How to cancel a long term Task in ViewModel?

Question

This is simple test but not simple. HomeView show TestView as a sheet, TestView will hide almost actions (no .task, onReceive...) into TestViewModel, TestViewModel will detect device orientation and show it on TestView. when dismiss TestView, stop the detection.

Almost works fine but when dismiss TestView, print("TestViewModel deinit.") not happened, and the detection is still working.

I think Task have a reference of TestViewModel, that causes TestViewModel cannot release. But how to fix?

OK, next question is how to cancel the Task in TestViewModel(not in TestView) when dismiss TestView?

Any suggestion?

struct HomeView: View {
    @State var showTestView = false
    
    var body: some View {
        Button("Show Test View") {
            showTestView = true
        }.sheet(isPresented: $showTestView) {
            TestView()
        }
    }
}

struct TestView: View {
    @StateObject private var vm = TestViewModel()
    @Environment(\.dismiss) var dismiss
    
    var body: some View {
        VStack {
            Text("isPortrait = \(vm.isPortrait.description)")
            Button("Dismiss") {
                dismiss()
            }
        }
        .onDisappear {
            print("TestView onDisappear.")
        }
    }
}

@MainActor
class TestViewModel: ObservableObject {
    @Published var isPortrait = false
    
    init() {
        print("TestViewModel init.")
        setup()
    }
    
    deinit {
        print("TestViewModel deinit.")
    }
    
    func setup() {
        Task {
            await observeNotification()
        }
    }
    
    private func observeNotification() async {
        let sequence = NotificationCenter.default.notifications(named: UIDevice.orientationDidChangeNotification)
            .map { _ in await UIDevice.current.orientation }
        for await value in sequence {
            print("orientationDidChangeNotification changed, orientation = \(value).")
            isPortrait = value == .portrait
        }
    }
}

Does this answer your question? [SwiftUI: @StateObject deinit NOT called?](https://stackoverflow.com/questions/71696080/swiftui-stateobject-deinit-not-called) — Ashley Mills, Feb 24 '23 at 09:17
try removing the view model object those aren't needed in SwiftUI that is what the View struct is for. — malhal, Feb 24 '23 at 10:50
What is the issue, you don't want to use the `.task()` modifier? — Louis Lac, Feb 24 '23 at 11:34
@LouisLac YES, I do not want to use .task{} in View that is just for show, I want to do almost anything in ViewModel. — foolbear, Feb 24 '23 at 13:00
@malhal I don't think so. View is for showing, ViewModel get any datas ready for View. I think this is typical/classic MVVM design pattern for SwiftUI. — foolbear, Feb 24 '23 at 13:08
@foolbear no, in SwiftUI the View is for getting the data ready. SwiftUI takes the View data and then it creates/updates/removes UIKit UIViews automatically for us. — malhal, Feb 24 '23 at 13:28
@malhal, I think viewmodel produce datas, view consume datas for showing. — foolbear, Feb 24 '23 at 14:35
@foolbear no, the model produces data, View struct transforms the data for showing as it passes down the View hierarchy. — malhal, Feb 24 '23 at 15:23
@foolbear In SwiftUI, you can consider the View to be its own ViewModel. No need for separate classes. Look at how much more complicated it becomes. — Ashley Mills, Feb 24 '23 at 15:25

Louis Lac · Accepted Answer · 2023-02-25T11:23:29.353

You need to manage the cancellation of the task yourself: keep a reference to the background task in TaskViewModel and cancel the task when the object is destroyed.

@MainActor
final class TestViewModel: ObservableObject {
    @Published var isPortrait = false
    private var task: Task<Void, Never>? = nil
    
    init() {
        print("TestViewModel init.")
        setup()
    }
    
    deinit {
        print("TestViewModel deinit.")
        task?.cancel()
        task = nil
    }
    
    func setup() {
        guard task == nil else { return }
        
        task = Task { [weak self] in
            let sequence = NotificationCenter.default.notifications(
                named: UIDevice.orientationDidChangeNotification
            )
            .map { _ in
                await UIDevice.current.orientation
            }
            
            for await value in sequence {
                if Task.isCancelled {
                    print("Task cancelled")
                    return
                }
                
                print("orientationDidChangeNotification changed, orientation = \(value).")
                self?.isPortrait = value == .portrait
            }
            
            print("Observation ended")
        }
    }
}

As you guessed, you must treat self as a weak reference inside the fire up background task, otherwise a reference cycle will keep both objects alive in memory.

A behavior I did not find well documented is that the async iterator returned by the .notification()method seems to handle cancellation, as such, the iterator returns nil when the task is cancelled and the for loop is ended.

That being said, the .task() view modifier is the most convenient and a safer way to achieve what you are trying to do. It is designed specifically for this purpose.

Great job. it totally works, thanks! And I know .task{} will automatic cancel the child task, it's a best practice. But .task{} works wired for list item and navigation stack and etc. And I want to keep view neatly and viewmodel do those routines itself. If a solution works, It will be easily reproduced to any other kind of thing. — foolbear, Feb 24 '23 at 15:02
But put the testview into list(as list item). the issue happen again, cannot deinit. — foolbear, Feb 24 '23 at 15:06
By putting multiple `TestView`s into a list (in `HomeView`) I got the correct behavior, all the models are being reinitialized. Could you share the code you are using? — Louis Lac, Feb 25 '23 at 11:33
add a new TestView item into a list, than delete it, but no print("TestViewModel deinit."). just put a list in HomeView, TestView as a list item instead of a sheet. — foolbear, Feb 25 '23 at 11:46
Please edit your question with your new code or create a new question, it's hard to understand what you are trying to do from comments. — Louis Lac, Feb 25 '23 at 12:01

How to cancel a long term Task in ViewModel?

1 Answers1