Cake Pattern Example in Swift (Demo at iOSCon 2017 in London)

CakePatternExample.swift

// Demo at iOSCon 2017 in London
// https://speakerdeck.com/yoichitgy/dependency-injection-in-practice-ioscon
//
// How to Use: Paste this program to a Playground.
// Tested with: Xcode 8.2.1

// Interface (protocol)
protocol Logger {
    func log(_ message: String)
}

protocol SocialService {
    func fetchFollowers(callback: ([String]) -> Void)
}

// Implementation
struct PrintLogger: Logger {
    func log(_ message: String) {
        print("LOG: \(message)")
    }
}

struct HardcodedSocialService: SocialService {
    var logger: Logger?
    
    func fetchFollowers(callback: ([String]) -> Void) {
        let followers = ["Anya", "Yoichi"]
        logger?.log("Fetch completed.")
        callback(followers)
    }
}

// Abstract Components
protocol LoggerComponent {
    static var logger: Logger { get }
    static func createLogger() -> Logger
}

protocol SocialServiceComponent {
    static var socialService: SocialService { get }
    static func createSocialService() -> SocialService
}

// Concrete Components
protocol PrintLoggerComponent: LoggerComponent { }
extension PrintLoggerComponent {
    static func createLogger() -> Logger {
        return PrintLogger()
    }
}

protocol HardcodedSocialServiceComponent: SocialServiceComponent { }
extension HardcodedSocialServiceComponent where Self: LoggerComponent {
    static func createSocialService() -> SocialService {
        return HardcodedSocialService(logger: logger)
    }
}

class Context: PrintLoggerComponent, HardcodedSocialServiceComponent {
    static let logger = createLogger()
    static let socialService = createSocialService()
}

let socialService = Context.socialService
socialService.fetchFollowers { (followers) in
    print(followers)
}