Swift 如何在自定义视图中读取修改符子元素

您需要使用一些带下划线前缀(表示它们不稳定)的API来读取ViewBuilder中的视图.然后，您可以使用您自己的特征键，同样使用下划线前缀的API来读取视图特征.特征可以存储表示传递给您的自定义修改器的视图的AnyView.

要从ViewBuilder中提取视图，可以使用Swift Package View Extractor.你可以看看他们是如何做到这一点的-这并不是很多代码.

例如，让我们创建一个VStack，它允许您使用自定义修饰符vstackButtonLabel在其子视图的底部添加按钮.用户可以使用它来指定他们想要的按钮标签的视图.

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是一个_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()))
    }
}

用法示例:

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")
        }
}

结果:

作为另一个例子，这里有一个非常简单的"选项卡视图"(如果你可以这样称呼它的话)，它允许你使用 Select 器来 Select 选项卡.

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
}