Even if you assign the `@MainActor` attribute to a closure, the processing inside the closure is not executed on the main thread

Using Swift
1

I was experimenting with writing Global Actor code on hand while watching SE-0316. At that time, even though @MainActor was given, it was not executed on the main thread. Here is the code:

import UIKit

class ViewController: UIViewController {
    var callback: () -> Void = { @MainActor in
        print(Thread.isMainThread)  // false
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        Task.detached {
            await self.callback()
        }
    }
}

Adding async to the closure's type annotation executed it in the main thread as intended. Is the earlier code a bug? Please let me know if there is any knowledge that is missing, not a bug.

import UIKit

class ViewController: UIViewController {
    var callback: () async -> Void = { @MainActor in
        print(Thread.isMainThread)  // true
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        Task.detached {
            await self.callback()
        }
    }
}
  • Swift version: 5.10
  • Xcode: 15.3
1 Like
2

Looks like a bug.

Making it a function also fixes it.

class ViewController: UIViewController {
    func callback() {
        ...
    }
    ...
}
3

Oh, thanks.

In that example, it would be true. It's ok to understand.
Detatched Task does not inherit actor context, but since UIViewContrller is implicitly isolated to MainActor, the callback is isolated to MainActor, so it is executed on the main thread.

import UIKit

class ViewController: UIViewController {
    func callback() {
        print(Thread.isMainThread)  // true
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        Task.detached {
            await self.callback()
        }
    }
}
4

You could also explicitly call over to the MainActor if that’s easier to read:

Task {
    MainActor.run {
        await self.callback()
    }
}

I sometimes find this easier for me to remember what I was aiming for. Kind of the equivalent to “main.async { }” from the DispatchQueue technique.

1 Like
5

That's for sure.

I checked additionally, and when I removed the type annotation as shown below, it was executed on the main thread. It seems that the type annotation I wrote was incorrect.

import UIKit

class ViewController: UIViewController {
    var callback = { @MainActor in
        print(Thread.isMainThread)  // true
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        Task.detached {
            await self.callback()
        }
    }
}
1 Like
6

Hmm, perhaps the type of callback is not "() -> Void" but "() async -> Void" in this case, as ViewController is implicitly an actor by virtue of UIViewController?

7

Yes, it seems so. It is probably no longer isolated to MainActor due to the () -> Void type annotation.

8

If you turn on strict concurrency warnings you will see that there is a problem with this code:

let callback: () -> Void = { @MainActor in … }

:warning: Converting function value of type '@MainActor () -> Void' to '() -> Void' loses global actor 'MainActor'; this is an error in Swift 6

10 Likes
9

The correct type annotation would be var callback: @MainActor () -> Void. (…As seen in the above compiler diagnostic.)

1 Like
10

Right. Actor isolation has to be represented in the type of a sync function, or else it just can’t be automatically honored — it’s sync, so only the caller can make sure it runs with the right isolation. In non-strict mode, Swift has to assume that you’re getting it right dynamically somehow, but strict mode makes the compiler diagnose it immediately. SE-0423, if accepted, will at least make Swift diagnose the failure dynamically.

8 Likes
11

Sorry for jumping with my topic but I have a slightly different yet similar problem with understanding how @MainActor annotation is preserved while passing around a closure.

Could someone look at why it is not respected even if the closure type is annotated with @MainActor?