iOS - persistence - core data stack

Core Data Stack

Managed Object Context

• NSManagedObjectContext
• an intelligent scratch pad for working with managed objects
• associate or register it with a context. Never instantiate directly

Managed Object Model

• NSManagedObjectModel
• a description of the app’s data, including entities, attributes, relationships, and class mappings

Persistent Store Coordinator

• NSPersistenceStoreCoordinator
• a mediator for turning persisted data into managed objects and turning managed objects into persisted data
• interfaces with the underlying persistent store, such as SQL, so we don’t have to

Persistent Container

• NSPersistentContainer
• a helper that handles creation of the stack, and provides convenience methods

Fetch Requests, Faults, and Uniquing

Fetch Requests

• An NSFetchRequest specifies the criteria needed to select and optionally sort a group of managed objects held in a persistent store.

• Do not want to fetch all the data at once. Ideal to make multiple requests to get the data you need because iOS devices have finite memory, apps can have a lot of data, and fetch requests help limit your app’s data consumption.
• A finely-tuned fetch request means we’ll only use the minimum memory required to display content to the user.

Faults

• Relationships are not immediately loaded. Instead, Core Data has a mechanism called faulting. Faulting allows any attribute or relationship to be in a special state where it is promised to load when needed. When you fetch a managed object, its relationships are initially represented as faults.
• Core Data automatically retrieves the data either from a cache or from the persistent store

Uniquing

• Allows you to avoid reload the same object in the object belongs in several different object graphs/ relationships.
  • Example: a song can belong to multiple playlists

Code Examples

You have an NSManagedObject subclass named Animal, and an NSManagedObjectContext in a local variable called context. Create a new managed object and save it in the persistent store.

class DataController {
    let persistentContainer: NSPersistentContainer
    
    var viewContext: NSManagedObjectContext {
        return persistentContainer.viewContext
    }
    
    init(modelName: String) {
        persistentContainer = NSPersistentContainer(name: modelName)
    }
    
    func load(completion: (() -> Void)? = nil) {
        persistentContainer.loadPersistentStores { (storeDescription, error) in
            guard error == nil else {
                fatalError(error!.localizedDescription)
            }
            self.autoSaveContext()
            completion?()
        }
    }
}

Inject DataController AppDelegate.swift

let dataController = DataController(modelName: "Animals")

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    dataController.load()
    
    let navigationController = window?.rootViewController as! UINavigationController
    let animalsViewController = navigationController.topViewController as! AnimalsViewController
    animalsViewController.dataController = dataController
    return true
}

Load Data via Fetch Requests

let fetchRequest: NSFetchRequest<Animal> = Animal.fetchRequest()
let sortDescriptor = NSSortDescriptor(key: "birthDate", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]

if let result = try? dataController.viewContext.fetch(fetchRequest) {
    aniamls = result
    tableView.reloadData()
}

Add a new animal to the end of the animals array

func addAnimal(name: String) {
    let animal =  Animal(context: dataController.viewContext)
    animal.name = name
    animal.birthDate = Date()
    try? dataController.viewContext.save()
    animals.insert(animal, at: 0)
    tableView.insertRows(at: [IndexPath(row: 0, section: 0)], with: .fade)
}

Delete a new animal to the end of the animals array

/// Deletes the animal at the specified index path
func deleteAnimal(at indexPath: IndexPath) {
    let animalToDelete = animal(at: indexPath)
    dataController.viewContext.delete(animalToDelete)
    try? dataController.viewContext.save()
    animals.remove(at: indexPath.row)
    tableView.deleteRows(at: [indexPath], with: .fade)
}