无法访问财产

Question

我想在MainView 中显示一个单独的SubjectView。

SubjectView 中subject 上的属性topics 不可访问。

我该怎么办？谢谢。

---

更新：

ForEach(subject.topics, id: \.self) { topic in
    Text(topic.title)
}

...可能是有问题的部分。

---

文件：SubjectView

import SwiftUI

struct SubjectView: View {
    @State var subject: Subject
    
    var body: some View {
        List {
            ForEach(subject.topics, id: \.self) { topic in
                Text(topic.title)
            }
        }
    }
}

文件MainView

import SwiftUI

struct MainView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    
    @FetchRequest(entity: Subject.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \Subject.name, ascending: true)]) var subjects: FetchedResults<Subject>
    
    @State var subjectName: String = ""
    
    var body: some View {
        NavigationView {
            Form {
                List {
                    ForEach(subjects, id: \.self) { subject in
                        NavigationLink(
                            destination: SubjectView(subject: subject),
                            label: {
                                Text(subject.name ?? "-")
                            })
                    }
                }
            }
            .navigationTitle("Schulfächer")
        }
    }
}

Screenshot of the Subject entity

SubjectView:var body: some View 处的错误为：

Failed to produce diagnostic for expression; please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the project

Answer 1

ForEach 的集合类型必须符合RandomAccessCollection。 topics 不是因为它是(NS)Set。

一个简单的解决方案是将sort Set 变成Array。或者显式创建一个数组，该行假定topics 是原生Swift Set<Topic>

Array(subject.topics)

注意：不要随意使用\.self。确定一个唯一的属性和/或采用Identifiable。例如，您可以使用唯一的 NSManagedObject 实例的 NSManagedObjectID。

Answer 2

@State var subject: Subject

在SubjectView 中应该是

@ObservedObject var subject: Subject

https://developer.apple.com/documentation/swiftui/managing-model-data-in-your-app

@StateObject 用于初始化ObservableObjects

至于你的话题

ForEach((subject.topics.allObjects as? [Topic]) ?? [], id: \.self) { topic in

应该有效