iOS – Is Awesome Design Pattern Valuable In Swift ?

Hello Readers, CoolMonkTechie heartily welcomes you in this article.

In this article, We will learn about Why design patterns are important and which one is the most popular frequently used design patterns in Swift. Swift is a programming language that allows developers to create versatile applications for multiple operating systems (though it is most frequently used to write applications for iOS). When we are new in programming languages, we don’t know which design patterns we should use with it and how to implement them.

Being able to use a relevant design pattern is a prerequisite to creating functional, high-quality, and secure applications. 

We’ve decided to help by taking an in-depth look at the design patterns most widely used in Swift and showing different approaches to solving common problems in mobile development with them.

A famous quote about learning is :

“ Anyone who stops learning is old, whether at twenty or eighty. Anyone who keeps learning stays young. ” 

So Let’s begin.

Design Patterns: What they are and why you should know them ?

A software design pattern is a solution to a particular problem you might face when designing an app’s architecture. But unlike out-of-the-box services or open-source libraries, we can’t simply paste a design pattern into our application because it isn’t a piece of code. Rather, it’s a general concept for how to solve a problem. A design pattern is a template that tells you how to write code, but it’s up to you to fit our code to this template.

Design patterns bring several benefits:

• Tested solutions. We don’t need to waste time and reinvent the wheel trying to solve a particular software development problem, as design patterns already provide the best solution and tell us how to implement it.
• Code unification. Design patterns provide us with typical solutions that have been tested for drawbacks and bugs, helping us make fewer mistakes when designing our app architecture.
• Common vocabulary. Instead of providing in-depth explanations of how to solve this or that software development problem, we can simply say what design pattern we used and other developers will immediately understand what solutions we implemented.

Types of Software Design Patterns

Before we describe the most common architecture patterns in Swift, you should first learn the three types of software design patterns and how they differ:

• Creational Design Patterns
• Structural Design Patterns
• Behavioral Design Patterns

1. Creational Design Patterns

Creational software design patterns deal with object creation mechanisms, which increase flexibility and reuse of existing code. They try to instantiate objects in a manner suitable for the particular situation. Here are several creational design patterns:

• Factory Method
• Abstract Factory
• Builder
• Singleton
• Prototype

2. Structural Design Patterns

Structural design patterns aim to simplify the design by finding an easy way of realizing relationships between classes and objects. These patterns explain how to assemble objects and classes into larger structures while keeping these structures flexible and efficient.These are some structural architecture patterns:

• Adapter
• Bridge
• Facade
• Decorator
• Composite
• Flyweight
• Proxy

3. Behavioral Design Patterns

Behaviour design patterns identify common communication patterns between entities and implement these patterns. 

These patterns are concerned with algorithms and the assignment of responsibilities between objects. Behavioral design patterns include:

• Chain of Responsibility
• Template Method
• Command
• Iterator
• Mediator
• Memento
• Observer
• Strategy
• State
• Visitor

Most of these design patterns, however, are rarely used, and you’re likely to forget how they work before you even need them. So we’ve handpicked the five design patterns most frequently used in Swift to develop applications for iOS and other operating systems.

Most frequently used design patterns in Swift

We’re going to provide only the essential information about each software design pattern – namely, how it works from the technical point of view and when it should be applied. We’ll also give an illustrative example in the Swift programming language.

1. Builder

The Builder pattern is a creational design pattern that allows us to create complex objects from simple objects step by step. This design pattern helps us use the same code for creating different object views.

Imagine a complex object that requires incremental initialization of multiple fields and nested objects. Typically, the initialization code for such objects is hidden inside a mammoth constructor with dozens of parameters. Or even worse, it can be scattered all over the client code.

The Builder design pattern calls for separating the construction of an object from its own class. The construction of this object is instead assigned to special objects called builders and split into multiple steps. To create an object, you successively call builder methods. And you don’t need to go through all the steps – only those required for creating an object with a particular configuration.

You should apply the Builder design pattern :

• when you want to avoid using a telescopic constructor (when a constructor has too many parameters, it gets difficult to read and manage);
• when your code needs to create different views of a specific object;
• when you need to compose complex objects.

Example:

Suppose you’re developing an iOS application for a restaurant and you need to implement ordering functionality. You can introduce two structures, Dish and Order, and with the help of the OrderBuilder object, you can compose orders with different sets of dishes.

