This document discusses saving data using the Codable protocol to encode and decode model objects and save them to files in the app's Documents directory. It shows how to encode and decode a Note object using a PropertyListEncoder and PropertyListDecoder. It also demonstrates encoding and decoding an array of Note objects to save multiple items at once. The key steps are to implement Codable on model objects, perform the encoding/decoding in a model controller, and archive the data during app life cycle events like entering the background. The goal of the lesson is to persist data between app launches using Codable and storing files in the Documents directory.

1. Unit 4—Lesson 7:
Saving Data

2. Saving data
View Model
User action Update
Update Notify
Controller

3. Encoding and decoding with Codable
class Note: Codable {…}
Use an Encoder object to encode
Use a Decoder object to decode

4. Encoding
Encoding and decoding with Codable
struct Note: Codable {
let title: String
let text: String
let timestamp: Date
}
let newNote = Note(title: "Dry cleaning", text: "Pick up suit from dry cleaners",
timestamp: Date())
let propertyListEncoder = PropertyListEncoder()
if let encodedNotes = try? propertyListEncoder.encode(newNote) {
. . .
}

5. Decoding
Encoding and decoding with Codable
let propertyListDecoder = PropertyListDecoder()
if let decodedNote = try? propertyListDecoder.decode(Note.self, from: encodedNote) {
. . .
}

6. App sandbox

7. App sandbox

8. Sandboxing
Writing data to a file
App File Manager
Documents
Documents
Documents

9. Documents directory
Writing data to a file
let documentsDirectory = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask).first!
let archiveURL =
documentsDirectory.appendingPathComponent("appData").appendingPathExtension("plist")

10. Writing the data
Writing data to a file
let documentsDirectory = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask).first!
let archiveURL =
documentsDirectory.appendingPathComponent("notes_data").appendingPathExtension("plist")
let propertyListEncoder = PropertyListEncoder()
let encodedData = try? propertyListEncoder.encode(data)
try? encodedData?.write(to: archiveURL, options: .noFileProtection)

11. Reading the data
Writing data to a file
let documentsDirectory = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask).first!
let archiveURL =
documentsDirectory.appendingPathComponent("appData").appendingPathExtension("plist")
let propertyListDecoder = PropertyListDecoder()
if let retrievedData = try? Data(contentsOf: archiveURL),
let decodedNote = try? propertyListDecoder.decode(Note.self, from: retrievedNoteData) {
. . .
}

12. Saving an array of model data
Writing data to a file
let notes = [note1, note2, note3]
let documentsDirectory = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask).first!
let archiveURL =
documentsDirectory.appendingPathComponent("notes_data").appendingPathExtension("plist")
let propertyListEncoder = PropertyListEncoder()
let encodedData = try? propertyListEncoder.encode(notes)
try? encodedData?.write(to: archiveURL, options: .noFileProtection)

13. Reading an array of model data
Writing data to a file
let documentsDirectory = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask).first!
let archiveURL =
documentsDirectory.appendingPathComponent("notes_data").appendingPathExtension("plist")
let propertyListDecoder = PropertyListDecoder()
if let retrievedNotesData = try? Data(contentsOf: archiveURL),
let decodedNotes = try? propertyListDecoder.decode(Array<Note>.self,
from: retrievedNotesData) {
...
}

14. Remember
Your model objects should implement the Codable protocol.
Reading and writing should happen in the model controller.
Archive in the correct app delegate life-cycle events. For example:
• When the app enters the background
• When the app is terminated

15. Saving Data
Unit 4—Lesson 7
Learn how to persist data using Codable, a protocol for saving files to your app’s
Documents directory.

16. Lab: Remember Your Best Friends
Unit 4—Lesson 7
Use the Codable protocol to persist information between launches of an app
listing your best friends.

17. © 2017 Apple Inc.
This work is licensed by Apple Inc. under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license.