developer Community,
I've noticed a significant change in concurrent task execution behavior when testing on macOS 15 beta 4 &amp; Xcode 16 Beta 4 compared to previous versions. Tasks that previously ran concurrently now appear to execute sequentially, impacting performance and potentially affecting apps relying on concurrent execution.
To illustrate this, I've created a simple toy example:
import SwiftUI
struct ContentView: View {
@State private var results: [String] = []
var body: some View {
VStack {
Button("Run Concurrent Tasks") {
results.removeAll()
runTasks()
}
ForEach(results, id: \.self) { result in
Text(result)
}
}
}
func runTasks() {
Task {
async let task1 = countingTask(name: "Task 1", target: 1000)
async let task2 = countingTask(name: "Task 2", target: 5000)
async let task3 = countingTask(name: "Task 3", target: 1500)
let allResults = await [task1, task2, task3]
results = allResults
}
}
func countingTask(name: String, target: Int) async -&gt; String {
print("\(name) started")
var count = 0
for _ in 0..&lt;target {
count += 1
}
print("\(name) finished. Count: \(count)")
return "\(name) completed. Count: \(count)"
}
}
Observed behavior (macOS 15 Beta 4 &amp; Xcode 16 Beta 4):
Tasks appear to execute sequentially:
Task 1 started
Task 1 finished. Count: 1000
Task 2 started
Task 2 finished. Count: 5000
Task 3 started
Task 3 finished. Count: 1500
Expected behavior:
Tasks start almost simultaneously and finish based on their workload:
Task 1 started
Task 2 started
Task 3 started
Task 1 finished. Count: 1000
Task 3 finished. Count: 1500
Task 2 finished. Count: 5000
Observed behavior in macOS 15 Beta:
The profile reveals that the tasks are executing sequentially. This is evidenced by each task starting only after the previous one has completed.
Hi everyone,
when I was doing some testing on macOS 15 + Xcode 16 Beta 4 I noticed that my app's performance took a significant hit. A simple task that previously was completed within 15 seconds or less now took about a minute to complete.
I came to the conclusion that the only plausible cause could be the way .task {} and asynchronous functions are handled.
Starting several .task{} and calling async functions from within using macOS 14.5 and Xcode 15.4 results in following log output:
task1 started
task3 started
task2 started
task4 started
--> task2 ended
--> task3 ended
--> task4 ended
--> task1 ended`
Running the same code on macOS 15.0 + Xcode 16 Beta 4 will result in the following log output:
task1 started
--> task1 ended
task2 started
--> task2 ended
task3 started
--> task3 ended
task4 started
--> task4 ended
In the first example the code is executed in 'parallel'. All tasks are started and doing there respective work. In second example a task is started and we are waiting for it to complete before the other tasks are started.
I could start to rewrite my code to get the results I desire, however I'm wondering if this is a bug in regards to macOS 15 + Xcode 16 Beta 4 and the way .task {} and asynchronous functions are handled. The output is quite different after all.
What's your take on this? If you want to try it out for yourself you can use the following sample code:
import SwiftUI
struct ContentView: View {
func func1() async -> Int {
print("task1 started")
var myInt: Int = 0
while myInt < 999999999 {
myInt += 1
}
print(" --> task1 ended")
return 1
}
func func2() async -> Int {
print("task2 started")
var myInt: Int = 0
while myInt < 999999 {
myInt += 1
}
print(" --> task2 ended")
return 2
}
func func3() async -> Int {
print("task3 started")
var myInt: Int = 0
while myInt < 999999 {
myInt += 1
}
print(" --> task3 ended")
return 3
}
func func4() async -> Int {
print("task4 started")
var myInt: Int = 0
while myInt < 999999999 {
myInt += 1
}
print(" --> task4 ended")
return 4
}
var body: some View {
VStack {
Text("Hello, world!")
}
.task {
await func1()
}
.task {
await func2()
}
.task {
await func3()
}
.task {
await func4()
}
}
}
#Preview {
ContentView()
}
According to the documentation (https://developer.apple.com/documentation/usernotificationsui/unnotificationcontentextension), a Notification Content Extension should consist of a UIViewController that adopts the UNNotificationContentExtension protocol.
