core-data - 为什么 @FetchRequest 在 ContentView 中失败

Question

我的 SwiftUI ContentView 中有以下代码

struct ContentView: View {

@State private var search = ""
@State private var selectedBookID: Int64? = 0

@FetchRequest(entity: Books.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \Books.bokName, ascending: true)])
var books: FetchedResults<Books>

var body: some View {
    NavigationView {
        VStack {
            HStack {
                NavigationLink("New book...", destination: BookView(isNew: true)).padding(.leading)
                Spacer()
                NavigationLink("Authors...", destination: AuthorView())
                NavigationLink("Genres...", destination: GenreView()).padding(.trailing)
            }.padding(.vertical)
            TextField("Search...", text: $search).padding(.horizontal)
            List(books, id: \Books.bokID, selection: $selectedBookID) { book in
                NavigationLink(book.bokName!, destination: BookView(isNew: false)).buttonStyle(PlainButtonStyle())
            }

        }
    }
}
}

并且它在应用程序启动期间失败，并出现错误“任何模型中没有 NSEntityDescriptions 声称 NSManagedObject 子类 'Books' 所以 +entity 很困惑。你加载你的 NSManagedObjectModel 了吗？”。它适用于同一应用程序中的所有其他视图。请注意，它是 macOS 应用程序。谢谢。

标签： core-dataswiftuinsfetchrequest

Answer 1

我找到了方法 - 只需将 @FetchRequest 部分从 ContentView 中取出并将其放入额外的结构中，如下所示

struct ContentView: View {

var body: some View {
    NavigationView {
        VStack {
            HStack {
                NavigationLink("New book...", destination: BookView(isNew: true)).padding(.leading)
                Spacer()
                NavigationLink("Authors...", destination: AuthorView())
                NavigationLink("Genres...", destination: GenreView()).padding(.trailing)
            }.padding(.vertical)
            BookList()
        }
    }
}
}

struct BookList: View {

@State private var search = ""
@State private var selectedBookID: Int64? = 0

@FetchRequest(entity: Books.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \Books.bokName, ascending: true)])
var books: FetchedResults<Books>

var body: some View {
    TextField("Search...", text: $search).padding(.horizontal)
    List(books, id: \Books.bokID, selection: $selectedBookID) { book in
        NavigationLink(book.bokName!, destination: BookView(isNew: false)).buttonStyle(PlainButtonStyle())
    }
}
}