如果包装在 NavigationView (macOS) 中，则 SwiftUI 列表选择绑定在每次单击时更新两次

Question

Given the below code, I made a strange observation (on macOS). If a List is wrapped in a NavigationView I suddenly get two updates per click on a row (one on mouseDown - aka holding the mouse click down and not releasing it, one on mouseUp - aka releasing the mouse click). This doesn't happen on a plain simple list, or if it's wrapped in an HStack instead. Does anyone know why and how I can control / change this behavior?

See HStack version in action:

See NavigationView version in action:

struct ContentView: View {
    @State var selection: Set<Int> = []

    var body: some View {
        List(0..<20, selection: Binding(get: {
            self.selection
        }, set: { val in
            print("set \(val)")
            self.selection = val
        })) { idx in
            Text("\(idx)")
        }

        Color.red
        Color.green
        Color.blue
    }
}

// Wrapped in a HStack, 1 update per row selection is triggered, as expected!
struct HStackVersion: View {
    var body: some View {
        HStack(spacing:0.0) {
            ContentView()
        }
    }
}

// Wrapped in a NavigationView, 2 updates per row selection are triggered??
struct NavigationViewVersion: View {
    var body: some View {
        NavigationView {
            ContentView()
        }
    }
}

Answer 1

The screenshot below shows the stacktrace for a breakpoint in the setter showing that SwiftUI.ListCoreCoordinator is setting the binding twice. I've seen a similar problem with Map here . I'm not sure if duplicate calls to binding setters is part of their design or not but it understandably could be, given that SwiftUI coalesces all state changes into a single call to body .

If you'd like to work around the issue then I would suggest going with the more standard onChange modifier instead of that custom Binding , onChange is only called once for the new value, eg

struct MacListProbView: View {
    @State var selection: Set<Int> = []

    var body: some View {        
        List(0..<20, selection: $selection) { idx in
            Text("\(idx)")
        }
        .onChange(of: selection) { newSelection in
            print("set \(newSelection)")
        }
    }
}

struct MacListProbViewNav: View {
    var body: some View {
        NavigationView {
            MacListProbView()
            Color.red // note it's more efficient to have these here because they are not affected by the selection value.
            Color.green
            Color.blue
        }
    }
}