Accessing Core Data Stack in MVVM application

You can use Core Data Singleton class

import CoreData

class CoreDataStack {
    static let shared = CoreDataStack()

    private init() {}

    var managedObjectContext: NSManagedObjectContext {
        return self.persistentContainer.viewContext
    }

    var workingContext: NSManagedObjectContext {
        let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
        context.parent = self.managedObjectContext
        return context
    }

    // MARK: - Core Data stack

    lazy var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentContainer(name: "MyStuff")
        container.loadPersistentStores(completionHandler: { storeDescription, error in
            if let error = error as NSError? {
                RaiseError.raise()
            }
        })
        return container
    }()

    // MARK: - Core Data Saving support

    func saveContext() {
        self.managedObjectContext.performAndWait {
            if self.managedObjectContext.hasChanges {
                do {
                    try self.managedObjectContext.save()
                    appPrint("Main context saved")
                } catch {
                    appPrint(error)
                    RaiseError.raise()
                }
            }
        }
    }

    func saveWorkingContext(context: NSManagedObjectContext) {
        do {
            try context.save()
            appPrint("Working context saved")
            saveContext()
        } catch (let error) {
            appPrint(error)
            RaiseError.raise()
        }
    }
}

Core Data is not thread safe. If you write something on manageObject and don’t want to save that, but some other thread save the context, then the changes that you don’t want to persist will also persist.

So to avoid this situation always create working context – which is private.

When you press save, then first private context get saved and after that you save main context.

In MVVM you should have DataLayer through which your ViewModel interact with Core Data singleton class.