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Core Data by Tutorials

Eighth Edition · iOS 14 · Swift 5.3 · Xcode 12

Before You Begin

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Core Data by Tutorials

Section 1: 11 chapters
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2. NSManagedObject Subclasses
Written by Pietro Rea

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You got your feet wet with a simple Core Data app in Chapter 1; now it’s time to explore more of what Core Data has to offer!

At the core of this chapter is the subclassing of NSManagedObject to make your own classes for each data entity. This creates a direct one-to-one mapping between entities in the data model editor and classes in your code. This means in some parts of your code, you can work with objects and properties without worrying too much about the Core Data side of things.

Along the way, you’ll learn about all the data types available in Core Data entities, including a few outside the usual string and number types. And with all the data type options available, you’ll also learn about validating data to automatically check values before saving.

Getting started

Head over to the files accompanying this book and open the sample project named BowTies in the starter folder. Like HitList, this project uses Xcode’s Core Data-enabled template. Like before, this means Xcode generated its own ready-to-use Core Data stack located in AppDelegate.swift.

Open Main.storyboard. Here you’ll find the sample project’s single-page UI:

As you can probably guess, BowTies is a lightweight bow tie management application. You can switch between the different colors of bow ties you own — the app assumes one of each — using the topmost segmented control. Tap “R” for red, “O” for orange and so on.

Tapping on a particular color pulls up an image of the tie and populates several labels on the screen with specific information about the tie. This includes:

  • The name of the bow tie (so you can tell similarly-colored ones apart)
  • The number of times you’ve worn the tie
  • The date you last wore the tie
  • Whether the tie is a favorite of yours

The Wear button on the bottom-left increments the number of times you’ve worn that particular tie and sets the last worn date to today.

Orange is not your color? Not to worry. The Rate button on the bottom-right changes a bow tie’s rating. This particular rating system uses a scale from 0 to 5, allowing for decimal values.

That’s what the application is supposed to do in its final state. Open ViewController.swift to see what it currently does:

import UIKit

class ViewController: UIViewController {

  // MARK: - IBOutlets
  @IBOutlet weak var segmentedControl: UISegmentedControl!
  @IBOutlet weak var imageView: UIImageView!
  @IBOutlet weak var nameLabel: UILabel!
  @IBOutlet weak var ratingLabel: UILabel!
  @IBOutlet weak var timesWornLabel: UILabel!
  @IBOutlet weak var lastWornLabel: UILabel!
  @IBOutlet weak var favoriteLabel: UILabel!
  @IBOutlet weak var wearButton: UIButton!
  @IBOutlet weak var rateButton: UIButton!

  // MARK: - View Life Cycle
  override func viewDidLoad() {
    super.viewDidLoad()
  }

  // MARK: - IBActions
  @IBAction func segmentedControl(
    _ sender: UISegmentedControl) {

  }

  @IBAction func wear(_ sender: UIButton) {

  }

  @IBAction func rate(_ sender: UIButton) {

  }
}

The bad news is in its current state, BowTies doesn’t do anything. The good news is you don’t need to do any Ctrl-dragging!

The segmented control and all the labels on the user interface are already connected to IBOutlets in code. In addition, the segmented control, Wear and Rate button all have corresponding IBActions.

It looks like you have everything you need to get started adding some Core Data — but wait, what are you going to display onscreen? There’s no input method to speak of, so the app must ship with sample data. That’s exactly right. BowTies includes a property list called SampleData.plist containing the information for seven sample ties, one for each color of the rainbow.

Furthermore, the application’s asset catalog Assets.xcassets contains seven images corresponding to the seven bow ties in SampleData.plist.

What you have to do now is take this sample data, store it in Core Data and use it to implement the bow tie management functionality.

Modeling your data

In the previous chapter, you learned one of the first things you have to do when starting a new Core Data project is create your data model.

