Dependency Injection by Protocol Extension

Dependency Injection by Protocol Extension

import UIKit
import PlaygroundSupport

//MARK: Networking
protocol NetworkServiceProtocol {
    func test()
}

struct NetworkService: NetworkServiceProtocol {
    func test() {
        print("Regular test")
    }
}

struct MockNetworkService: NetworkServiceProtocol {
    func test() {
        print("Mock test")
    }
}

//MARK: Dependencies
struct Dependencies {
    let networkService: NetworkServiceProtocol
}

extension Dependencies {
    #if false
    static let shared = Dependencies()
    private init() {
        networkService = NetworkService()
    }
    #else
    static var shared = Dependencies()
    private init() {
        networkService = MockNetworkService()
    }
    static func make(
        networkService: NetworkServiceProtocol = MockNetworkService()
    ) -> Dependencies {
        return Dependencies(
            networkService: networkService
        )
    }
    #endif
}

//MARK: Injection Protocols
protocol NetworkServiceInjected {
    var networkService: NetworkServiceProtocol { get }
}

extension NetworkServiceInjected {
    var networkService: NetworkServiceProtocol {
        return Dependencies.shared.networkService
    }
}

//MARK: Instance
class V: UIViewController, NetworkServiceInjected { }

let v = V()
v.networkService.test()
Dependencies.shared = Dependencies.make(
    networkService: NetworkService()
)
v.networkService.test()