未找到 <TypeName> 类型的 ObservableObject。 <TypeName> 的 View.environmentObject(_:) 作为该视图的祖先可能会丢失

Question

我有三个视图（AddMatchView、TeamPickerView 和 TeamsOfCountryView）。一切都应该像这样工作：从 AddTeamView 我转到 TeamPickerView，在那里我选择国家并转到 TeamsOfCountryView，在点击我需要的团队后，两个视图（TeamPickerView 和 TeamsOfCountryView）都应该立即关闭，这要归功于公共 Bool 变量，并且选定的团队（Team 类型的对象）传递给父 AddMatchView。但是选择团队后，只关闭了TeamsOfCountryView，TeamPickerView出现错误：Fatal error: No ObservableObject of type DBService found。 DBService 的 View.environmentObject (_ :) 作为该视图的祖先可能会丢失。

在我决定为我的视图创建一个 ViewModel 之前，一切都正常运行。即，如果我将 @State 变量从 AddMatchView 转移到 TeamPickerView，那么错误不会很糟糕，但我使用来自 @ObservedObject 的属性

主父视图：

struct AddMatchView: View {
@ObservedObject var viewModel = AddMatchViewModel()

@State isPresented = false //This works!
@State team: Team? //This works!

var body: some View {
    //...
    Button(action: { viewModel.isPresented.toggle() }){
        //...
    }.fullScreenCover(isPresented: $viewModel.isPresented) { TeamPickerView(team: $viewModel.home, isPresented: $viewModel.isPresented)}
//.fullScreenCover(isPresented: $isPresented) { TeamPickerView(team: $home, isPresented: $isPresented)} THIS WORKS!!
//...
}

使用后出现错误的ViewModel：

class AddMatchViewModel: ObservableObject{
@Published var home: Team?
@Published var isPresented = false
//...
}

TeamsPickerView：

struct TeamPickerView: View {
    @EnvironmentObject var db: DBService 
    
    @Binding var team: Team?
    
    @Binding var isPresented: Bool

    @State private var searchText = ""
    
    var body: some View {
        NavigationView {
            VStack{
                SearchBar(text: $searchText)
                Form{
                    //ERROR in below line after selecting team in child TeamsOfCountryView: No ObservableObject of type DBService found. A View.environmentObject(_:) for DBService may be missing as an ancestor of this view.
                    List (db.countries.filter({ searchText.isEmpty ? true : $0.name.contains(searchText) })) { country in
                        NavigationLink(destination: TeamsOfCountryView(countryID: country.documentID, team: $team, isPresented: $isPresented)) {
                            HStack{
                                Image(uiImage: Flag(countryCode: country.code)!.image(style: .roundedRect)).resizable().scaledToFit().frame(maxWidth: 30, maxHeight: 30)
                                Text(country.name)
                            }
                        }
                    }
                }
                
            }
            .navigationBarTitle(Text("Countries"))
            .navigationBarItems(trailing: Button(action: {isPresented = false }){
                Text("Close")
            })
        }
    }
}

TeamsOfCountryView：

struct TeamsOfCountryView: View {
    @EnvironmentObject var db: DBService
    
    @Binding var team: Team?

    @Binding var isPresented: Bool

    //...

    var body: some View {
        VStack{
            //...
            Form{
                List (teams.filter({ searchText.isEmpty ? true : $0.name.contains(searchText) })) { team in
                    Button(action: {
                        //After that, an error occurs in the parent TeamPickerView
                        self.team = team
                        self.isPresented = false
                    }){
                        //...
                    }
                }
            }
            //...
        }
    }

小更新：尽可能简化示例

class ViewModel: ObservableObject{
    @Published var isPresented = false
}

class EnvObj: ObservableObject{
    @Published var foo = "test"
}

struct ContentView: View {
    @ObservedObject var viewModel = ViewModel()

    //@State var isPresented = false   no error if we use it instead of viewModel.isPresented
    
    var body: some View {
        Button("Open Child View A"){
            viewModel.isPresented.toggle()
        }.fullScreenCover(isPresented: $viewModel.isPresented){ChildViewA(isPresented: $viewModel.isPresented)}
    }
}

struct ChildViewA: View {
    @EnvironmentObject var envObj: EnvObj
    @Binding var isPresented: Bool
    
    @State var openChildB = false

    var body: some View {
        NavigationView{
            VStack{
            //ERROR HERE: No ObservableObject of type EnvObj found. A View.environmentObject(_:) for EnvObj may be missing as an ancestor of this view.
            Text(envObj.foo)
            NavigationLink(destination: ChildViewB(isPresented: $isPresented)){
                Text("Open Child View B")
                
            }
            }
        }
    }
}

struct ChildViewB: View {
    @EnvironmentObject var envObj: EnvObj
    @Binding var isPresented: Bool
    
    var body: some View {
        Button("Close Child View A and B"){
            isPresented = false
        }
    }
}

Answer 1

您需要将@EnvironmentObject 注入到每个环境中（记住每个.sheet 或.fullScreenCover 都会创建一个新环境）：

Button("Open Child View A"){
    viewModel.isPresented.toggle()
}
.fullScreenCover(isPresented: $viewModel.isPresented) {
    ChildViewA(isPresented: $viewModel.isPresented)
        .environmentObject(EnvObj()) // inject here
}

注意：必须先创建 EnvironmentObject。如果它不是一开始就创建和注入的，你就无法通过@EnvironmentObject var envObj: EnvObj 访问它。

此外，您实际上并不需要直接在 fullScreenCover 闭包中创建依赖项。您可以将它们放在根级别并相应地注入。

Answer 2

我最好的猜测是，当您将 DBService 对象传递给 ContentView 时，您需要对其进行强引用。在声明内容视图的SceneDelegate（或App 对象）中，而不是ContentView(DBService())，执行以下操作：

let dbService = DBService() // <- strong reference, stored property at class/struct level

func scene() {
    ContentView(dbService) // <- this is the stored instance, not a brand new DBService
}

// (or)

var body: some Scene {
    ContentView(dbService) // <- this is the stored instance, not a brand new DBService
}