在SwiftUI视图中使用开关/枚举

Question

从Xcode11.4开始，SwiftUI不允许在像VStack {}这样的函数构造器块中使用switch语句，失败时会出现像Generic parameter 'Content' could not be inferred这样的通用错误。如何在SwiftUI中使用switch语句根据枚举值创建不同的视图？

Answer 1

从Xcode12开始支持SwiftUI视图构建器中的switch：

enum Status {
    case loggedIn, loggedOut, expired
}

struct SwiftUISwitchView: View {

    @State var userStatus: Status = .loggedIn

    var body: some View {
        VStack {
            switch self.userStatus {
            case .loggedIn:
                Text("Welcome!")
            case .loggedOut:
                Image(systemName: "person.fill")
            case .expired:
                Text("Session expired")
            }

        }
    }
}

对于Xcode 11，您可以使用以下解决方法：

a)使用显式返回类型将其包装在单个Group块中-如果switch语句是函数构建器块中的唯一语句，则允许这样做：

enum Status {
    case loggedIn, loggedOut, expired
}

struct SwiftUISwitchView: View {

    @State var userStatus: Status = .loggedIn

    var body: some View {
        VStack {
            Group { () -> Text in
                switch(self.userStatus) {
                case .loggedIn:
                    return Text("Welcome!")
                case .loggedOut:
                    return Text("Please log in")
                case .expired:
                    return Text("Session expired")
                }
            }
        }
    }
}

struct SwitchUsageInSwiftUI_Previews: PreviewProvider {
    static var previews: some View {
        SwiftUISwitchView()
    }
}

备选b)创建一个单独的函数来根据枚举计算View：

struct SwiftUISwitchView: View {

    @State var userStatus: Status = .loggedIn

    // if it's always the same View, you can use some View
    func viewFor(status: Status) -> some View {
        switch(status) {
        case .loggedIn:
            return Text("Welcome!")
        case .loggedOut:
            return Text("Please log in")
        case .expired:
            return Text("Session expired")
        }
    }

    var body: some View {
        VStack {
            viewFor(status: userStatus)
        }
    }
}

如果返回的视图可以有不同的类型，则需要将其包装在AnyView中，因为some View要求返回类型在所有情况下都相同：

// if it's different types, you have to erase to AnyView
func viewForStatusDifferentViews(status: Status) -> AnyView {
    switch(status) {
    case .loggedIn:
        return AnyView(Text("Welcome!"))
    case .loggedOut:
        return AnyView(Image(systemName: "person.fill"))
    case .expired:
        return AnyView(Text("Session expired"))
    }
}

备选方案c)创建一个单独的View来计算View by enum值：

// Alternative: A separate view
struct StatusView: View {
 
    var status : Status
    
    var body: some View {
        switch(status) {
        case .loggedIn:
            return AnyView(Text("Welcome!"))
        case .loggedOut:
            return AnyView(Image(systemName: "person.fill"))
        case .expired:
            return AnyView(Text("Session expired"))
        }
    }
    
}

Runnable example code at Github: SwiftUIPlayground

Answer 2

您可以将枚举与@ViewBuilder一起使用，如下所示...

Declear枚举

enum Destination: CaseIterable, Identifiable {
  case restaurants
  case profile
  
  var id: String { return title }
  
  var title: String {
    switch self {
    case .restaurants: return "Restaurants"
    case .profile: return "Profile"
    }
  }
  
}

现在在视图文件中

struct ContentView: View {

   @State private var selectedDestination: Destination? = .restaurants

    var body: some View {
        NavigationView {
          view(for: selectedDestination)
        }
     }

  @ViewBuilder
  func view(for destination: Destination?) -> some View {
    switch destination {
    case .some(.restaurants):
      CategoriesView()
    case .some(.profile):
      ProfileView()
    default:
      EmptyView()
    }
  }
}

如果您想对NavigationLink使用相同的大小写...您可以使用它，如下所示

struct ContentView: View {
  
  @State private var selectedDestination: Destination? = .restaurants
  
  var body: some View {
    NavigationView {

      List(Destination.allCases,
           selection: $selectedDestination) { item in
        NavigationLink(destination: view(for: selectedDestination),
                       tag: item,
                       selection: $selectedDestination) {
          Text(item.title).tag(item)
        }
      }
        
    }
  }
  
  @ViewBuilder
  func view(for destination: Destination?) -> some View {
    switch destination {
    case .some(.restaurants):
      CategoriesView()
    case .some(.profile):
      ProfileView()
    default:
      EmptyView()
    }
  }
}