Question

在SwiftUI视图中，我有一个基于@FetchRequest的列表，该列表显示了主要实体的数据以及连接辅助实体的通孔关系。当我添加具有新的相关辅助实体的新主实体时，视图及其列表已正确更新。

问题是，当我在详细视图中更新连接的辅助项目时，数据库得到了更新，但是更改未反映在“主要列表”中。显然，@ FetchRequest不会被另一个视图中的更改触发。

此后，当我在主视图中添加新项目时，以前更改的项目最终将得到更新。

作为一种解决方法，我另外在详细信息视图中更新了主要实体的属性，并且更改正确地传播到了主要视图。

我的问题是：如何在SwiftUI核心数据中对所有相关的@FetchRequests强制进行更新？特别是当我无法直接访问相关实体/ @ Fetchrequests时？

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

我对此也很挣扎，找到了一个非常干净的解决方案：

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：

WineList：

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")
    }
}

WineRow：

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，该发布者将生成有关上下文更改的事件以及主视图中的一些状态变量，以在从该发布者接收事件时强制重新构建视图。
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某些更改。

这里是对代码受影响部分的简单修改，可以提供所需的行为。

@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

要解决此问题，您必须将 @ObservedObject 添加到 var primary: Primary 中的 SecondaryView 才能正常工作 List。 Primary 属于 NSManagedObject 类，它已经符合 @ObservableObject 协议。这样可以观察到 Primary 实例的变化。

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

    @ObservedObject 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

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

Answer 4

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。

Answer 5

我试图像这样在细节视图中触摸主要对象：

// TODO: ❌ workaround to trigger update on primary @FetchRequest

if let primary = secondary.primary {
   secondary.managedObjectContext?.refresh(primary, mergeChanges: true)
}

然后主列表将更新。但是细节视图必须知道父对象。这将起作用，但是可能不是SwiftUI或Combine方法...

编辑：

基于上述解决方法，我使用全局save（managedObject :)函数修改了我的项目。这将涉及所有相关的实体，从而更新所有相关的@FetchRequest。

import SwiftUI
import CoreData

extension Primary: Identifiable {}

// MARK: - Primary View

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

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

    var body: some View {
        print("body PrimaryListView"); return
        List {
            ForEach(fetchedResults) { primary in
                NavigationLink(destination: SecondaryView(secondary: primary.secondary!)) {
                    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)
        }
    }
}

// MARK: - Detail View

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

    var secondary: Secondary

    @State private var newSecondaryName = ""

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

    private func saveChanges() {
        secondary.secondaryName = newSecondaryName

        // save Secondary and touch Primary
        (UIApplication.shared.delegate as! AppDelegate).save(managedObject: secondary)

        presentationMode.wrappedValue.dismiss()
    }
}

extension AppDelegate {
    /// save and touch related objects
    func save(managedObject: NSManagedObject) {

        let context = persistentContainer.viewContext

        // if this object has an impact on related objects, touch these related objects
        if let secondary = managedObject as? Secondary,
            let primary = secondary.primary {
            context.refresh(primary, mergeChanges: true)
            print("Primary touched: \(primary.primaryName ?? "no name")")
        }

        saveContext()
    }
}

SwiftUI中相关实体更改时，如何更新@FetchRequest？

5 个答案: