Getting Started with Core Data Tutorial

Fetching from Core Data

To get data from your persistent store into the managed object context, you have to fetch it. Open ViewController.swift and add the following below viewDidLoad():

override func viewWillAppear(_ animated: Bool) {
  super.viewWillAppear(animated)
  
  //1
  guard let appDelegate =
    UIApplication.shared.delegate as? AppDelegate else {
      return
  }
  
  let managedContext =
    appDelegate.persistentContainer.viewContext
  
  //2
  let fetchRequest =
    NSFetchRequest<NSManagedObject>(entityName: "Person")
  
  //3
  do {
    people = try managedContext.fetch(fetchRequest)
  } catch let error as NSError {
    print("Could not fetch. \(error), \(error.userInfo)")
  }
}

Step by step, this is what the code does:

Fetch requests have several qualifiers used to refine the set of results returned. For now, you should know NSEntityDescription is one of these required qualifiers.

Setting a fetch request’s entity property, or alternatively initializing it with init(entityName:), fetches all objects of a particular entity. This is what you do here to fetch all Person entities. Also note NSFetchRequest is a generic type. This use of generics specifies a fetch request’s expected return type, in this case NSManagedObject.

1. Before you can do anything with Core Data, you need a managed object context. Fetching is no different! Like before, you pull up the application delegate and grab a reference to its persistent container to get your hands on its NSManagedObjectContext.
2. As the name suggests, NSFetchRequest is the class responsible for fetching from Core Data. Fetch requests are both powerful and flexible. You can use fetch requests to fetch a set of objects meeting the provided criteria (i.e. give me all employees living in Wisconsin and have been with the company at least three years), individual values (i.e. give me the longest name in the database) and more.
3. You hand the fetch request over to the managed object context to do the heavy lifting. fetch(_:) returns an array of managed objects meeting the criteria specified by the fetch request.

Build and run the application. Immediately, you should see the list of names you added earlier:

Great! They’re back from the dead (pun intended). Add a few more names to the list and restart the app to verify saving and fetching are working. Short of deleting the app, resetting the Simulator or throwing your phone off a tall building, the names will appear in the table view no matter what.

Key Points

• Core Data provides on-disk persistence, which means your data will be accessible even after terminating your app or shutting down your device. This is different from in-memory persistence, which will only save your data as long as your app is in memory, either in the foreground or in the background.
• Xcode comes with a powerful Data Model editor, which you can use to create your managed object model.
• A managed object model is made up of entities, attributes and relationships
• An entity is a class definition in Core Data.
• An attribute is a piece of information attached to an entity.
• A relationship is a link between multiple entities.
• An NSManagedObject is a run-time representation of a Core Data entity. You can read and write to its attributes using Key-Value Coding.
• You need an NSManagedObjectContext to save() or fetch(_:) data to and from Core Data.

Where to Go From Here?

You can download the completed project for this tutorial using the “Download materials” buttons at the top or bottom of this tutorial.

In this tutorial, you’ve already experienced several fundamental Core Data concepts: Data Models, entities, attributes, managed objects, managed object contexts and fetch requests.

If you enjoyed what you learned in this tutorial, why not check out the complete Core Data by Tutorials book, available in our store?

Here’s a taste of what’s in the book:

1. Chapter 1, Your First Core Data App: You’ll click File ▸ New Project and write a Core Data app from scratch! This chapter covers the basics of setting up your data model and then adding and fetching records.

2. Chapter 2, NSManagedObject Subclasses: NSManagedObject is the base data storage class of your Core Data object graphs. This chapter will teach you how you customize your own managed object subclasses to store and validate data.

3. Chapter 3, The Core Data Stack: Under the hood, Core Data is made up of many parts working together. In this chapter, you’ll learn about how these parts fit together, and move away from the starter Xcode template to build your own customizable system.

4. Chapter 4, Intermediate Fetching: Your apps will fetch data all the time, and Core Data offers many options for getting the data to you efficiently. This chapter covers more advanced fetch requests, predicates, sorting and asynchronous fetching.

5. Chapter 5, NSFetchedResultsController: Table views are at the core of many iOS apps, and Apple wants to make Core Data play nicely with them! In this chapter, you’ll learn how NSFetchedResultsController can save you time and code when your table views are backed by data from Core Data.

6. Chapter 6, Versioning and Migration: As you update and enhance your app, its data model will almost certainly need to change. In this chapter, you’ll learn how to create multiple versions of your data model and then migrate your users forward so they can keep their existing data as they upgrade.

7. Chapter 7, Unit Tests: Testing is an important part of the development process, and you shouldn’t leave Core Data out of that! In this chapter, you’ll learn how to set up a separate test environment for Core Data and see examples of how to test your models.

8. Chapter 8, Measuring and Boosting Performance: No one ever complained that an app was too fast, so it’s important to be vigilant about tracking performance. In this chapter, you’ll learn how to measure your app’s performance with various Xcode tools and then pick up some tips for dealing with slow spots in your code.

9. Chapter 9, Multiple Managed Object Contexts: In this final chapter, you’ll expand the usual Core Data stack to include multiple managed object contexts. You’ll learn how this can improve perceived performance and help make your app architecture less monolithic and more compartmentalized.

And to help sweeten the deal, the digital edition of the book is on sale for $44.99! But don’t wait — this sale price is only available for a limited time.

Speaking of sweet deals, be sure to check out the great prizes we’re giving away this year with the iOS 11 Launch Party, including over $9,000 in giveaways!

To be eligible for for this epic iOS 12 giveaway, all you have to do is leave a comment on the original launch post, letting us know which book or course is your favorite on this list — or which upcoming book or course you’re most excited about!

We hope you enjoy this update, and stay tuned for more book releases and updates!