如何解决"environment object"这个问题?

How to solve this problem of "environment object"?

//环境对象class

class AppData: ObservableObject {
    @Published var studs : [StudentModel]   
}

    var body: some View {
        
            VStack{
                List(appData.studs,id:\.rollNo){ s in  //causing error
                    Text("\(s.rollNo)")
                    NavigationLink("", destination: StudentView(s: s))
                }
            }.navigationBarItems(trailing:
                                    Button(action: {
                                        self.addStud.toggle()
                                        
                                    }){
                                        Image(systemName: "plus")
                                            .renderingMode(.original)
                                    }
                .sheet(isPresented: $addStud, content: {
                    AddStudent()
                })
            )
            .navigationBarTitle(Text("Students"),displayMode: .inline)
    }

Fatal error: No ObservableObject of type AppData found. A View.environmentObject(_:) for AppData may be missing as an ancestor of this view.

您的示例代码在视图的开头缺少一些行。根据错误消息的声音,您已经有了类似的东西:

struct MyView: View {
  @EnvironmentObject var appData: AppData
  // ...rest of view ...
}

除了用于从环境中为您的对象获取引用的代码外,您还需要确保在链的更上游某处将其放入。您的错误消息告诉你这就是问题所在 - 它正在环境中寻找一种 AppData 对象,但那里什么也没有。

假设您将其声明为应用级别;它可能看起来像这样:

@main
struct TestDemoApp: App {
    // 1. Instantiate the object, using `@StateObejct` to make sure it's "owned" by the view
    @StateObject var appData = AppData()

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environmentObject(appData) // 2. make it available to the hierarchy of views
        }
    }
}

您还需要做的是确保任何使用您的环境对象的视图也可以访问其 Xcode 预览中的一个。您可能想要创建一个包含示例数据的 AppData 版本,这样您的预览就不会与实时数据混淆。

extension AppData {
  static var preview: AppData = ...
}

struct MyView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView()
      .environmentObject(AppData.preview)
  }
}