如何复刻SwiftUI Picker的选择绑定逻辑，实现自定义Menu的选中态管理

SwiftUI不支持直接遍历@ViewBuilder传入的不透明子视图读取tag属性，系统Picker依赖内部私有能力实现该逻辑，以下是两种可落地的实现方案：

方案1：模拟系统Picker的Tag声明风格 #

通过环境变量传递选中状态、自定义MenuItem视图来模拟系统API使用体验，和你期望的调用方式基本一致：

1. 定义环境键与MenuItem #

// 定义选中状态的环境键
private struct MenuSelectionKey: EnvironmentKey {
    static let defaultValue: AnyHashable? = nil
}

extension EnvironmentValues {
    var menuSelection: AnyHashable? {
        get { self[MenuSelectionKey.self] }
        set { self[MenuSelectionKey.self] = newValue }
    }
}

// 自定义MenuItem视图，自动读取环境中的选中状态匹配自身tag
struct MenuItem<Content: View, Tag: Hashable>: View {
    let tag: Tag
    let content: Content
    @Environment(\.menuSelection) private var selection
    
    init(tag: Tag, @ViewBuilder content: () -> Content) {
        self.tag = tag
        self.content = content()
    }
    
    var body: some View {
        content
            .foregroundColor(selection as? Tag == tag ? .accentColor : .primary)
    }
}

// 给View加扩展，方便类似系统.tag()的调用
extension View {
    func menuTag<Tag: Hashable>(_ tag: Tag) -> some View {
        MenuItem(tag: tag) { self }
    }
}

2. 实现Menu容器 #

struct Menu<Selection: Hashable, Content: View>: View {
    @Binding var selection: Selection
    @ViewBuilder var content: () -> Content
    
    var body: some View {
        VStack(content: content)
            .environment(\.menuSelection, AnyHashable(selection))
    }
}

3. 调用示例 #

struct ScreenView: View {
    @State private var selectedMenu: String = "Home"
    
    var body: some View {
        Menu(selection: $selectedMenu) {
            Text("Home")
                .menuTag("Home")
            Text("Settings")
                .menuTag("Settings")
            Text("Profile")
                .menuTag("Profile")
        }
    }
}

方案2：数据源驱动实现（更简洁易维护） #

如果不需要完全模拟系统Picker的声明风格，直接传入数据源给Menu内部用ForEach生成子视图是更简单的方案：

struct Menu<Item: Hashable, Content: View>: View {
    @Binding var selection: Item
    let items: [Item]
    @ViewBuilder var itemContent: (Item) -> Content
    
    var body: some View {
        VStack {
            ForEach(items, id: \.self) { item in
                itemContent(item)
                    .foregroundColor(selection == item ? .accentColor : .primary)
                    .onTapGesture {
                        selection = item
                    }
            }
        }
    }
}

// 调用示例
struct ScreenView: View {
    @State private var selectedMenu: String = "Home"
    private let menuItems = ["Home", "Settings", "Profile"]
    
    var body: some View {
        Menu(selection: $selectedMenu, items: menuItems) { item in
            Text(item)
        }
    }
}

内容的提问来源于stack exchange，提问作者robertsan