Storing non-standard data types in Core Data

Still, there are many other types of data you may want to save. For example, what would you do if you had to store an instance of UIColor?

import UIKit

class ColorAttributeTransformer:
  NSSecureUnarchiveFromDataTransformer {

  //1
  override static var allowedTopLevelClasses: [AnyClass] {
    [UIColor.self]
  }

  //2
  static func register() {
    let className =
      String(describing: ColorAttributeTransformer.self)
    let name = NSValueTransformerName(className)

    let transformer = ColorAttributeTransformer()
    ValueTransformer.setValueTransformer(
      transformer, forName: name)
  }
}

func application(_ application: UIApplication,
                 didFinishLaunchingWithOptions
  launchOptions: [UIApplication.LaunchOptionsKey: Any]?)
                 -> Bool {

  ColorAttributeTransformer.register()

  return true
}

Managed object subclasses

In the sample project from the last chapter, you used key-value coding to access the attributes on the Person entity. It looked similar to the following:

// Set the name
person.setValue(aName, forKeyPath: "name")

// Get the name
let name = person.value(forKeyPath: "name")


import Foundation
import CoreData

@objc(BowTie)
public class BowTie: NSManagedObject {

}

import Foundation
import CoreData

extension BowTie {

  @nonobjc public class func fetchRequest() 
    -> NSFetchRequest<BowTie> {
    
    return NSFetchRequest<BowTie>(entityName: "BowTie")
  }

  @NSManaged public var name: String?
  @NSManaged public var isFavorite: Bool
  @NSManaged public var lastWorn: Date?
  @NSManaged public var rating: Double
  @NSManaged public var searchKey: String?
  @NSManaged public var timesWorn: Int32
  @NSManaged public var id: UUID?
  @NSManaged public var url: URL?
  @NSManaged public var photoData: Data?
  @NSManaged public var tintColor: UIColor?
}

extension BowTie : Identifiable {
  
}

func application(_ application: UIApplication,
                 didFinishLaunchingWithOptions
  launchOptions: [UIApplication.LaunchOptionsKey: Any]?)
                 -> Bool {

  ColorAttributeTransformer.register()
  
  // Save test bow tie
  let bowtie = NSEntityDescription.insertNewObject(
    forEntityName: "BowTie",
    into: self.persistentContainer.viewContext) as! BowTie
  bowtie.name = "My bow tie"
  bowtie.lastWorn = Date()
  saveContext()
  
  // Retrieve test bow tie
  let request: NSFetchRequest<BowTie> = BowTie.fetchRequest()
  
  if let ties =
    try? self.persistentContainer.viewContext.fetch(request),
    let testName = ties.first?.name,
    let testLastWorn = ties.first?.lastWorn {
    print("Name: \(testName), Worn: \(testLastWorn)")
  } else {
    print("Test failed.")
  }
  
  return true
}

Name: My bow tie, Worn: 2020-09-02 15:05:13 +0000

Propagating a managed context

Open ViewController.swift and add the following below import UIKit:

import CoreData

// MARK: - Properties
var managedContext: NSManagedObjectContext!

func application(_ application: UIApplication,
                  didFinishLaunchingWithOptions
  launchOptions: [UIApplication.LaunchOptionsKey: Any]?)
  -> Bool {
  ColorAttributeTransformer.register()
  return true
}

// Insert sample data
func insertSampleData() {

  let fetch: NSFetchRequest<BowTie> = BowTie.fetchRequest()
  fetch.predicate = NSPredicate(format: "searchKey != nil")

  let tieCount = (try? managedContext.count(for: fetch)) ?? 0

  if tieCount > 0 {
    // SampleData.plist data already in Core Data
    return
  }
  
  let path = Bundle.main.path(forResource: "SampleData",
                              ofType: "plist")
  let dataArray = NSArray(contentsOfFile: path!)!

  for dict in dataArray {
    let entity = NSEntityDescription.entity(
      forEntityName: "BowTie",
      in: managedContext)!
    let bowtie = BowTie(entity: entity,
                        insertInto: managedContext)
    let btDict = dict as! [String: Any]

    bowtie.id = UUID(uuidString: btDict["id"] as! String)
    bowtie.name = btDict["name"] as? String
    bowtie.searchKey = btDict["searchKey"] as? String
    bowtie.rating = btDict["rating"] as! Double
    let colorDict = btDict["tintColor"] as! [String: Any]
    bowtie.tintColor = UIColor.color(dict: colorDict)

    let imageName = btDict["imageName"] as? String
    let image = UIImage(named: imageName!)
    bowtie.photoData = image?.pngData()
    bowtie.lastWorn = btDict["lastWorn"] as? Date

