AC-iOS Codable and UserDefaults (Swift 4.x)

Readings

1. Ultimate Guide to Parsing With Swift
2. An Introduction to NSUserDefaults

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0. Objectives

1. Become familiar with using UserDefaults to store data
2. Understand that UserDefaults is a light-weight, persistent storage option for small amounts of data that relate to how your app should be configured, based on the user's selection/choices.
3. Become with the Codable protocol that allows for easy conversion between Swift objects and storeable Data

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1. Codable: What a time to be alive!

Many of you will be starting your journey of Swift data serialization with some incredibly powerful and easy-to-use tools. I, of course, am refering to the Codable protocol which was introduced in Swift 4 to save developers from the agony that is de/serializing Data.

Serialization: What is it?

Serialization, in development, refers to converting an object/struct/var into raw data, essentially 0's and 1's. Serialization is performed when you need to store language/code-specific objects into persistant storage. Then later, when you need to retrieve the object from memory you de-serialize it, meaning you convert it from its raw binary back into useable objects in code.

Serialization: Where does it happen?

All over. But one very common scenario is making URLRequests. Remember how the completion handlers for our network requests looked like:

func makeRequest(completion: @escaping (Data?)-> Void) {
	// ... awesome code ...
}

They took a parameter of Data? because a response sent back from an API is all raw data! It's up to our code to deserialize it so that we can use it in our apps. Thats what JSONSerialization was being used for! It should be easy to see why that class was named the way it was: it's tasked with deserializing json-formatted Data into a Swift-useable type, Any.

Another place it is commonly used is to store/retrieve information from UserDefaults, which is what we're going to take a look at first.

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2. UserDefaults: Light-weight, simple persistant storage

The intended use of UserDefaults is in the name: you want to store a user's default preferences when using your app. For example, suppose your app offered a "night mode". A user might only want to use your app in "night mode" because it's easier for them to read. It wouldn't be a great experience for them, however, if they had to switch the app into "night mode" every time they opened the app. Instead, you store the user's preference for using "night mode" in UserDefaults and then every time your app launches you retrieve that stored information and update your app before it finishes launching.

via Slashgear

Where does Codable come in?

UserDefaults is a really great option for storing simple data types -- in fact, it only can natively store Swift or Foundation types. This includes (these Foundation objects are bridged to their Swift equivalents):

- NSData
- NSString
- NSNumber
	- UInt
	- Int
	- Float
	- Double
	- Bool
- NSDate
- NSArray
- NSDictionary

The tricky part is if you want to store custom objects that you've defined in code.

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3. Trying out UserDefaults

Note: We're going to be writing this code inside of didFinishLaunching in the AppDelegate.swift file.

Let's take a look at a few basic examples of storing and retrieving data.

It all begins with the standard singleton:

let defaults = UserDefaults.standard

Note: It's possible to define a separate UserDefaults by creating one with a new id, but unnecessary for our purposes.

From here, it's as simple as calling set(_:forKey:) function on defaults in order to store data:

	let defaults = UserDefaults.standard

	// store a string
	let string = "Hello, World"
	defaults.set(string, forKey: "stringKey")

	// store a number
	let number = 10
	defaults.set(number, forKey: "numbersKey")

	// store an array
	let arr = ["Hello", "World"]
	defaults.set(arr, forKey: "arrKey")

	// store a dict
	let dict = ["Hello" : "World"]
	defaults.set(dict, forKey: "dictKey")

Retrieving that data is only slightly more work since you need to account for optionals and types:

	// retrieve a string
	if let aString = defaults.value(forKey: "stringKey") as? String {
		print("String Retrieved: \(aString)")
	}

	// retrieve a number
	if let aNumber = defaults.value(forKey: "numbersKey") as? Int {
		print("Number Retrieved: \(aNumber)")
	}

	// retrieve an array
	if let aArr = defaults.value(forKey: "arrKey") as? [String] {
		print("Array Retrieved: \(aArr)")
	}

	// retrieve a dict
	if let aDict = defaults.value(forKey: "dictKey") as? [String:String] {
		print("Dictionary Retrieved: \(aDict)")
	}

