Update to ContentView.swift to synchronize Core Data context with Swift Data

ContentView.swift

// Modified version of ContentView.swift from Pol Piella's blog
// post "Using Core Data and Swift Data side by side".
//
// See: https://www.polpiella.dev/core-data-and-swift-data
//
// Crux is in the `onAppear`. 
// ```
//         .onAppear() {
//             NotificationCenter.default.addObserver(forName: Notification.Name.NSManagedObjectContextDidSave,
//                                                    object: nil,
//                                                    queue: nil) { [self]  (notification) in
//                 self.coreDataContext.mergeChanges(fromContextDidSave: notification)
//             }
// ```
import SwiftUI
import SwiftData

struct ContentView: View {
    @FetchRequest(sortDescriptors: [SortDescriptor(\.date, order: .reverse)])
    var entries: FetchedResults<CoreDataDiaryEntry>
    @Query
    var swiftDataEntries: [DiaryEntry]

    @Environment(\.modelContext) var swiftDataContext
    @Environment(\.managedObjectContext) var coreDataContext
    
    var body: some View {
        VStack(spacing: 10) {
            HStack {
                VStack {
                    Text("Core data entity count")
                    Text("\(entries.count)")
                }

                VStack {
                    Text("Swift data entity count")
                    Text("\(swiftDataEntries.count)")
                }
            }
            Spacer()
            Button("New Swift Data entry", action: createSwiftDataEntry)
            Button("New Core Data entry", action: createCoreDataEntry)
        }
        .buttonStyle(.borderedProminent)
        .padding()
        .onAppear() {
            NotificationCenter.default.addObserver(forName: Notification.Name.NSManagedObjectContextDidSave,
                                                   object: nil,
                                                   queue: nil) { [self]  (notification) in
                self.coreDataContext.mergeChanges(fromContextDidSave: notification)
            }
        }
    }

    private func createSwiftDataEntry() {
        let entry = DiaryEntry(title: "New entry!", body: "This is a Swift Data entry")
        swiftDataContext.insert(entry)
    }

    private func createCoreDataEntry() {
        let entry = CoreDataDiaryEntry(context: coreDataContext)
        entry.date = .now
        entry.title = "New entry!"
        entry.body = "This is a CoreData entry"

        try? coreDataContext.save()
    }
}