CODEWITHSUNDEEP
iosswiftasync-await
Jordan H
Jordan H

Reputation: 55715

Defer block in Task causes "Main actor-isolated property 'someProperty' can not be mutated from the main actor"

I'm writing a cancelable async function which does a lot of work with many early returns interspersed if the task has been canceled. I need to perform some post processing tear-down work when the task successfully runs to completion or is canceled. To accomplish that, I tried to put this logic in a defer block but this reveals an odd error message:

Main actor-isolated property 'isPerformingAsyncWork' can not be mutated from the main actor

Is this not a direct contradiction, saying this property can only be modified on the main actor and you cannot mutate it from the main actor? 😅 How do you resolve this or otherwise execute some code when the task finishes/gets canceled?

Here's some sample code to demonstrate the problem:

class ViewController: UIViewController {
    
    var isPerformingAsyncWork = false

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        
        performAsyncWork()
    }
    
    func performAsyncWork() {
        guard !isPerformingAsyncWork else { return }
        
        let alert = UIAlertController(title: "Working on it…", message: nil, preferredStyle: .alert)
        
        let task = Task {
            defer {
                //called upon task completion or cancelation
                
                //FIXME: Main actor-isolated property 'isPerformingAsyncWork' can not be mutated from the main actor
                isPerformingAsyncWork = false
            }
            
            isPerformingAsyncWork = true
            
            let url = URL(string: "https://hws.dev/user-favorites.json")!
            let (data, _) = try await URLSession.shared.data(from: url)
            
            guard !Task.isCancelled else { return }
            
            let values = try JSONDecoder().decode([Int].self, from: data)
            
            guard !Task.isCancelled else { return }
            
            //more async work here, more isCancelled checks, etc...

            alert.presentingViewController?.dismiss(animated: true)
        }
        
        alert.addAction(UIAlertAction(title: "Cancel", style: .cancel) { action in
            task.cancel()
        })
        present(alert, animated: true)
    }

}

Upvotes: 14

Views: 4648

Answers (1)

vadian
vadian

Reputation: 285064

You can spawn another Task dispatched explicitly to the MainActor

let task = Task {
    defer {
        //called upon task completion or cancelation
        Task { @MainActor in
            isPerformingAsyncWork = false
        }
    }
...

Upvotes: 14

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