core-data - 当 SwiftUI 中的相关实体发生更改时，如何更新 @FetchRequest？

Question

在 SwiftUI 中 View我有一个 List基于 @FetchRequest显示 Primary 的数据实体和via关系连接Secondary实体。View和它的 List正确更新，当我添加一个新的 Primary具有新的相关次要实体的实体。

问题是，当我更新连接时 Secondary在详细 View 中的项目，数据库得到更新，但更改未反射(reflect)在 Primary 中列表。
显然，@FetchRequest不会被另一个 View 中的更改触发。

当我此后在主 View 中添加新项目时，先前更改的项目最终会得到更新。

作为一种解决方法，我另外更新了 Primary 的一个属性。详细 View 中的实体，更改正确传播到 Primary看法。

我的问题是:
如何强制更新所有相关 @FetchRequests在 SwiftUI 核心数据中？
特别是当我无法直接访问相关实体时/@Fetchrequests ?

import SwiftUI

extension Primary: Identifiable {}

// Primary View

struct PrimaryListView: View {
    @Environment(\.managedObjectContext) var context

    @FetchRequest(
        entity: Primary.entity(),
        sortDescriptors: [NSSortDescriptor(key: "primaryName", ascending: true)]
    )
    var fetchedResults: FetchedResults<Primary>

    var body: some View {
        List {
            ForEach(fetchedResults) { primary in
                NavigationLink(destination: SecondaryView(primary: primary)) {
                VStack(alignment: .leading) {
                    Text("\(primary.primaryName ?? "nil")")
                    Text("\(primary.secondary?.secondaryName ?? "nil")").font(.footnote).foregroundColor(.secondary)
                }
                }
            }
        }
        .navigationBarTitle("Primary List")
        .navigationBarItems(trailing:
            Button(action: {self.addNewPrimary()} ) {
                Image(systemName: "plus")
            }
        )
    }

    private func addNewPrimary() {
        let newPrimary = Primary(context: context)
        newPrimary.primaryName = "Primary created at \(Date())"
        let newSecondary = Secondary(context: context)
        newSecondary.secondaryName = "Secondary built at \(Date())"
        newPrimary.secondary = newSecondary
        try? context.save()
    }
}

struct PrimaryListView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext

        return NavigationView {
            PrimaryListView().environment(\.managedObjectContext, context)
        }
    }
}

// Detail View

struct SecondaryView: View {
    @Environment(\.presentationMode) var presentationMode

    var primary: Primary

    @State private var newSecondaryName = ""

    var body: some View {
        VStack {
            TextField("Secondary name:", text: $newSecondaryName)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .padding()
                .onAppear {self.newSecondaryName = self.primary.secondary?.secondaryName ?? "no name"}
            Button(action: {self.saveChanges()}) {
                Text("Save")
            }
            .padding()
        }
    }

    private func saveChanges() {
        primary.secondary?.secondaryName = newSecondaryName

        // TODO: ❌ workaround to trigger update on primary @FetchRequest
        primary.managedObjectContext.refresh(primary, mergeChanges: true)
        // primary.primaryName = primary.primaryName

        try? primary.managedObjectContext?.save()
        presentationMode.wrappedValue.dismiss()
    }
}

Answer 1

我也为此苦苦挣扎，并找到了一个非常好的和干净的解决方案:
您必须将行包装在单独的 View 中，并在实体的该行 View 中使用 @ObservedObject。
这是我的代码:
酒单:

struct WineList: View {
    @FetchRequest(entity: Wine.entity(), sortDescriptors: [
        NSSortDescriptor(keyPath: \Wine.name, ascending: true)
        ]
    ) var wines: FetchedResults<Wine>

    var body: some View {
        List(wines, id: \.id) { wine in
            NavigationLink(destination: WineDetail(wine: wine)) {
                WineRow(wine: wine)
            }
        }
        .navigationBarTitle("Wines")
    }
}

葡萄酒行:

struct WineRow: View {
    @ObservedObject var wine: Wine   // !! @ObserveObject is the key!!!

    var body: some View {
        HStack {
            Text(wine.name ?? "")
            Spacer()
        }
    }
}