// Design Patterns: Builder

import Foundation

// Models

enum DishCategory: Int {
    case firstCourses, mainCourses, garnishes, drinks
}

struct Dish {
    var name: String
    var price: Float
}

struct OrderItem {
    var dish: Dish
    var count: Int
}

struct Order {
    var firstCourses: [OrderItem] = []
    var mainCourses: [OrderItem] = []
    var garnishes: [OrderItem] = []
    var drinks: [OrderItem] = []
    
    var price: Float {
        let items = firstCourses + mainCourses + garnishes + drinks
        return items.reduce(Float(0), { $0 + $1.dish.price * Float($1.count) })
    }
}

// Builder

class OrderBuilder {
    private var order: Order?
    
    func reset() {
        order = Order()
    }
    
    func setFirstCourse(_ dish: Dish) {
        set(dish, at: order?.firstCourses, withCategory: .firstCourses)
    }
    
    func setMainCourse(_ dish: Dish) {
        set(dish, at: order?.mainCourses, withCategory: .mainCourses)
    }
    
    func setGarnish(_ dish: Dish) {
        set(dish, at: order?.garnishes, withCategory: .garnishes)
    }
    
    func setDrink(_ dish: Dish) {
        set(dish, at: order?.drinks, withCategory: .drinks)
    }
    
    func getResult() -> Order? {
        return order ?? nil
    }
    
    private func set(_ dish: Dish, at orderCategory: [OrderItem]?, withCategory dishCategory: DishCategory) {
        guard let orderCategory = orderCategory else {
            return
        }
        
        var item: OrderItem! = orderCategory.filter( { $0.dish.name == dish.name } ).first
        
        guard item == nil else {
            item.count += 1
            return
        }
        
        item = OrderItem(dish: dish, count: 1)
        
        switch dishCategory {
        case .firstCourses:
            order?.firstCourses.append(item)
        case .mainCourses:
            order?.mainCourses.append(item)
        case .garnishes:
            order?.garnishes.append(item)
        case .drinks:
            order?.drinks.append(item)
        }
    }
}

// Usage

let steak = Dish(name: "Steak", price: 2.30)
let chips = Dish(name: "Chips", price: 1.20)
let coffee = Dish(name: "Coffee", price: 0.80)

let builder = OrderBuilder()
builder.reset()
builder.setMainCourse(steak)
builder.setGarnish(chips)
builder.setDrink(coffee)

let order = builder.getResult()
order?.price

// Result:
// 4.30

2. Adapter

Adapter is a structural design pattern that allows objects with incompatible interfaces to work together. In other words, it transforms the interface of an object to adapt it to a different object.

An adapter wraps an object, therefore concealing it completely from another object. For example, you could wrap an object that handles meters with an adapter that converts data into feet.

You should use the Adapter design pattern:

• when you want to use a third-party class but its interface doesn’t match the rest of your application’s code;
• when you need to use several existing subclasses but they lack particular functionality and, on top of that, you can’t extend the superclass.

Example:

Suppose you want to implement a calendar and event management functionality in your iOS application. To do this, you should integrate the EventKit framework and adapt the Event model from the framework to the model in your application. An Adapter can wrap the model of the framework and make it compatible with the model in your application.

// Design Patterns: Adapter

import EventKit

// Models

protocol Event: class {
    var title: String { get }
    var startDate: String { get }
    var endDate: String { get }
}

extension Event {
    var description: String {
        return "Name: \(title)\nEvent start: \(startDate)\nEvent end: \(endDate)"
    }
}

class LocalEvent: Event {
    var title: String
    var startDate: String
    var endDate: String
    
    init(title: String, startDate: String, endDate: String) {
        self.title = title
        self.startDate = startDate
        self.endDate = endDate
    }
}

// Adapter

class EKEventAdapter: Event {
    private var event: EKEvent
    
    private lazy var dateFormatter: DateFormatter = {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "MM-dd-yyyy HH:mm"
        return dateFormatter
    }()
    
    var title: String {
        return event.title
    }
    var startDate: String {
        return dateFormatter.string(from: event.startDate)
    }
    var endDate: String {
        return dateFormatter.string(from: event.endDate)
    }
    
    init(event: EKEvent) {
        self.event = event
    }
}

// Usage

let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "MM/dd/yyyy HH:mm"

let eventStore = EKEventStore()
let event = EKEvent(eventStore: eventStore)
event.title = "Design Pattern Meetup"
event.startDate = dateFormatter.date(from: "06/29/2018 18:00")
event.endDate = dateFormatter.date(from: "06/29/2018 19:30")

let adapter = EKEventAdapter(event: event)
adapter.description

// Result:
// Name: Design Pattern Meetup
// Event start: 06-29-2018 18:00
// Event end: 06-29-2018 19:30

3. Decorator

The Decorator pattern is a structural design pattern that allows you to dynamically attach new functionalities to an object by wrapping them in useful wrappers.