The only problem is that UNNotificationContentExtension's methods are not @MainActor isolated, but UIViewController is, which produces this error when you try to build your code with Complete concurrency checking turned on:
Main actor-isolated instance method 'didReceive' cannot be used to satisfy nonisolated protocol requirement
If you add nonisolated, you are then left with another problem in that UNNotification is not Sendable.
What is the recommended solution to this problem?
Let's say I have a Task that I want to extend into the background with beginBackgroundTask(expirationHandler:). Furthermore, I'd like to leverage cooperative cancelation of subtasks when responding to the expiration handler. Unfortunately, the expirationHandler: closure parameter is not async, so I'm unable to do something like:
actor MyTaskManagerOne {
var backgroundID = UIBackgroundTaskIdentifier.invalid
func start() {
Task {
let doTheWorkTask = Task {
await self.doTheWork()
}
backgroundID = await UIApplication.shared.beginBackgroundTask {
doTheWorkTask.cancel()
// next line: compile error, since not an async context
await doTheWorkTask.value // ensure work finishes up
// next line: generates MainActor compilation warnings despite docs allowing it
UIApplication.shared.endBackgroundTask(self.backgroundID)
}
await doTheWorkTask.value
}
}
func doTheWork() async {}
}
So instead, I think I have to do something like this. It, however, generates runtime warnings, since I'm not directly calling endBackgroundTask(_:) at the end of the expirationHandler:
actor MyTaskManagerTwo {
var backgroundID = UIBackgroundTaskIdentifier.invalid
func start() {
Task {
let doTheWorkTask = Task {
await self.doTheWork()
}
backgroundID = await UIApplication.shared.beginBackgroundTask {
doTheWorkTask.cancel()
// 1. not calling endBackgroundTask here generates runtime warnings
}
await doTheWorkTask.value
// 2. even though endBackgroundTask gets called
// here (as long as my cooperative cancellation
// implementations abort quickly in `doTheWork()`)
await UIApplication.shared.endBackgroundTask(self.backgroundID)
}
}
func doTheWork() async {}
}
As best I can tell, the MyTaskManagerTwo actor works and does not cause a watchdog termination (as long as cancellation is sufficiently fast). It is, however, producing the following runtime warning:
Background task still not ended after expiration handlers were called: <_UIBackgroundTaskInfo: 0x302753840>: taskID = 2, taskName = Called by libswift_Concurrency.dylib, from <redacted>, creationTime = 9674 (elapsed = 28). This app will likely be terminated by the system. Call UIApplication.endBackgroundTask(_:) to avoid this.
Is the runtime warning ok to ignore in this case?
AVAudioFormat has no Swift concurrency annotations but the documentation states "Instances of this class are immutable."
This made me always assume it was safe to pass AVAudioFormat instances around. Is this the case? If so can it be marked as Sendable? Am I missing something?
Usage of multiple async lets crashes the app in a nondeterministic fashion. We are experiencing this crash in production, but it is rare.
0 libswift_Concurrency.dylib 0x20a8b89b4 swift_task_create_commonImpl(unsigned long, swift::TaskOptionRecord*, swift::TargetMetadata<swift::InProcess> const*, void (swift::AsyncContext* swift_async_context) swiftasynccall*, void*, unsigned long) + 384
1 libswift_Concurrency.dylib 0x20a8b6970 swift_asyncLet_begin + 36
We managed to isolate the issue, and we submitted a technical incident (Case-ID: 8007727). However, we were completely ignored, and referred to the developer forums.
To reproduce the bug you need to run the code on a physical device and under instruments (we used swift concurrency). This bug is present on iOS 17 and 18, Xcode 15.1, 15.4 and 16 beta, swift 5 and 6, including strict concurrency.
Here's the code for Swift 6 / Xcode 16 / strict concurrency:
(I wanted to attach the project but for some reason I am unable to)
typealias VoidHandler = () -> Void
enum Fetching { case inProgress, idle }
protocol PersonProviding: Sendable {
func getPerson() async throws -> Person
}
actor PersonProvider: PersonProviding {
func getPerson() async throws -> Person {
async let first = getFirstName()
async let last = getLastName()
async let age = getAge()
async let role = getRole()
return try await Person(firstName: first,
lastName: last,
age: age,
familyMemberRole: role)
}
private func getFirstName() async throws -> String {
try await Task.sleep(nanoseconds: 1_000_000_000)
return ["John", "Kate", "Alex"].randomElement()!
