Say you have a SwiftData object that doesn't allow optionals:
@Model
class MySet: Identifiable {
var id: UUID
var weight: Int
var reps: Int
var isCompleted: Bool
var exercise: Exercise
Then you get from your MySet data a list of these MySets and append them into a State:
@State var sets: [MySet]
if let newSets = exercise.sets { //unwrapping the passed exercises sets
if (!newSets.isEmpty) { //we passed actual sets in so set them to our set
sets = newSets
}
And you use that State array in a ForEach and bind the Textfield directly to the data like so:
ForEach($sets){ $set in
TextField("\(set.weight)", value: $set.weight, formatter: NumberFormatter())
.keyboardType(.decimalPad)
TextField("\(set.reps)", value: $set.reps,formatter: NumberFormatter())
.keyboardType(.decimalPad)
}
This will bind whatever is written into the TextField directly to the SwiftData object which is a problem because say you have 50 written and you want to change it to 60 you need to be able to clear the 50 to write the 60 and since the SwiftData object doesn't allow nil it will allow you to remove the 0 but not the 5. Is there anyway to remedy this without having the swiftData object allow optionals for example catching the clearing of the TextField and making it 0 instead of nil when its cleared?
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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.
This bites me a lot. I'm looking at the documentation for, say, UNUserNotificationCenter.
And NOWHWERE but NOWHERE do I see anything that says, "hey, on platform *** you should import YYY to use this class."
Am I just not looking in the right place in Apple documentation to find this?
Surely, somewhere at the top level of documentation, it must tell you want the proper package to import is, per platform?
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
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.
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
hi
im fairly new to coding.. about a month, just so u know.. 😇
I am going through the tasks in (Swift Playgrounds) and I am currently in (Learning to code 2) the chapter is (Random gems everywhere) and I tried every possible solution to complete it, but to no avail!
any suggestions, comments, or corrections or tips, would be greatly appreciated!
I will attach some screenshots for a reference of what I'm trying to accomplish..
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()
}
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
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.
Problem Statement:
Unable to import .h file from an ObjC SPM to a .h file in an ObjC file in a framework with mix of ObjC and Swift code
The issue is: in order to support access of ObjC file in Swift code in a framework we need to use umbrella header (in place of bridging header). Once the file is imported in Umbrella header and made public it will no longer allow import of .h file from package in its interface
Project Structure:
a. Package: ObjCPackage
ObjCPackage
|- Package.swift
|- ObjCPackage
MathsUtilities.h (class interface)
MathsUtilities.m (class implementation)
NiceLogs.h (protocol)
b. Project: ObjCSwiftFramework
ObjCSwiftFramework
|- ObjCSwiftFramework.h (umbrella header)
|- Calculation.h (objc class interface)
|- Calculation.m (objc class implementation)
|- SwiftCalci.swift (swift class)
Details:
#import <ObjCSwiftFramework/Calculation.h> added in ObjCSwiftFramework.h as Calculation has to be used in SwiftCalci
Calculation.h marked as public in target membership in Xcode so that it can be added in umbrella header ObjCSwiftFramework.h
#import "NiceLogs.h" in Calculation.h gives error
Here is a small sample which I created to demonstrate the problem:
ObjCSwiftFramework
So any time I create a class that's both @Observable and Codable, e.g.
@Observable class GameLocationManager : Codable {
I get a warning in the macro expansion code:
@ObservationIgnored private let _$observationRegistrar = Observation.ObservationRegistrar()
Immutable property will not be decoded because it is declared with an initial value which cannot be overwritten.
I've been ignoring them for now, but there are at least a half a dozen of them now in my (relatively small) codebase, and I'd like to find a solution (ideally one that doesn't require me to write init(decoder:) for every @Observable class in my project...), especially since I'm not sure what the actual consequences of ignoring this might be.
Example1:
let num = NSDecimalNumber(string: "0.123456789012345678909")
let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.usesGroupingSeparator = true
formatter.maximumFractionDigits = 25
formatter.minimumFractionDigits = 25
formatter.minimumIntegerDigits = 1
let str = formatter.string(from: num) ?? ""
print(str)
output
"0.1234567890123460000000000"
Example2:
let num = NSDecimalNumber(string: "12323.123456789012345678909")
let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.usesGroupingSeparator = true
formatter.maximumFractionDigits = 25
formatter.minimumFractionDigits = 25
formatter.minimumIntegerDigits = 1
let str = formatter.string(from: num) ?? ""
print(str)
output
"12,323.1234567890000000000000000"
How to correctly format the contents of the above two inputs？
For some time now Xcode has been downloading crash reports from users of my app about crashes related to arrays. One of them looks like this:
...
