Lecture Mobile Application Development (119310)

iOS Framework Basics

Agenda

Framework Components - Storage Options
- NSUser Defaults
- Property List
- Core Data

Storing Data on iOS devices locally

Data persistence is one of the most important functions in application development. Options to store data on iOS devices are (amonst others):
- NSUser Defaults
- Property List
- Core Data
- SQLite
- Key Chain
We will focus here on the first three and most often used options to persist data on iOS

NSUser Defaults

Simple built in data dictionary for storing small amounts of data such as e.g. user settings
Example how to store simple data types:


let defaults = UserDefaults.standard
// String
defaults.set("Hans Mueller", forKey: "Name")
// Int
defaults.set(65, forKey: "Age")
// Date object
defaults.set(Date(), forKey: "AccessTime")
// Array
let array = ["Hello", "HdM"]
defaults.set(array, forKey: "StoredArray")
// Dictionary
let dict = ["Firstname": "Hans", "Lastname": "Mueller"]
defaults.set(dict, forKey: "StoredDict")

NSUser Defaults

Reading objects will never fail but return default values if the key does not exist:
- integer(forKey:) returns an integer if the key existed, or 0 if not.
- bool(forKey:) returns a boolean if the key existed, or false if not.
- float(forKey:) returns a float if the key existed, or 0.0 if not.
- double(forKey:) returns a double if the key existed, or 0.0 if not.
When retrieving objects, the result is optional. This means you can either accept the optionality, or typecast it to a non-optional type and use the nil coalescing operator to handle missing values. For example object(forKey:) returns AnyObject? so you need to conditionally typecast it to your data type:
```
let savedArray = defaults.object(forKey: "SavedArray") as? [String] ?? [String]()
```

Property Lists

Property lists are XML files that comply to Apple’s PropertyList DTD
An example for a property list is the Info.plist file that resides in all app projects
To create your own .plist file go in the Xcode menu to File->New->File->Resource->Property List

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Property Lists

To access a property file within the app project use:

let path = Bundle.main.path(forResource: "myprop", ofType: "plist")
let dict = NSDictionary(contentsOfFile: path!)

Now you can read the contents like:

// reading a String item
let tableData = dict!.object(forKey: "testitem") as! String
// reading an Array of Strings
let devices = dict!.object(forKey: "devices") as! [String]

Property Lists

To write to a property file, it needs to be loaded into a mutable dictionary

// load the property file
let writableProp = NSMutableDictionary(contentsOfFile: path!)
// change a value
writableProp?.setValue("Ansgar Gerlicher", forKey: "Name")
// write the property file atomically
writableProp?.write(toFile: path!, atomically: true)

Core Data

Core Data is an ORM framework to persist objects in iOS
To activate it just check the CoreData box, when creating your app project

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This will add some extra code to your AppDelegate class allowing you to access the NSPersistentContainer instance to read and store data

Core Data

In order to read and store data using Core Data the following steps are necessary:
- Create a Data Model using the integrated Data Model editor in XCode (here we create a Person entity):

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Inside your ViewController (or whereever you need to access your data model):
- Import CoreData
- Obtain an reference to the NSPersistentContainer instance and to the persistence context (viewContext) for this container:

let appDelegate = UIApplication.shared.delegate as! AppDelegate
let container = appDelegate.persistentContainer
let context = container.viewContext

Core Data

With Xcode version > 8.0 the classes for the entities that are defined in the model editor are automatically generated in the background.
Thus it is very easy to create a new instance of our Person class and set it’s properties:

let person = Person(context: context)
person.name = "Ansgar R. S. Gerlicher"
person.age = 22

Core Data

In order to save the data persistently all you need to do is call save()on the context:

 do {
   try context.save()
 } catch {
   print("Failed saving context")
 }

Core Data

To read stored data from the context, you need to use the NSFetchRequest class to define what objects you want to read
Then use the fetch()function to load the objects from the store

 let request : NSFetchRequest<Person> = Person.fetchRequest()
       
 do {
    let result = try context.fetch(request)
    for personObject in result {
        print(personObject)
    }
} catch {
    print("Loading Person failed")
}

Summary

In order to persist data on an iOS device you have many options:
Little information such as settings can be stored using the NSUser Defaults
App specific information and predefined data can be stored in Property list files
Images and binary data can be stored on the file systems
App specific data or user generated data can be stored in databases using Core Data ORM framework

Recab Questions

What kind of data would you store how?
When is the best time to persist data?
What is the best way to load data from a storage?