}
private func getLastName() async throws -> String {
try await Task.sleep(nanoseconds: 1_400_000_000)
return ["Kowalski", "McMurphy", "Grimm"].randomElement()!
}
private func getAge() async throws -> Int {
try await Task.sleep(nanoseconds: 2_100_000_000)
return [56, 24, 11].randomElement()!
}
private func getRole() async throws -> Person.Role {
try await Task.sleep(nanoseconds: 500_000_000)
return Person.Role.allCases.randomElement()!
}
}
@MainActor
final class ViewModel {
private let provider: PersonProviding = PersonProvider()
private var fetchingTask: Task<Void, Never>?
let onFetchingChanged: (Fetching) -> Void
let onPersonFetched: (Person) -> Void
init(onFetchingChanged: @escaping (Fetching) -> Void,
onPersonFetched: @escaping (Person) -> Void) {
self.onFetchingChanged = onFetchingChanged
self.onPersonFetched = onPersonFetched
}
func fetchData() {
fetchingTask?.cancel()
fetchingTask = Task {
do {
onFetchingChanged(.inProgress)
let person = try await provider.getPerson()
guard !Task.isCancelled else { return }
onPersonFetched(person)
onFetchingChanged(.idle)
} catch {
print(error)
}
}
}
}
struct Person {
enum Role: String, CaseIterable { case mum, dad, brother, sister }
let firstName: String
let lastName: String
let age: Int
let familyMemberRole: Role
init(firstName: String, lastName: String, age: Int, familyMemberRole: Person.Role) {
self.firstName = firstName
self.lastName = lastName
self.age = age
self.familyMemberRole = familyMemberRole
}
}
import UIKit
class ViewController: UIViewController {
@IBOutlet private var first: UILabel!
@IBOutlet private var last: UILabel!
@IBOutlet private var age: UILabel!
@IBOutlet private var role: UILabel!
@IBOutlet private var spinner: UIActivityIndicatorView!
private lazy var viewModel = ViewModel(onFetchingChanged: { [weak self] state in
switch state {
case .idle:
self?.spinner.stopAnimating()
case .inProgress:
self?.spinner.startAnimating()
}
}, onPersonFetched: { [weak self] person in
guard let self else { return }
first.text = person.firstName
last.text = person.lastName
age.text = "\(person.age)"
role.text = person.familyMemberRole.rawValue
})
@IBAction private func onTap() {
viewModel.fetchData()
}
}
Good day.
I'm having problems debugging hangs. Watched the video WWDC Analyze hangs with Instruments. Apparently the main thread is blocked. It’s just not possible to analyze what exactly is blocking the flow. Please help me figure it out.
https://drive.google.com/file/d/1QvFK4y79CG0fBMrJPOrBCTuWsOaqichY/view?usp=share_link <- trace
I'm continuing with the migration towards Swift 6. Within one of our libraries, I want to check whether a parameter object: Any? confirms to Sendable.
I tried the most obvious one:
if let sendable = object as? Sendable {
}
But that results into the compiler error "Marker protocol 'Sendable' cannot be used in a conditional cast".
Is there an other way to do this?
I have the following code:
extension AssetGridViewController: PHPhotoLibraryChangeObserver {
func photoLibraryDidChange(_ changeInstance: PHChange) {
Task { @MainActor in
guard let changes = changeInstance.changeDetails(for: fetchResult) else { return }
fetchResult = changes.fetchResultAfterChanges
}
}
}
With Swift 6, this generates a compilation error: Main actor-isolated instance method 'photoLibraryDidChange' cannot be used to satisfy nonisolated protocol requirement. The error includes to fix-it suggestions:
Adding nonisolated to the function (nonisolated func photoLibraryDidChange(_ changeInstance: PHChange))
Adding @preconcurrency to the protocol conformance (extension AssetGridViewController: @preconcurrency PHPhotoLibraryChangeObserver {)
Both options generate a runtime error: EXC_BREAKPOINT (code=1, subcode=0x105b7c400). For context, AssetGridViewController is a regular UIViewController.
Any ideas on how to fix this?