No wonder this design pattern is also called the Wrapper design pattern. This name describes more precisely the core idea behind this pattern: you place a target object inside another wrapper object that triggers the basic behavior of the target object and adds its own behavior to the result.

Both objects share the same interface, so it doesn’t matter for a user which of the objects they interact with − clean or wrapped. You can use several wrappers simultaneously and get the combined behavior of all these wrappers.

You should use the Decorator design pattern :

• when you want to add responsibilities to objects dynamically and conceal those objects from the code that uses them;
• when it’s impossible to extend responsibilities of an object through inheritance.

Example :

Imagine you need to implement data management in your iOS application. You could create two decorators: EncryptionDecorator for encrypting and decrypting data and EncodingDecorator for encoding and decoding.

// Design Patterns: Decorator

import Foundation

// Helpers (may be not include in blog post)

func encryptString(_ string: String, with encryptionKey: String) -> String {
    let stringBytes = [UInt8](string.utf8)
    let keyBytes = [UInt8](encryptionKey.utf8)
    var encryptedBytes: [UInt8] = []
    
    for stringByte in stringBytes.enumerated() {
        encryptedBytes.append(stringByte.element ^ keyBytes[stringByte.offset % encryptionKey.count])
    }
    
    return String(bytes: encryptedBytes, encoding: .utf8)!
}

func decryptString(_ string: String, with encryptionKey: String) -> String {
    let stringBytes = [UInt8](string.utf8)
    let keyBytes = [UInt8](encryptionKey.utf8)
    var decryptedBytes: [UInt8] = []
    
    for stringByte in stringBytes.enumerated() {
        decryptedBytes.append(stringByte.element ^ keyBytes[stringByte.offset % encryptionKey.count])
    }
    
    return String(bytes: decryptedBytes, encoding: .utf8)!
}

// Services

protocol DataSource: class {
    func writeData(_ data: Any)
    func readData() -> Any
}

class UserDefaultsDataSource: DataSource {
    private let userDefaultsKey: String
    
    init(userDefaultsKey: String) {
        self.userDefaultsKey = userDefaultsKey
    }
    
    func writeData(_ data: Any) {
        UserDefaults.standard.set(data, forKey: userDefaultsKey)
    }
    
    func readData() -> Any {
        return UserDefaults.standard.value(forKey: userDefaultsKey)!
    }
}

// Decorators

class DataSourceDecorator: DataSource {
    let wrappee: DataSource
    
    init(wrappee: DataSource) {
        self.wrappee = wrappee
    }
    
    func writeData(_ data: Any) {
        wrappee.writeData(data)
    }
    
    func readData() -> Any {
        return wrappee.readData()
    }
}

class EncodingDecorator: DataSourceDecorator {
    private let encoding: String.Encoding
    
    init(wrappee: DataSource, encoding: String.Encoding) {
        self.encoding = encoding
        super.init(wrappee: wrappee)
    }
    
    override func writeData(_ data: Any) {
        let stringData = (data as! String).data(using: encoding)!
        wrappee.writeData(stringData)
    }
    
    override func readData() -> Any {
        let data = wrappee.readData() as! Data
        return String(data: data, encoding: encoding)!
    }
}

class EncryptionDecorator: DataSourceDecorator {
    private let encryptionKey: String
    
    init(wrappee: DataSource, encryptionKey: String) {
        self.encryptionKey = encryptionKey
        super.init(wrappee: wrappee)
    }
    
    override func writeData(_ data: Any) {
        let encryptedString = encryptString(data as! String, with: encryptionKey)
        wrappee.writeData(encryptedString)
    }
    
    override func readData() -> Any {
        let encryptedString = wrappee.readData() as! String
        return decryptString(encryptedString, with: encryptionKey)
    }
}

// Usage

var source: DataSource = UserDefaultsDataSource(userDefaultsKey: "decorator")
source = EncodingDecorator(wrappee: source, encoding: .utf8)
source = EncryptionDecorator(wrappee: source, encryptionKey: "secret")
source.writeData("Design Patterns")
source.readData() as! String

// Result:
// Design Patterns

4. Facade

Facade is a structural design pattern that provides a simplified interface to a library, a framework, or any other complex set of classes.

Imagine that your code has to deal with multiple objects of a complex library or framework. You need to initialize all these objects, keep track of the right order of dependencies, and so on. As a result, the business logic of your classes gets intertwined with implementation details of other classes. Such code is difficult to read and maintain.

The Facade pattern provides a simple interface for working with complex subsystems containing lots of classes. The Facade pattern offers a simplified interface with limited functionality that you can extend by using a complex subsystem directly. This simplified interface provides only the features a client needs while concealing all others.

