如何将核心数据模型项传递到视图中进行编辑

Question

有一个最小的示例项目

我的主要 (ContentView) 看起来像：

import SwiftUI
import CoreData

struct ContentView: View {
    @Environment( \.managedObjectContext ) private var viewContext
    
    @FetchRequest( sortDescriptors: [ NSSortDescriptor( keyPath: \Item.name, ascending: true ) ],
                   animation:       .default )
    private var items: FetchedResults<Item>
    
    var body: some View {
        NavigationView {
            List {
                ForEach( items ) { item in
                    NavigationLink {
                        NameViewer( itemID: item.objectID )
                    } label: {
                        Text( item.name! )
                    }
                }
                .onDelete( perform: deleteItems )
            }
            .toolbar {
                ToolbarItem( placement: .navigationBarTrailing ) {
                    EditButton()
                }
                
                ToolbarItem {
                    Button() {
                        print( "Add Item" )
                    } label: {
                        NavigationLink {
                            NameViewer();
                        } label: {
                            Label( "Add Item", systemImage: "plus" )
                        }
                    }
                }
            }
        }
    }
    
    
    
    private func deleteItems(offsets: IndexSet) {
        withAnimation {
            offsets.map { items[[=10=]] }.forEach(viewContext.delete)
            
            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}


struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
    }
}

和 NameViewer 看起来像：

import SwiftUI
import CoreData

enum TrustReason: String, Identifiable, CaseIterable
{
    var id: UUID
    {
        return UUID();
    }
    
    case unknown         = "Unknown";
    case legalOnly       = "Legal Only";
    case goodLabeling    = "Good Labeling";
    case facilityClean   = "Facility Clean";
    case detailedAnswers = "Detailed Answers";
    case unresponsive    = "Unresponsive";
}



extension TrustReason
{
    var title: String
    {
        switch self
        {
            case .unknown:
                return "Unknown";
                
            case .legalOnly:
                return "Legal Only";
                
            case .goodLabeling:
                return "Good Labeling";
                
            case .facilityClean:
                return "Facility Clean";
                
            case .detailedAnswers:
                return "Detailed Answers";
                
            case .unresponsive:
                return "Unresponsive";
        }
    }
}



struct NameViewer: View {
    @Environment( \.presentationMode )     var         presentationMode
    @Environment( \.managedObjectContext ) private var moc
    
    @State private var name: String = ""
    @State private var reason: TrustReason = .unknown

    var itemID: NSManagedObjectID?
    
    var body: some View {
        Form {
            Section( header: Text( "Information" ) ) {
                TextField( "Name", text: $name )
            }
            
            Section( header: Text( "Trust" ) ) {
                Picker( "Reason", selection: $reason ) {
                    ForEach( TrustReason.allCases ) { trustReason in
                        Text( trustReason.title ).tag( trustReason )
                    }
                }
            }
        }
        .toolbar {
            Button() {
                if ( saveName() ) {
                    self.presentationMode.wrappedValue.dismiss()
                }
            } label: {
                Text( "Save" )
            }
        }
        .onAppear {
            print( "on appear" )
            
            guard let theID = itemID,
                  let item = moc.object( with: theID ) as? Item else {
                      return
                  }
            
            print( "passed guard" )
            
            if let itemName = item.name {
                name = itemName
            }
            
            print( name )
        }
    }
    
    
    
    private func saveName() -> Bool {
        let item = Item( context: moc )
        
        do {
            print( self.name )
            
            item.name = self.name
            
            try moc.save()
            
            return true
        } catch {
            print( error )
            print( error.localizedDescription )
        }
        
        self.moc.rollback();
        
        return false
    }
}



struct NameViewer_Previews: PreviewProvider {
    static var previews: some View {
        NameViewer()
    }
}

我可以创建要在 ContentView 列表中显示的新项目。

然后，当我 select 列表中的一个项目时，我将该项目传递给 NameViewer。我可以确认我在 .onAppear 代码中成功找到了正确的对象。

但是，有两个问题：

如果我 select 列表中的项目，项目名称不会出现在名称文本字段中，除非我先单击文本字段。'
使用 .onAppear 似乎不是放置该代码的正确位置。原因是 Picker 将另一个视图推入堆栈，一旦项目被选中，.onAppear 再次运行并且我丢失了对名称字段的更改名称。

