Saving Local Data with Core Data

Overview

To store data persistently within an application, Xcode provides the Core Data framework. It's a powerful and flexible object graph and persistence framework that allows you to manage the model layer of your application.

Reference:

• Complete iOS Development Series 1-14

Step 1: Enabling Core Data in Your Project

The easiest way to set up Core Data is to enable it when you create a new project. Simply check the "Use Core Data" box in the project setup screen.

This will automatically generate the necessary Core Data stack setup code in your AppDelegate.swift file.

Step 2: Defining Your Data Model

Core Data uses a data model file (with a .xcdatamodeld extension) to define your app's data structures. Think of this as defining the schema for your local database.

1. Open the .xcdatamodeld file in your project.
2. Click "Add Entity" to create a new entity (similar to a table in a database or a collection in Firebase). For this example, we'll name it Shopping.
3. With the Shopping entity selected, add attributes (like columns in a table). We'll add name (String), size (String), and id (UUID).

Step 3: Accessing the Core Data Context

To interact with Core Data (saving, fetching, etc.), you need access to the NSManagedObjectContext. The boilerplate code for this is in AppDelegate. Here’s how you get a reference to it from your UIViewController:

// In your UIViewController
import UIKit
import CoreData

// Get a reference to the AppDelegate
let appDelegate = UIApplication.shared.delegate as! AppDelegate
// Get the managed object context
let context = appDelegate.persistentContainer.viewContext

Step 4: Creating and Saving Data

Now, let's write the code to save a new Shopping item.

1. Create a New Managed Object

First, create a new instance of your entity.

let newShoppingItem = NSEntityDescription.insertNewObject(forEntityName: "Shopping", into: context)

2. Set Attribute Values

Use the setValue(_:forKey:) method to populate the attributes of your new object.

// Assuming you have nameTextField and sizeTextField outlets
newShoppingItem.setValue(nameTextField.text!, forKey: "name")
newShoppingItem.setValue(sizeTextField.text!, forKey: "size")
newShoppingItem.setValue(UUID(), forKey: "id") // Assign a unique ID

3. Save the Context

The changes you've made so far are only in memory. To persist them to the device's storage, you must save the context. This is a "throwing" operation, so it needs to be done within a do-try-catch block.

do {
    try context.save()
    print("Success: Data saved!")
} catch {
    print("Error saving context: \(error)")
}

Here is the complete function for saving the data:

@IBAction func saveButtonTapped(_ sender: UIButton) {
    // 1. Get reference to AppDelegate and context
    guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else { return }
    let context = appDelegate.persistentContainer.viewContext

    // 2. Create a new object
    let newShoppingItem = NSEntityDescription.insertNewObject(forEntityName: "Shopping", into: context)

    // 3. Set its values
    newShoppingItem.setValue(nameTextField.text!, forKey: "name")
    newShoppingItem.setValue(sizeTextField.text!, forKey: "size")
    newShoppingItem.setValue(UUID(), forKey: "id")

    // 4. Save the context
    do {
        try context.save()
        print("Successfully saved data.")
        // Optionally, clear text fields or navigate away
    } catch {
        print("Failed to save data: \(error)")
    }
}