iOS Beginners Lesson 2

1. DATA MODEL
Building our App

2. OVERVIEW
• Lesson 1: Introductions
• Lesson 2: iOS specifics
• Lesson 3: Data Model
• Lesson 4: Logic (Controller) & Interface

3. LESSON 3: DATA MODEL
• Hour 1: Local Storage and Data Class
• Hour 2: Dictionary and User Defaults
• Hour 3: Save to File

4. Local Storage and Data Class

5. • Instantiating objects from classes (NSObject)
keep your object’s data in your app/device
memory
• These data “disappears” when your app is
shut down or if you shut down/reboot your
phone
• To persist your data for your user, you need
“local storage” (i.e. data is written into our
Local Storage?

6. Local Storage?

7. <Application Home>/AppName.app
• Known as the bundle directory
• Do not write anything into this directory
• To prevent tampering, the bundle directory is
signed at installation time
• If you write into this directory after your app
launches, your app cannot be launched again
Commonly used directories for
Storage

8. <Application Home>/Documents/
• Store critical user documents and app data files
• Critical data refers to any data that cannot be re-
created by our app
• Contents of this directory can be made available to
user through file sharing
• Contents of this directory are backed up by iTunes
Commonly used directories for
Storage

9. <Application Home>/Documents/Inbox/
• Use this directory to access files that your app was
asked to open by outside entities, e.g. Mail program
• Your app can read and delete files in this directory but
cannot create new files or write into existing files
• If user needs to edit file in this directory, app must
silently move it out of directory before making changes
• Contents of this directory are backed up by iTunes
Commonly used directories for
Storage

10. <Application Home>/Library/
• Non-user data files. This directory should not be used for user data files. Use it for your app
support files.
• The contents of this directory, except for the Caches subdirectory, are backed up by iTunes
• iOS 5.0 & earlier, put app support files in <Application Home>/Library/Caches so these files
don’t get backed up
• iOS 5.0.1 and later, app support files go into <Application Home>/Library/Application Support
directory and is applied with the com.apple.MobileBackup extended attribute to each support
file so files do not get backed up to either iTunes or iCloud
• If you have a lot of support files, store them all in a custom sub-directory and apply the
extended attribute to that sub-directory
• Data cache files are placed in <Application Home>/Library/Caches directory. Database cache
files, downloadable content used by magazine, newspaper and map apps are also stored here.
Your app needs to be able to gracefully handle situations where cached data is deleted by the
system to free up disk space
Commonly used directories for
Storage

11. <Application Home>/tmp/
• This directory is used to place data which we
do not need to keep for extended periods of
time
• It’s a good habit to delete files in this tmp
directory when we no longer need to use the
file we have placed there temporarily.
Commonly used directories for
Storage

12. • User Defaults (the ‘standard’ plist)
• Different file types (PDFs, text, doc, images
as png, jpg etc etc)
• CoreData (SQLite database)
• 3rd party solutions like Realm.io
Options for Local Storage?

13. User Defaults, also known as “Shared
Preferences”
• Key/value pairs
• Managed through Objective-C
• Stored in XML files that are managed through
Apple’s API
• Stored in unencrypted format (“plain text”)
Options for Local Storage?

14. File Storage
• Files can be created and read on disk
• No special file type, so store any file type that
you like (XML, JSON, delimited data csv, PDF,
png, jpg etc)
• Store these files “internal to app” or “external to
app” (in a shared file locations, such a Photo
Albums)
Options for Local Storage?

15. SQLite and Core Data
• Structured data with relationships can be saved into
an SQLite database
• iOS provides built-in support for SQLite
(www.sqlite.org)
• All classes and interfaces are in CoreData package
• Database files are stored in “internal location” in
app (within the app sandbox)
Options for Local Storage?

16. • We can use User Defaults
• Each note can be represented as a key-value pair in User
Defaults
‣ key: a unique date-time stamp
‣ value: string
• Practical example: User defaults (NSUserDefaults)
• Practical example: File Storage
How do we store Notes?

17. • Representation of what your app does (using custom
classes, subclass from NSObject for example)
• Spaceship game app will have Spaceship class,
TakeNotes app will have Note class, Painting app
could define a Image class
• Can subclass from UIDocument as well if we want to
define a document-based data model
• Other possibilities if using CoreData
(NSManagedObject) or Realm.io (RLMObject)
Data model objects

18. Create our “Data” class for
Notes

19. Create our “Data” class for
Notes

20. Create our “Data” class for
Notes

21. Create our “Data” class for
Notes

22. Create our “Data” class for
Notes

23. Dictionary and User Defaults

24. Our first method

25. Our first method

26. Other methods

27. Save to File

28. Save into Filesystem

29. Retrieve from Filesystem

30. WHAT’S NEXT
• Lesson 1: Introductions
• Lesson 2: iOS specifics
• Lesson 3: Data Model
• Lesson 4: Logic (Controller) & Interface