如何更改代码以解决这些问题？

Answer 1

为了实现所需的功能，我会在 UI 和核心数据方面更改您的架构。

在用户界面方面，最好使用导航链接来显示静态数据详细视图，并使用模式来执行数据操作，例如创建和编辑对象。因此，有一个视图显示对象详细信息（例如 NameViewer），另一个视图用于编辑对象（例如 NameEditor）。此外，将 NSManagedObject 子类的属性直接绑定到 SwiftUI 控件。不要创建额外的 @State 属性，然后复制值。您正在引入共享状态，这是 SwiftUI 可以消除的东西。

在核心数据方面，为了执行创建和更新操作，您需要使用子上下文。每当您创建或更新对象时，都会显示一个注入了子上下文的模态编辑器视图。这样，如果我们对我们的更改不满意，我们可以简单地忽略该模式，并且更改会被神奇地丢弃，而无需调用 rollback()，因为该子上下文会随视图一起被破坏。由于您现在正在使用子上下文，因此不要忘记将主视图上下文也保存在某个地方，例如当用户导航出您的应用程序时。

所以要在代码中实现它，我们需要一些结构来存储我们新创建的对象以及它们的子上下文：

struct CreateOperation<Object: NSManagedObject>: Identifiable {
    let id = UUID()
    let childContext: NSManagedObjectContext
    let childObject: Object
    
    init(with parentContext: NSManagedObjectContext) {
        let childContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
        childContext.parent = parentContext
        let childObject = Object(context: childContext)
        
        self.childContext = childContext
        self.childObject = childObject
    }
}

struct UpdateOperation<Object: NSManagedObject>: Identifiable {
    let id = UUID()
    let childContext: NSManagedObjectContext
    let childObject: Object
    
    init?(
        withExistingObject object: Object,
        in parentContext: NSManagedObjectContext
    ) {
        let childContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
        childContext.parent = parentContext
        guard let childObject = try? childContext.existingObject(with: object.objectID) as? Object else { return nil }
        
        self.childContext = childContext
        self.childObject = childObject
    }
}

而UI代码如下：

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.name, ascending: true)], animation: .default
    ) private var items: FetchedResults<Item>
    @State private var itemCreateOperation: CreateOperation<Item>?
    
    var body: some View {
        NavigationView {
            List {
                ForEach(items) { item in
                    NavigationLink {
                        NameViewer(item: item)
                    } label: {
                        Text(item.name ?? "")
                    }
                }
            }
            .toolbar {
                ToolbarItemGroup(placement: .navigationBarTrailing) {
                    EditButton()
                    Button(action: {
                        itemCreateOperation = CreateOperation(with: viewContext)
                    }) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            .sheet(item: $itemCreateOperation) { createOperation in
                NavigationView {
                    NameEditor(item: createOperation.childObject)
                        .navigationTitle("New Item")
                }
                .environment(\.managedObjectContext, createOperation.childContext)
            }
        }
    }
}

struct NameViewer: View {
    @Environment(\.managedObjectContext) private var viewContext
    @State private var itemUpdateOperation: UpdateOperation<Item>?
    
    @ObservedObject var item: Item
    
    var body: some View {
        Form {
            Section {
                Text(item.name ?? "")
            }
        }
        .navigationTitle("Item")
        .toolbar  {
            Button("Update") {
                itemUpdateOperation = UpdateOperation(withExistingObject: item, in: viewContext)
            }
        }
        .sheet(item: $itemUpdateOperation) { updateOperation in
            NavigationView {
                NameEditor(item: updateOperation.childObject)
                    .navigationTitle("Update Item")
            }
            .environment(\.managedObjectContext, updateOperation.childContext)
        }
    }
}

struct NameEditor: View {
    @Environment(\.dismiss) private var dismiss
    @Environment(\.managedObjectContext) private var childContext
    
    @ObservedObject var item: Item
    
    var body: some View {
        Form {
            Section(header: Text("Information")) {
                if let name = Binding($item.name) {
                    TextField("Name", text: name)
                }
            }
        }
        .toolbar {
            Button() {
                try? childContext.save()
                dismiss()
            } label: {
                Text("Save")
            }
        }
    }
}

更多信息见我的相关回答：

如何将核心数据模型项传递到视图中进行编辑

How to pass a CoreData model item into a view for editing

core-data

ios

swift

swiftui