Code Type: ARM-64
Parent Process: launchd [1]
User ID: 501
Date/Time: 2024-07-18 14:59:40.4375 +0800
OS Version: macOS 15.0 (24A5289h)
...
Crashed Thread: 0
Exception Type: EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000001, 0x00000001045048b8
Termination Reason: Namespace SIGNAL, Code 5 Trace/BPT trap: 5
Terminating Process: exc handler [1771]
Thread 0 Crashed:
0 MyApp 0x00000001045048b8 specialized Collection.map<A>(_:) + 596
1 MyApp 0x00000001045011e4 MyViewController.validateToolbarButtons() + 648 (MyViewController.swift:742)
...
The relevant code looks like this:
class MyViewController {
func validateToolbarButtons() {
let indexes = tableView.clickedRow == -1 || tableView.selectedRowIndexes.contains(tableView.clickedRow) ? tableView.selectedRowIndexes : IndexSet(integer: tableView.clickedRow)
let items = indexes.map({ myArray[$0] })
...
}
}
The second crash looks like this:
...
Code Type: X86-64 (Native)
Parent Process: launchd [1]
User ID: 502
Date/Time: 2024-07-15 15:53:35.2229 -0400
OS Version: macOS 15.0 (24A5289h)
...
Crashed Thread: 0
Exception Type: EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes: 0x0000000000000001, 0x0000000000000000
Termination Reason: Namespace SIGNAL, Code 4 Illegal instruction: 4
Terminating Process: exc handler [13244]
Thread 0 Crashed:
0 libswiftCore.dylib 0x00007ff812904fc0 _assertionFailure(_:_:flags:) + 288
1 MyApp 0x0000000101a31e04 specialized _ArrayBuffer._getElementSlowPath(_:) + 516
2 MyApp 0x00000001019d04eb MyObject.myProperty.setter + 203 (MyObject.swift:706)
3 MyApp 0x000000010192f66e MyViewController.controlTextDidChange(_:) + 190 (MyViewController.swift:166)
...
And the relevant code looks like this:
class MyObject {
var myProperty: [MyObject] {
get {
...
}
set {
let items = newValue.map({ $0.id })
...
}
}
}
What could cause such crashes? Could they be caused by anything other than concurrent access from multiple threads (which I'm quite sure is not the case here, as I only access these arrays from the main thread)?
Hi, I'm new to swift but have experience with coding in general. Following the app dev training tutorial, came across this line of code:
var wrapper: ErrorWrapper {
ErrorWrapper(error: someVal)
}
My question is, why not just do this...
var wrapper: ErrorWrapper = ErrorWrapper(error: someVal)
Is it a conventions thing or is there some purpose, code seems to work either way. My understanding of closures is that they are just lambda functions, so in the first codeblock, all it's doing is calling a function that returns the instantiated ErrorWrapper object. Why not just assign the variable to it?
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).
Hi all,
I just wanted to ask how people were using ModelActor with the Swift 6 language mode enabled. My current implementation involves passing the ModelContainer to my ModelActor, which worked in Sonoma and previous betas of Sequoia, however in the current Beta 3, I get this error:
"Sending 'self.modelContext.container' risks causing data races"
I am a bit confused by this, as from what I understand, ModelContainer conforms to Sendable, so ideally this error should not be thrown. Is this a bug in Beta 3?
Thanks in advance.
When i try to run break statement in default case, it doesn't run and shows 'Closure containing control flow statement cannot be used with result builder 'ViewBuilder'' error.Why it doesn't run and how to solve this trouble?
Hi, everyone. I am trying to print out “Hello, world” by creating a Command Line tool project. I try to type “swiftc Hello.swift” to compile it in the terminal but instead of having a file named Hello.swift, I have a file named by default Hello.xcodeprojec. How I can solve this issue?
Just when I think I am finally starting to understand Swift I come across a gotcha like the following:
I have two object, a swiftui display and one of data to be displayed. Ok, sounds easy.
The data is read out of a JSON file so I have a set of arrays and dictionaries. The data is valid when read, it is definitely there, but when I go to display it, its gone. Just vanished. Wasted about a day on this so far, and I’ve seen it before, the inability to pass out of an object an array or dictionary with contents intact.
If I create an array var, and not let the system do it, contents are preserved. So, in the data object I’ll have something like this:
struct DataObject{
var item: [String:Any]
item=JSONData serialized out of memory, and may have say, 12 fields
}
In my SwiftUI module I have:
var item=dataObject.item
dataObject.item now has 0 fields.
I can allocate and initialize a dictionary in DataObject and those elements come through fine. So it seems like the stuff being serialized from JSON is being deleted out from under me.