Let's try a few out now:

Practice

Instructions: For each of these data structures, you must store them to UserDefaults and ensure that you can retrieve and print their properties just as we did in the first set of sample code:

// 1.
let numbersArr = [2, 4, 5, 6, 7, 10]

// 2.
let dates = [Date(), Date(), Date(), Date()]

// 3.
let largerDict = [
	"name" : "Louis",
	"cats_name" : "Miley",
	"location" : "Queens"
	]

// 4.
let mixedTypeDict: [String : Any] = [
	"name" : "Miley",
	"breed" : "American Shorthair",
	"age" : 5
	]

// 5.
let nestedArray = [
	[1, 2, 3, 4, 5],
	[10, 20, 30, 40, 50],
	[11, 12, 13, 14]
	]

// 6.
let nestedDictionary = [
	"cat" : [
		"name" : "Miley"
		],

	"dog" : [
		"name" : "Spot"
		]
	]

// 7.
let nestedStructure = [
	"cats" : [
		[ 	"name" : "Miley",
			"age"  : 5 ],
		[	"name" : "Bale",
			"age" : 14]
		]
	]

// 8. ~~ Advanced ~~
let advancedStructure: [String : Any] = [
	"cats" : [
		[	"name" : "Miley",
			"age"  : 5 		],
		[	"name" : "Bale",
			"age" : 14		]	],

	"dogs" : [
		[	"breed" : "Basset Hound",
			"age"  : 7 	],
		[	"breed" : "Greyhound",
			"age" : 3	]	],

	"stats" : [
		"scale" : "miles",
		"distance_to_sun" : 92.96,
		"distance_to_moon" : 238900
	]
]

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4. PropertyListEncoder/Decoder

Let's say we have an object called ReadingPreference that is suppose to track a user's preferences when reading in an app:

class ReadingPreference {
	var fontName: String
	var fontSize: Float
	var darkMode: Bool

	init(fontName: String, fontSize: Float, darkMode: Bool) {
		self.fontName = fontName
		self.fontSize = fontSize
		self.darkMode = darkMode
	}
}

let preferences = ReadingPreference(fontName: "Menlo", fontSize: 16.0, darkMode: true)

Ideally, we'd like to store this object in UserDefaults so that we can load this information every time the app launches. We know that UserDefaults.set(_:forKey:) accepts an object of type Any and UserDefaults.object(forKey:) returns Any, so why don't we try to store that info like so:

let defaults = UserDefaults.standard
defaults.set(preferences, forKey: "readingPreferencesKey")

// later to load it up:
let preference = defaults.object(forKey: "readingPreferencesKey") as! ReadingPreference

Go ahead and run this...

What happened exactly? Remember that UserDefaults has no problem storing native Swift data types listed earlier. What it cannot do is store custom objects that you've defined in code. If you try to do this, an exception is thrown and your app will crash. To get around this, we can (very) simply implement the Codable protocol:

// WHAT? all we do is add the Codable protocol keyword and nothing else??? 👍
class ReadingPreference: Codable {
	var fontName: String
	var fontSize: Float
	var darkMode: Bool

	init(fontName: String, fontSize: Float, darkMode: Bool) {
		self.fontName = fontName
		self.fontSize = fontSize
		self.darkMode = darkMode
	}
}

The core component of the Codable protocol is that so long as the properties of an object that conforms to Codable are Swift types, you get a free Dictionary representation of your object.

Important: Codable is a protocol that combines two other relavent protocols, Encodable and Decodable. Semantically, this means that an object which is "codable" should be both able to be "encoded" and "decoded." Remember that here, encoding/decoding are synonyms for serializing/deserializing

Now, there's still a little work that needs to be done in order to convert a ReadingPreference object into a data type that works with UserDefaults, but not terribly much work:

let preferences = ReadingPreference(fontName: "Menlo", fontSize: 16.0, darkMode: true)

// 1.
let encodedPreferences = try! PropertyListEncoder().encode(preferences)

// 2.
defaults.set(encodedPreferences, forKey: "encodedPreferences")