You should use the Facade design pattern:

• when you want to provide a simple or unified interface to a complex subsystem;
• when you need to decompose a subsystem into separate layers.

Example:

Lots of modern mobile applications support audio recording and playback, so let’s suppose you need to implement this functionality. You could use the Facade pattern to hide the implementation of services responsible for the file system (FileService), audio sessions (AudioSessionService), audio recording (RecorderService), and audio playback (PlayerService). The Facade provides a simplified interface for this rather complex system of classes.

// Design Patterns: Facade

import AVFoundation

// Services (may be not include in blog post)

struct FileService {
    private var documentDirectory: URL {
        return FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
    }
    
    var contentsOfDocumentDirectory: [URL] {
        return try! FileManager.default.contentsOfDirectory(at: documentDirectory, includingPropertiesForKeys: nil)
    }
    
    func path(withPathComponent component: String) -> URL {
        return documentDirectory.appendingPathComponent(component)
    }
    
    func removeItem(at index: Int) {
        let url = contentsOfDocumentDirectory[index]
        try! FileManager.default.removeItem(at: url)
    }
}

protocol AudioSessionServiceDelegate: class {
    func audioSessionService(_ audioSessionService: AudioSessionService, recordPermissionDidAllow allowed: Bool)
}

class AudioSessionService {
    weak var delegate: AudioSessionServiceDelegate?
    
    func setupSession() {
        try! AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord, with: [.defaultToSpeaker])
        try! AVAudioSession.sharedInstance().setActive(true)
        
        AVAudioSession.sharedInstance().requestRecordPermission { [weak self] allowed in
            DispatchQueue.main.async {
                guard let strongSelf = self, let delegate = strongSelf.delegate else {
                    return
                }
                
                delegate.audioSessionService(strongSelf, recordPermissionDidAllow: allowed)
            }
        }
    }
    
    func deactivateSession() {
        try! AVAudioSession.sharedInstance().setActive(false)
    }
}

struct RecorderService {
    private var isRecording = false
    private var recorder: AVAudioRecorder!
    private var url: URL
    
    init(url: URL) {
        self.url = url
    }
    
    mutating func startRecord() {
        guard !isRecording else {
            return
        }
        
        isRecording = !isRecording
        recorder = try! AVAudioRecorder(url: url, settings: [AVFormatIDKey: kAudioFormatMPEG4AAC])
        recorder.record()
    }
    
    mutating func stopRecord() {
        guard isRecording else {
            return
        }
        
        isRecording = !isRecording
        recorder.stop()
    }
}

protocol PlayerServiceDelegate: class {
    func playerService(_ playerService: PlayerService, playingDidFinish success: Bool)
}

class PlayerService: NSObject, AVAudioPlayerDelegate {
    private var player: AVAudioPlayer!
    private var url: URL
    weak var delegate: PlayerServiceDelegate?
    
    init(url: URL) {
        self.url = url
    }
    
    func startPlay() {
        player = try! AVAudioPlayer(contentsOf: url)
        player.delegate = self
        player.play()
    }
    
    func stopPlay() {
        player.stop()
    }
    
    func audioPlayerDidFinishPlaying(_ player: AVAudioPlayer, successfully flag: Bool) {
        delegate?.playerService(self, playingDidFinish: flag)
    }
}

// Facade

protocol AudioFacadeDelegate: class {
    func audioFacadePlayingDidFinish(_ audioFacade: AudioFacade)
}

class AudioFacade: PlayerServiceDelegate {
    private let audioSessionService = AudioSessionService()
    private let fileService = FileService()
    private let fileFormat = ".m4a"
    private var playerService: PlayerService!
    private var recorderService: RecorderService!
    weak var delegate: AudioFacadeDelegate?
    
    private lazy var dateFormatter: DateFormatter = {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd_HH:mm:ss"
        return dateFormatter
    }()
    
    init() {
        audioSessionService.setupSession()
    }
    
    deinit {
        audioSessionService.deactivateSession()
    }
    
    func startRecord() {
        let fileName = dateFormatter.string(from: Date()).appending(fileFormat)
        let url = fileService.path(withPathComponent: fileName)
        recorderService = RecorderService(url: url)
        recorderService.startRecord()
    }
    
    func stopRecord() {
        recorderService.stopRecord()
    }
    
    func numberOfRecords() -> Int {
        return fileService.contentsOfDocumentDirectory.count
    }
    
    func nameOfRecord(at index: Int) -> String {
        let url = fileService.contentsOfDocumentDirectory[index]
        return url.lastPathComponent
    }
    
    func removeRecord(at index: Int) {
        fileService.removeItem(at: index)
    }
    
    func playRecord(at index: Int) {
        let url = fileService.contentsOfDocumentDirectory[index]
        playerService = PlayerService(url: url)
        playerService.delegate = self
        playerService.startPlay()
    }
    
    func stopPlayRecord() {
        playerService.stopPlay()
    }
    
    func playerService(_ playerService: PlayerService, playingDidFinish success: Bool) {
        if success {
            delegate?.audioFacadePlayingDidFinish(self)
        }
    }
}

