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Code Block 
struct MyApp: App {
    @Environment(\.scenePhase) private var scenePhase
    @StateObject private var persistentStore = PersistentStore.shared
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(\.managedObjectContext, persistentStore.context)
        }
        .onChange(of: scenePhase) { phase in
            switch phase {
            case .active:
                print("\(#function) REPORTS - App change of scenePhase to ACTIVE")
            case .inactive:
                print("\(#function) REPORTS - App change of scenePhase to INACTIVE")
            case .background:
                print("\(#function) REPORTS - App change of scenePhase to BACKGROUND")
                savePersistentStore()
            @unknown default:
                fatalError("\(#function) REPORTS - fatal error in switch statement for .onChange modifier")
            }
        }
    }
    func savePersistentStore() {
        persistentStore.save()
    }
}

Code Block 
import SwiftUI
import CoreData
class PersistentStore: ObservableObject {
    var context: NSManagedObjectContext { persistentContainer.viewContext }
		
		// One line singleton
    static let shared = PersistentStore()
		
		// Mark the class private so that it is only accessible through the singleton `shared` static property
    private init() {}
    private let persistentStoreName: String = "YourPersistentStoreName"
    // MARK: - Core Data stack
    lazy var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentContainer(name: persistentStoreName)
				
        // Enable history tracking
        // (to facilitate previous NSPersistentCloudKitContainer to load as NSPersistentContainer)
        // (not required when using NSPersistentCloudKitContainer)
        guard let persistentStoreDescriptions = container.persistentStoreDescriptions.first else {
            fatalError("\(#function): Failed to retrieve a persistent store description.")
        }
        persistentStoreDescriptions.setOption(true as NSNumber,
                                              forKey: NSPersistentHistoryTrackingKey)
        persistentStoreDescriptions.setOption(true as NSNumber,
                                              forKey: NSPersistentStoreRemoteChangeNotificationPostOptionKey)
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error {
                fatalError("Unresolved error \(error)")
            }
        })
        container.viewContext.automaticallyMergesChangesFromParent = true
				// Add the following line to set your merge policy...
				container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
        return container
    }()
   
    // MARK: - Core Data Saving and Undo support
    func save() {
        let context = persistentContainer.viewContext
        if !context.commitEditing() {
            NSLog("\(NSStringFromClass(type(of: self))) unable to commit editing before saving")
        }
        if context.hasChanges {
            do {
                try context.save()
            } catch {
                let nserror = error as NSError
                NSApplication.shared.presentError(nserror)
            }
        }
    }
}