a2-6.swift

lazy var mockPersistantContainer: NSPersistentContainer = {
        
        let container = NSPersistentContainer(name: "PersistentTodoList", managedObjectModel: self.managedObjectModel)
        let description = NSPersistentStoreDescription()
        description.type = NSInMemoryStoreType
        description.shouldAddStoreAsynchronously = false // Make it simpler in test env
        
        container.persistentStoreDescriptions = [description]
        container.loadPersistentStores { (description, error) in
            // Check if the data store is in memory
            precondition( description.type == NSInMemoryStoreType )
                                        
            // Check if creating container wrong
            if let error = error {
                fatalError("Create an in-mem coordinator failed \(error)")
            }
        }
        return container
    }()