How to read modifier child elements in custom view?

How can I read the value of modifier used in child elements in my custom view, I want to achieve the effect like TabView ?

struct TabItem: View {
    var body: some View {
        TabView {
            View1()
                .tabItem {
                    Label("Menu", systemImage: "list.dash")
                }


            View2()
                .tabItem {
                    Label("Order", systemImage: "square.and.pencil")
                }
        }
    }
}

I want the parent view, i.e. my CustomView, to read whether the child element has a custom modifier, e.g. .inCustomView, and read the content from it and create a button or something else based on it, just like in the case of e.g. TabView

In iOS 18, you can now read ContainerValues using ForEach(subview:) or Group(subviews:).

First, declare a container value that you want to read like this:

extension ContainerValues {
    @Entry var someCustomValue: Int = 0
}

extension View {
    func customValue(_ x: Int) -> some View {
        containerValue(\.someCustomValue, x)
    }
}

You can read this in your container view like this:

struct CustomView<Content: View>: View {
    let content: Content
    
    init(@ViewBuilder content: () -> Content) {
        self.content = content()
    }
    
    var body: some View {
        Group(subviews: content) { subviews in
            // here you can write
            let customValueOfFirstView = subviews[0].containerValues.someCustomValue
        }
    }
}

// usage:

CustomView {
    // customValueOfFirstView will be 1
    Text("Foo").customValue(1)
}

The rest of the answer is for iOS 17 and below.

---

You would need to use some underscore-prefixed (meaning they are unstable) APIs to read the views that are in a ViewBuilder. Then you can read a view trait using your own trait key, again using underscore-prefixed APIs. The trait can store an AnyView representing the view passed to your custom modifier.

To extracting the views from a ViewBuilder, you can use the Swift Package View Extractor. You can look at how they do this - it's not a lot of code.

As an example, let's create a VStack that allows you to add buttons at the bottom of its subviews by using a custom modifier, vstackButtonLabel. Users can use this to specify the view they want for the button's label.

struct VStackWithButtons<Content: View>: View {
    @ViewBuilder let content: () -> Content
    
    var body: some View {
        VStack {
            content()
            HStack {
                ExtractMulti(content) { views in
                    // "views" is a RandomAccessCollection
                    ForEach(views) { view in
                        // You can also access "view.id" here if needed 
                        if let buttonLabel = view[VStackButtonLabelTrait.self] {
                            Button {
                                print("Do something")
                            } label: {
                                buttonLabel
                            }
                        }
                    }
                }
            }
        }
    }
}

VStackButtonLabelTrait is a _ViewTraitKey:

struct VStackButtonLabelTrait: _ViewTraitKey {
    static let defaultValue: AnyView? = nil
}

extension View {
    func vstackButtonLabel<Content: View>(@ViewBuilder content: () -> Content) -> some View {
        _trait(VStackButtonLabelTrait.self, AnyView(content()))
    }
}

Example usage:

VStackWithButtons {
    Text("This has no buttons")
    
    Text("This has a button")
        .vstackButtonLabel {
            Label("Foo", systemImage: "globe")
        }
    
    Text("This also has a button")
        .vstackButtonLabel {
            Label("Bar", systemImage: "rectangle")
        }
}

Result:

As another example, here is a very simple "tab view" (if you can even call it that) that allows you to select tabs using a picker.

struct ContentView: View {
    @State var selectedTab = 0
    var body: some View {
        CustomTabView(selectedTab: $selectedTab) {
            Text("Tab 1")
                .id(0)
            
            Text("Tab 2")
                .id(1)
                .customTabItem {
                    Label("Foo", systemImage: "globe")
                }
            
            Text("Tab 3")
                .id(2)
                .customTabItem {
                    Label("Bar", systemImage: "rectangle")
                }
        }
    }
}

struct CustomTabView<Content: View, Selection: Hashable>: View {
    
    @Binding var selectedTab: Selection
    
    @ViewBuilder let content: () -> Content
    
    var body: some View {
        VStack {
            ExtractMulti(content) { views in
                ForEach(views) { view in
                    if view.id(as: Selection.self) == selectedTab {
                        view
                    }
                }
            }
            Picker("Pick", selection: $selectedTab) {
                ExtractMulti(content) { views in
                    ForEach(views) { view in
                        if let id = view.id(as: Selection.self) {
                            if let label = view[CustomTabItemTrait.self] {
                                label.tag(id)
                            } else {
                                Text("Unnamed").tag(id)
                            }
                        }
                    }
                }
            }
        }
    }
}

extension View {
    func customTabItem<Content: View>(@ViewBuilder content: () -> Content) -> some View {
        _trait(CustomTabItemTrait.self, AnyView(content()))
    }
}

struct CustomTabItemTrait: _ViewTraitKey {
    static let defaultValue: AnyView? = nil
}