Use reference to captured variable in concurrently-executing code

Question

Update: This question gets a lot of views. If you think the question can be enhanced with the situation in which you encountered the error yourself, please briefly describe your situation in the comments so we can make this Q&A more valuable. And if you have a solution to your version of the problem, please add it as an answer.

Update 2: I suspect the question gets upvoted because of the possible solution that I describe. Highlighted it for clarity.

---

I want to update the UI after doing async background work using Task.detached and an async function.

However, I get a build error Reference to captured var 'a' in concurrently-executing code error during build.

I tried some things and turning the variable into a let constant before updating the UI is the only thing that works. Why do I need to make a let constant before being able to update the UI? Are there alternatives?

class ViewModel: ObservableObject {
    
    @Published var something: String?
    
    init() {
        Task.detached(priority: .userInitiated) {
            await self.doVariousStuff()
        }
    }
    
    private func doVariousStuff() async {
        var a = "a"
        let b = await doSomeAsyncStuff()
        a.append(b)
        
        something = a /* Not working,
        Gives
           - runtime warning `Publishing changes from background threads 
                is not allowed; make sure to publish values from the main 
                thread (via operators like receive(on:)) on model updates.`
         or, if `something` is @MainActor:
           - buildtime error `Property 'something' isolated to global 
                actor 'MainActor' can not be mutated from this context`
         */



        await MainActor.run { 
            something = a 
        } /* Not working, 
        Gives buildtime error "Reference to captured var 'a' in 
         concurrently-executing code" error during build
         */


        DispatchQueue.main.async { 
            self.something = a 
        } /* Not working,
        Gives buildtime error "Reference to captured var 'a' in 
         concurrently-executing code" error during build
         */


        /*
         This however, works!
         */
        let c = a
        await MainActor.run {
            something = c
        }

    }
    
    private func doSomeAsyncStuff() async -> String {
        return "b"
    }
} 

Answer 1

Make your observable object as main actor, like

@MainActor                                // << here !!
class ViewModel: ObservableObject {

    @Published var something: String?

    init() {
        Task.detached(priority: .userInitiated) {
            await self.doVariousStuff()
        }
    }

    private func doVariousStuff() async {
        var a = "a"
        let b = await doSomeAsyncStuff()
        a.append(b)

        something = a         // << now this works !!
    }

    private func doSomeAsyncStuff() async -> String {
        return "b"
    }
}

Tested with Xcode 13 / iOS 15

backup

Answer 2

You can use @State and .task as follows:

struct ContentView: View {
    @State var result = ""

    var body: some View {
        HStack {
            Text(result)
        }
        .task {
            result = await Something.doSomeAsyncStuff()  
        }
    }
}

The task is started when view appears and is cancelled when it disappears. Also if you use .task(id:) it will restart (also cancelling previous task) when the value of id changes.

The async func can go in a few different places, usually somewhere so it can tested independently.

struct Something {
    static func doSomeAsyncStuff() async -> String {
        return "b"
    }
}