// 3.
let encoded = defaults.object(forKey: "encodedPreferences") as! Data

// 4.
let storedPrefs = try! PropertyListDecoder().decode(ReadingPreference.self, from: encoded)

print("\n\n\n\n\n\n\n")
print("Stored Preferences: \n\(storedPrefs.fontName), \(storedPrefs.fontSize), \(storedPrefs.darkMode)")

1. We make use of PropertyListEncoder which works with objects that conform to Encodable in order to create a Data object. encode(_:) can throw so we mark it with try
2. Data is one of the Swift types that UserDefaults can store, so we call set(_:forKey:)
3. When we wish to retrieve the object, we call object(forKey:) which returns Any?. We know it is Data so we force cast it.
4. Lastly, we use PropertyListDecoder.decode(_:from:). decode(_:from:) takes an object that is type of Decodable.Protocol in order to know what it should try to make the Data into. This also can throw so it is marked with a try

Kinda amazing... there's a lot of powerful magic going on with this protocol. Someday, you should go and look up the source code for it. But for now, let's just play around with some code examples.

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5. Codable Exercises

Ex 1. WarmUp: LoggedInUser

Now that we're storing a user's reading preferences successfully, let's also try storing some of their authentication information. Create a new class called LoggedInUser:

class LoggedInUser {
	var username: String
	var isPremium: Bool // if the user has a paid subscription
	var lastLogin: Date
}

Task: Attempt to store and retrieve a new LoggedInUser from UserDefaults.

Ex 2. In the Mix: Cart

Let's now imagine that our app offers some in-app purchases that we'd like to store in UserDefaults so that if the user adds some items to their cart but doesn't purchase them right away, it will still be there when they return to the app.

Let's begin with a CartItem, which will have a name for the product, a sku which will act as its unique identifier, and a price in dollars:

struct CartItem: Codable {
	let name: String
	let sku: Int
	let price: Double

	// Note: Adding the Codable protocol to this struct results in losing a struct's free initializer
	// This is because Codable comes with an initiaizer: init(decoder:)
	init(name: String, sku: Int, price: Double) {
		self.name = name
		self.sku = sku
		self.price = price
	}
}

CartItems should be managed by a Cart object:

class Cart {
	var items: [CartItem] = []

	init(items: [CartItem]) {
		self.items = items
	}

	// adds an items to the .items property
	func addItem(_ item: CartItem) {
	}

	// attempts remove an item, returns true if successful. false otherwise
	func removeItem(_ item: CartItem)  -> Bool {
	}
}

And lastly, saving and retrieving a cart should be done with a CartStorageManager

class CartStorageManager {
	let cart: Cart

	init(cart: Cart) {
		self.cart = cart
	}

	// Saves the cart to UserDefaults
	func saveCart() {
	}

	// Loads a Cart from UserDefaults if possible. If no cart is found stored in UserDefaults,
	// this returns an empty Cart
	class func loadCart() -> Cart {
	}
}

You're tasked with the following:

Note: Rememeber to uncomment the tests to ensure your implementation works.

1. Ensure that the necessary classes conform to Codable protocol in order to be able to save a Cart.
   • Note: A Cart consists of itself and its CartItems.
   • Note: Do all of the classes listed need to conform to Codable, or just some of them?
2. Create a few CartItems and add them to a Cart, test to make sure you're also able to remove them.
3. Add in the code for saveCart and loadCart in the CartStorageManager. The code should add the ability to save a Cart to UserDefaults and then later load it.
   • Be sure to test saving and loading! You should create a Cart with a few CartItems in it, save the Cart to UserDefaults and then retrieve the Cart. After retrieving the Cart, make sure that it contains all of the items that were stored with it.
   • Note: Nested Codable items work totally fine with UserDefaults!
4. (Advanced) Have your CartItem and Cart objects conform to Equatable. Implement anyway you see fit, but take into account that sku property of CartItem is suppose to be unique. Be sure to update your code where needed to make use of your new protocol conformance.

Reading Resource for Advanced:

1. Equatable and Comparable - Use Your Loaf