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UIKit Apprentice

Third Edition · iOS 18 · Swift 5.10 · Xcode 16

Before You Begin

Section 0: 5 chapters
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Getting Started

Section 1: 8 chapters
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Checklists

Section 2: 12 chapters
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My Locations

Section 3: 11 chapters
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Store Search

Section 4: 13 chapters
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27. Saving Locations
Written by Fahim Farook

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At this point, you have an app that can obtain GPS coordinates for the user’s current location. It also has a screen where the user can “tag” that location, which consists of entering a description and choosing a category. Later on, you’ll also allow the user to pick a photo.

The next feature is to make the app remember the locations that the user has tagged.

This chapter covers the following:

  • Core Data overview: A brief overview of what Core Data is and how it works.
  • Add Core Data: Add the Core Data framework to the app and use it.
  • The data store: Initializing the data store used by Core Data.
  • Pass the context: How to pass the context object used to access Core Data between view controllers.
  • Browse the data: Looking through the saved data.
  • Save the locations: Saving entered location information using Core Data.
  • Handle Core Data errors: Handling Core Data errors when there’s an issue with saving.

Core Data overview

You have to persist the data for these captured locations somehow — they need to be remembered even when the app terminates.

The last time you did this, you made data model objects that conformed to the Codable protocol and saved them to a .plist file. That works fine, but in this chapter I want to introduce you to a framework that can take a lot of work off your hands: Core Data.

Core Data is an object persistence framework for iOS apps. If you’ve looked at Core Data before, you may have found the official documentation a little daunting, but the principle is quite simple.

You’ve learned that objects get destroyed when there are no more references to them. In addition, all objects get destroyed when the app terminates.

With Core Data, you can designate some objects as being persistent so they will always be saved to a data store. Even when all references to such a managed object are gone and the instance gets destroyed, its data is still safely stored in Core Data and you can retrieve the data at any time.

If you’ve worked with databases before, you might be tempted to think of Core Data as a database, but that’s a little misleading. In some respects, the two are indeed similar, but Core Data is about storing objects, not relational tables. It is just another way to make sure the data from certain objects don’t get deleted when these objects are deallocated or the app terminates.

Using Core Data

Core Data requires the use of a data model. This is a special file that you add to your project to describe the objects that you want to persist. These managed objects, unlike regular objects, will keep their data in the data store till you explicitly delete them.

The data model

Back in Chapter 22, when you first created the MyLocations project, the project settings had an option named Storage which you set to Core Data. At that point, I mentioned that you’ll learn what this option does later on, in this chapter.

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Generate the code

➤ Click on the Location entity to select it and go to the Data Model inspector.

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Fix the code

Unfortunately, Xcode made a few small boo-boos in the types of the properties, so you’ll have to make some changes to Location+CoreDataProperties.swift.

The data store

On iOS, Core Data stores all of its data into an SQLite — pronounced “SQL light” — database. It’s OK if you have no idea what SQLite is. You’ll take a peek into that database later, but you don’t really need to know what goes on inside the data store in order to use Core Data.

Pass the context

When the user presses the Done button in the Tag Location screen, the app currently just closes the screen. Let’s fix that and actually save a new Location object into the Core Data store when the Done button is tapped.

Get the context

➤ Switch to LocationDetailsViewController.swift. First, import Core Data at the top, and then add a new instance variable:

Pass the context from SceneDelegate

SceneDelegate.swift now needs some way to pass the NSManagedObjectContext object to CurrentLocationViewController.

Browse the data

Core Data stores the data in an SQLite database. That file is named MyLocations.sqlite and it lives in the app’s Library folder. That’s similar to the Documents folder that you saw previously.

Core Data data store location

➤ The easiest way to find the location of the Core Data folder is to add the following to Functions.swift:

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Browse the Core Data store using a GUI app

You will use Liya to examine the data store file. Download it from the Mac App Store or cutedgesystems.com/software/liya/.

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Troubleshoot Core Data issues

There is another handy tool for troubleshooting Core Data. By setting a special flag on the app, you can see the SQL statements that Core Data uses under the hood to talk to the data store.

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Save the locations

You’ve successfully initialized Core Data and passed the NSManagedObjectContext to the Tag Location screen. Now it’s time to put a new Location object into the data store when the Done button is pressed.

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Handle Core Data errors

To save the contents of the context to the data store, you did:

Fake errors for testing purposes

Here’s a way to fake such a fatal error, just to illustrate what happens.

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Alert the user about crashes

➤ Add the following code to Functions.swift:

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26. Adding Polish
28. The Locations Tab
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