    let timesNumber = btDict["timesWorn"] as! NSNumber
    bowtie.timesWorn = timesNumber.int32Value
    bowtie.isFavorite = btDict["isFavorite"] as! Bool
    bowtie.url = URL(string: btDict["url"] as! String)
  }
  try? managedContext.save()
}

private extension UIColor {
  
  static func color(dict: [String: Any]) -> UIColor? {
    guard 
      let red = dict["red"] as? NSNumber,
      let green = dict["green"] as? NSNumber,
      let blue = dict["blue"] as? NSNumber else {
        return nil
    }
    
    return UIColor(
      red: CGFloat(truncating: red) / 255.0,
      green: CGFloat(truncating: green) / 255.0,
      blue: CGFloat(truncating: blue) / 255.0,
      alpha: 1)
  }
}

// MARK: - View Life Cycle
override func viewDidLoad() {
  super.viewDidLoad()

  let appDelegate = 
    UIApplication.shared.delegate as? AppDelegate
  managedContext = appDelegate?.persistentContainer.viewContext

  //1
  insertSampleData()

  //2
  let request: NSFetchRequest<BowTie> = BowTie.fetchRequest()
  let firstTitle = segmentedControl.titleForSegment(at: 0) ?? ""
  request.predicate = NSPredicate(
    format: "%K = %@",
    argumentArray: [#keyPath(BowTie.searchKey), firstTitle])

  do {
    //3
    let results = try managedContext.fetch(request)

    //4
    if let tie = results.first {
      populate(bowtie: tie)
    }
  } catch let error as NSError {
    print("Could not fetch \(error), \(error.userInfo)")
  }
}

func populate(bowtie: BowTie) {
  
  guard let imageData = bowtie.photoData as Data?,
    let lastWorn = bowtie.lastWorn as Date?,
    let tintColor = bowtie.tintColor else {
      return
  }
  
  imageView.image = UIImage(data: imageData)
  nameLabel.text = bowtie.name
  ratingLabel.text = "Rating: \(bowtie.rating)/5"
  
  timesWornLabel.text = "# times worn: \(bowtie.timesWorn)"
  
  let dateFormatter = DateFormatter()
  dateFormatter.dateStyle = .short
  dateFormatter.timeStyle = .none
  
  lastWornLabel.text =
    "Last worn: " + dateFormatter.string(from: lastWorn)
  
  favoriteLabel.isHidden = !bowtie.isFavorite
  view.tintColor = tintColor
}


var currentBowTie: BowTie!

currentBowTie = tie

@IBAction func wear(_ sender: UIButton) {  
  currentBowTie.timesWorn += 1
  currentBowTie.lastWorn = Date()
  
  do {
    try managedContext.save()
    populate(bowtie: currentBowTie)    
  } catch let error as NSError {    
    print("Could not fetch \(error), \(error.userInfo)")
  }
}


@IBAction func rate(_ sender: UIButton) {

  let alert = UIAlertController(title: "New Rating",
                                message: "Rate this bow tie",
                                preferredStyle: .alert)

  alert.addTextField { textField in
    textField.keyboardType = .decimalPad
  }

  let cancelAction = UIAlertAction(title: "Cancel",
                                   style: .cancel)

  let saveAction = UIAlertAction(
    title: "Save",
    style: .default
    ) { [unowned self] _ in
      if let textField = alert.textFields?.first {
        self.update(rating: textField.text)
      }
    }

  alert.addAction(cancelAction)
  alert.addAction(saveAction)
  
  present(alert, animated: true)
}

func update(rating: String?) {

  guard let ratingString = rating,
    let rating = Double(ratingString) else {
      return
  }

  do {
    currentBowTie.rating = rating
    try managedContext.save()
    populate(bowtie: currentBowTie)
  } catch let error as NSError {    
    print("Could not save \(error), \(error.userInfo)")
  }
}

Data validation in Core Data

Your first instinct may be to write client-side validation—something like, “Only save the new rating if the value is greater than 0 and less than 5.” Fortunately, you don’t have to write this code yourself. Core Data supports validation for most attribute types out of the box.

