Call to main actor-isolated instance method [...] in a synchronous nonisolated context; this is an error in the Swift 6 language mode

Using Swift
1

Hi all, I need some guidance to understand this warning I am getting while migrating the app to Swift 6. I've enabled the Struct Concurrency Checking to Complete. I have the following SwiftUI view:

struct HolidaysListView: View {
    let holidays: [Holiday]
    @Binding var path: NavigationPath
    
    var body: some View {
        VStack {
            GeometryReader { proxy in
                let size = proxy.size
                ScrollView(.horizontal) {
                    HStack(spacing: 0) {
                        ForEach(holidays) { holiday in
                            HolidayListViewItem(holiday: holiday, path: $path)
                                .padding(.horizontal)
                                .frame(width: size.width)
                                .visualEffect { content, geometryProxy in
                                    content
                                        .scaleEffect(scale(geometryProxy), anchor: .trailing)
                                        .rotationEffect(rotation(geometryProxy))
                                        .offset(x: minX(geometryProxy))
                                        .offset(x: excessMinX(geometryProxy))
                                }
                                .zIndex(holidays.zIndex(of: holiday))
                        }
                    }
                    .padding(.vertical)
                }
                .scrollTargetBehavior(.paging)
                .scrollIndicators(.hidden)
            }
            .aspectRatio(contentMode: .fit)
        }
    }
    
    private func minX(_ proxy: GeometryProxy) -> CGFloat {
        let minX = proxy.frame(in: .scrollView(axis: .horizontal)).minX
        return minX < 0 ? 0 : -minX
    }
    
    private func progess(_ proxy: GeometryProxy) -> CGFloat {
        let maxX = proxy.frame(in: .scrollView(axis: .horizontal)).maxX
        guard let width = proxy.bounds(of: .scrollView(axis: .horizontal))?.width else {
            return 0
        }
        let visibleCards = CGFloat(2)
        return min((maxX / width) - 1, visibleCards)
    }
    
    private func scale(_ proxy: GeometryProxy) -> CGFloat {
        let scaleFactor = 0.1
        return 1 - (progess(proxy) * scaleFactor)
    }
    
    private func excessMinX(_ proxy: GeometryProxy) -> CGFloat {
        let offset: CGFloat = 10
        return progess(proxy) * offset
    }
    
    private func rotation(_ proxy: GeometryProxy) -> Angle {
        let rotation: CGFloat = 5
        return Angle(degrees: progess(proxy) * rotation)
    }
}

And in the visualEffect modifier, I am getting 4 warnings like this:

Screenshot 2024-06-17 at 12.09.29
Screenshot 2024-06-17 at 12.09.291458×400 76.1 KB

It seems GeometryProxy is not Sendable.
I am lost on what to do here. Any feedback and explanations are much appreciated.

2

By default, methods of any struct conforming to View inherit the Main Actor isolation. So here scale, rotation, minX and excessMinX are implicitly @MainActor as well.

You don't want that because visualEffect's closure is not Main Actor isolated, so attempting to call any of those methods from within said closure raises a "Call to main actor-isolated instance method 'rotation' in a synchronous nonisolated context" warning.

All you need to do is mark the methods as nonisolated:

nonisolated private func minX(_ proxy: GeometryProxy) -> CGFloat { ... }
  
nonisolated private func progess(_ proxy: GeometryProxy) -> CGFloat { ... }
  
nonisolated private func scale(_ proxy: GeometryProxy) -> CGFloat { ... }
  
nonisolated private func excessMinX(_ proxy: GeometryProxy) -> CGFloat { ... }
  
nonisolated private func rotation(_ proxy: GeometryProxy) -> Angle { ... }

By removing this implicit @MainActor isolation, these methods will be able to be called from inside visualEffect { ... }. It's safe to pass a GeometryProxy to them because you're not crossing an actor boundary by doing so.

1 Like
3

Thanks @Andropov
I appreciate the time to answer my question. Need to learn better Concurrency.

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