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实体和通过关系连接Secondary实体的数据。当我添加一个带有新的相关次要实体的新实体时，View和它List的更新正确Primary。

问题是，当我Secondary在详细视图中更新连接的项目时，数据库会更新，但更改未反映在Primary列表中。显然，@FetchRequest不会被另一个视图中的更改触发。

当我此后在主视图中添加新项目时，先前更改的项目最终会得到更新。

作为一种解决方法，我还更新Primary了详细信息视图中实体的属性，并将更改正确传播到Primary视图。

我的问题是：如何强制更新@FetchRequestsSwiftUI Core Data 中的所有相关内容？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视图中，并在实体的该行视图中使用 @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()
        }
    }
}

Answer 2

您需要一个发布者，它会在主视图中生成有关上下文更改和某些状态变量的事件，以强制在接收来自该发布者的事件时重建视图。
重要提示：视图构建器代码中必须使用状态变量，否则渲染引擎将不知道发生了什么变化。

这是对代码受影响部分的简单修改，提供了您需要的行为。

@State private var refreshing = false
private var didSave =  NotificationCenter.default.publisher(for: .NSManagedObjectContextDidSave)

var body: some View {
    List {
        ForEach(fetchedResults) { primary in
            NavigationLink(destination: SecondaryView(primary: primary)) {
                VStack(alignment: .leading) {
                    // below use of .refreshing is just as demo,
                    // it can be use for anything
                    Text("\(primary.primaryName ?? "nil")" + (self.refreshing ? "" : ""))
                    Text("\(primary.secondary?.secondaryName ?? "nil")").font(.footnote).foregroundColor(.secondary)
                }
            }
            // here is the listener for published context event
            .onReceive(self.didSave) { _ in
                self.refreshing.toggle()
            }
        }
    }
    .navigationBarTitle("Primary List")
    .navigationBarItems(trailing:
        Button(action: {self.addNewPrimary()} ) {
            Image(systemName: "plus")
        }
    )
}

Answer 3

另一种方法：使用 Publisher 和 List.id()：

struct ContentView: View {
  /*
    @FetchRequest...
  */

  private var didSave =  NotificationCenter.default.publisher(for: .NSManagedObjectContextDidSave)  //the publisher
  @State private var refreshID = UUID()

  var body: some View {
      List {
        ...
      }
      .id(refreshID)
      .onReceive(self.didSave) { _ in   //the listener
          self.refreshID = UUID()
          print("generated a new UUID")
      }    
  }
}

每次在上下文中调用 NSManagedObjects 的 save() 时，它都会为列表视图生成一个新的 UUID，并强制刷新列表视图。