NSOrderedSet和SwiftUI ForEach

Question

我正在尝试使用CoreData和SwiftUI，并且有两个实体:狗和行走。Dog之间是一对多关系

public class Dog: NSManagedObject, Identifiable {
  @NSManaged public var name: String?
  @NSManaged public var walks: NSOrderedSet?
}

和漫步

public class Walk: NSManagedObject, Identifiable {
  @NSManaged public var date: Date?
  @NSManaged public var dog: Dog? 
}

我遇到的问题是在列表中显示选定狗的所有散步。以下WalksView将显示所有当前漫游

struct WalksView: View {
@ObservedObject var dogWalksVM:DogWalkVM
var selectedIndex:Int
var body: some View {
    let currentDog = dogWalksVM.dogs[selectedIndex]
    return  List {
        if currentDog.walks == nil {
            EmptyView()
        } else {
            ForEach(0..<currentDog.walks!.count) { index in
                Text("Date \(self.walkDate(currentDog.walks![index] as! Walk))")
            }
        }
    }
    .navigationBarTitle(Text("Walks for \(dogWalksVM.dogs[selectedIndex].name ?? "")"), displayMode: .inline)
        .navigationBarItems(trailing: Button(action: {
            self.addWalk(for: currentDog)
        }, label: {
            Image(systemName: "plus")
        }))

}

func walkDate(_ walk:Walk) -> String{
    let dateFormatter = DateFormatter()
    dateFormatter.dateStyle = .medium
    guard let walkDate = walk.date as Date?  else { return ""}
    return dateFormatter.string(from: walkDate)

}

func addWalk(for currentDog:Dog) {
    self.dogWalksVM.addWalk(for: currentDog)

}
}

但当我添加一个新的应用程序时，应用程序崩溃，并显示以下错误：

 ForEach<Range<Int>, Int, Text> count (1) != its initial count (0).
 `ForEach(_:content:)` should only be used for *constant* data.
 Instead conform data to `Identifiable` or use `ForEach(_:id:content:)` and provide an explicit `id`!

如果我尝试在漫步中执行ForEach，如下所示：

 ForEach(Array(currentDog.walks!),id: \.self) { walk in
                Text("Date \(self.walkDate(walk))")
            }

有人告诉我

Protocol type 'NSOrderedSet.Element' (aka 'Any') cannot conform to 'Hashable' because only concrete types can conform to protocols

Answer 1

下面的内容应该会对你有所帮助

 ForEach(Array(currentDog.walks!.set),id: \.self) { walk in
                Text("Date \(self.walkDate(walk))")
            }

Answer 2

正如本response中所述，有一种更好的方法可以遍历NSOrderedSet。数组(my_nsorderedset.set)不会保留顺序！

相反，使用mynsorderedset.array as！ContainedType。