Could not save Error Domain=NSCocoaErrorDomain Code=1610 "rating is too large." UserInfo={NSValidationErrorObject=<BowTie: 0x600002b8ab20> (entity: BowTie; id: 0xcef31f910384f2ad <x-coredata://A64812B6-5D4D-4934-805C-72F6A345EC7B/BowTie/p5>; data: {
    id = "800C3526-E83A-44AC-B718-D36934708921";
    isFavorite = 0;
    lastWorn = "2019-07-28 04:08:02 +0000";
    name = "Red Bow Tie";
    photoData = "{length = 50, bytes = 0x89504e47 0d0a1a0a 0000000d 49484452 ... aece1ce9 00000078 }";
    rating = 6;
    searchKey = R;
    timesWorn = 28;
    tintColor = "UIExtendedSRGBColorSpace 0.937255 0.188235 0.141176 1";
    url = "https://en.wikipedia.org/wiki/Bow_tie";
}), NSLocalizedDescription=rating is too large., NSValidationErrorKey=rating, NSValidationErrorValue=6}, ["NSValidationErrorKey": rating, "NSLocalizedDescription": rating is too large., "NSValidationErrorValue": 6, "NSValidationErrorObject": <BowTie: 0x600002b8ab20> (entity: BowTie; id: 0xcef31f910384f2ad <x-coredata://A64812B6-5D4D-4934-805C-72F6A345EC7B/BowTie/p5>; data: {
    id = "800C3526-E83A-44AC-B718-D36934708921";
    isFavorite = 0;
    lastWorn = "2019-07-28 04:08:02 +0000";
    name = "Red Bow Tie";
    photoData = "{length = 50, bytes = 0x89504e47 0d0a1a0a 0000000d 49484452 ... aece1ce9 00000078 }";
    rating = 6;
    searchKey = R;
    timesWorn = 28;
    tintColor = "UIExtendedSRGBColorSpace 0.937255 0.188235 0.141176 1";
    url = "https://en.wikipedia.org/wiki/Bow_tie";
}), "NSValidationErrorValue": 8, "NSValidationErrorKey": rating, "NSLocalizedDescription": rating is too large.]

func update(rating: String?) {

  guard let ratingString = rating,
    let rating = Double(ratingString) else {
      return
  }

  do {

    currentBowTie.rating = rating
    try managedContext.save()
    populate(bowtie: currentBowTie)

  } catch let error as NSError {

    if error.domain == NSCocoaErrorDomain &&
      (error.code == NSValidationNumberTooLargeError ||
        error.code == NSValidationNumberTooSmallError) {
      rate(rateButton)
    } else {
      print("Could not save \(error), \(error.userInfo)")
    }
  }
}

Could not save Error Domain=NSCocoaErrorDomain Code=1610 "rating is too large."

Tying everything up

The Wear and Rate buttons are working properly, but the app can only display one tie. Tapping the different values on the segmented control is supposed to switch ties. You’ll finish up this sample project by implementing that feature.

@IBAction func segmentedControl(_ sender: UISegmentedControl) {
  guard let selectedValue = sender.titleForSegment(
    at: sender.selectedSegmentIndex) else {
      return
  }

  let request: NSFetchRequest<BowTie> = BowTie.fetchRequest()
  request.predicate = NSPredicate(
    format: "%K = %@",
    argumentArray: [#keyPath(BowTie.searchKey), selectedValue])

  do {
    let results = try managedContext.fetch(request)
    currentBowTie = results.first
    populate(bowtie: currentBowTie)
  } catch let error as NSError {
    print("Could not fetch \(error), \(error.userInfo)")
  }
}

Key points

  • Core Data supports different attribute data types, which determines the kind of data you can store in your entities and how much space they will occupy on disk. Some common attribute data types are String, Date, and Double.
  • The Binary Data attribute data type gives you the option of storing arbitrary amounts of binary data in your data model.
  • The Transformable attribute data type lets you store any object that conforms to NSSecureCoding in your data model.
  • Using an NSManagedObject subclass is a better way to work with a Core Data entity. You can either generate the subclass manually or let Xcode do it automatically.
  • You can refine the set entities fetched by NSFetchRequest using an NSPredicate.
  • You can set validation rules (e.g. maximum value and minimum value) to most attribute data types directly in the data model editor. The managed object context will throw an error if you try to save invalid data.
1. Your First Core Data App
3. The Core Data Stack
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