// Usage

let audioFacade = AudioFacade()
audioFacade.numberOfRecords()

// Result:
// 0

5. Template Method

The Template Method pattern is a behavioral design pattern that defines a skeleton for an algorithm and delegates responsibility for some steps to subclasses. This pattern allows subclasses to redefine certain steps of an algorithm without changing its overall structure.

This design pattern splits an algorithm into a sequence of steps, describes these steps in separate methods, and calls them consecutively with the help of a single template method.

You should use the Template Method design pattern:

• when subclasses need to extend a basic algorithm without modifying its structure;
• when you have several classes responsible for quite similar actions (meaning that whenever you modify one class, you need to change the other classes).

Example:

Suppose you’re working on an iOS app that must be able to take and save pictures. Therefore, your application needs to get permissions to use the iPhone (or iPad) camera and image gallery. To do this, you can use the PermissionService base class that has a specific algorithm.

To get permission to use the camera and gallery, you can create two subclasses, CameraPermissionService and PhotoPermissionService, that redefine certain steps of the algorithm while keeping other steps the same.

// Design Patterns: Template Method

import AVFoundation
import Photos

// Services

typealias AuthorizationCompletion = (status: Bool, message: String)

class PermissionService: NSObject {
    private var message: String = ""
    
    func authorize(_ completion: @escaping (AuthorizationCompletion) -> Void) {
        let status = checkStatus()
        
        guard !status else {
            complete(with: status, completion)
            return
        }
        
        requestAuthorization { [weak self] status in
            self?.complete(with: status, completion)
        }
    }

    func checkStatus() -> Bool {
        return false
    }
    
    func requestAuthorization(_ completion: @escaping (Bool) -> Void) {
        completion(false)
    }
    
    func formMessage(with status: Bool) {
        let messagePrefix = status ? "You have access to " : "You haven't access to "
        let nameOfCurrentPermissionService = String(describing: type(of: self))
        let nameOfBasePermissionService = String(describing: type(of: PermissionService.self))
        let messageSuffix = nameOfCurrentPermissionService.components(separatedBy: nameOfBasePermissionService).first!
        message = messagePrefix + messageSuffix
    }
    
    private func complete(with status: Bool, _ completion: @escaping (AuthorizationCompletion) -> Void) {
        formMessage(with: status)
        
        let result = (status: status, message: message)
        completion(result)
    }
}

class CameraPermissionService: PermissionService {
    override func checkStatus() -> Bool {
        let status = AVCaptureDevice.authorizationStatus(for: .video).rawValue
        return status == AVAuthorizationStatus.authorized.rawValue
    }
    
    override func requestAuthorization(_ completion: @escaping (Bool) -> Void) {
        AVCaptureDevice.requestAccess(for: .video) { status in
            completion(status)
        }
    }
}

class PhotoPermissionService: PermissionService {
    override func checkStatus() -> Bool {
        let status = PHPhotoLibrary.authorizationStatus().rawValue
        return status == PHAuthorizationStatus.authorized.rawValue
    }
    
    override func requestAuthorization(_ completion: @escaping (Bool) -> Void) {
        PHPhotoLibrary.requestAuthorization { status in
            completion(status.rawValue == PHAuthorizationStatus.authorized.rawValue)
        }
    }
}

// Usage

let permissionServices = [CameraPermissionService(), PhotoPermissionService()]

for permissionService in permissionServices {
    permissionService.authorize { (_, message) in
        print(message)
    }
}

// Result:
// You have access to Camera
// You have access to Photo

That’s all about in this article.

Conclusion

In this article, We understood about the five design patterns most frequently used in Swift. The ability to pick a design pattern in Swift that’s relevant for building a particular project allows you to build fully functional and secure applications that are easy to maintain and upgrade. You should certainly have design patterns in your skillset, as they not only simplify software development but also optimize the whole process and ensure high code quality.

Thanks for reading ! I hope you enjoyed and learned about the most frequently used Design Patterns in Swift. Reading is one thing, but the only way to master it is to do it yourself.

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