Dependency injection example with tests

DependencyInjectionExample.swift

import Foundation
import XCTest

//////////////////////////////////////////////////////////////////////

// Preferences

protocol UserDefaults {
    func stringForKey(key: String) -> String
}

class Preferences {
    let userDefaults: UserDefaults

    init(userDefaults: UserDefaults) {
        self.userDefaults = userDefaults
    }

    var username: String {
        return userDefaults.stringForKey("Username")
    }
}

class PreferencesTests: XCTestCase {
    func testGetsUsernameFromUserDefaults() {
        let userDefaultsSpy = UserDefaultsSpy()
        userDefaultsSpy.stringForKeyResult = "john"
        let preferences = Preferences(userDefaults: userDefaultsSpy)

        let username = preferences.username

        XCTAssertTrue(userDefaultsSpy.didCallStringForKey)
        XCTAssertEqual(userDefaultsSpy.keyPassedToStringForKey, "Username")
        XCTAssertEqual(username, "john")
    }
}

//////////////////////////////////////////////////////////////////////

// Preferences logger

protocol LogOutput {
    func logMessage(message: String)
}

class PreferencesLogger {
    let preferences: Preferences
    let output: LogOutput

    init(preferences: Preferences, output: LogOutput) {
        self.preferences = preferences
        self.output = output
    }

    func log() {
        output.logMessage("Username: \(preferences.username)")
    }
}

class PreferencesLoggerTests: XCTestCase {
    func testLogsUsername() {
        let userDefaultsStub = UserDefaultsSpy()
        userDefaultsStub.stringForKeyResult = "john"
        let preferences = Preferences(userDefaults: userDefaultsStub)
        let logOutputSpy = LogOutputSpy()
        let preferencesLogger = PreferencesLogger(preferences: preferences, output: logOutputSpy)

        preferencesLogger.log()

        XCTAssertEqual(logOutputSpy.loggedMessage, "Username: john")
    }
}

//////////////////////////////////////////////////////////////////////

// Test doubles

class UserDefaultsSpy: UserDefaults {
    var stringForKeyResult = ""
    private(set) var didCallStringForKey = false
    private(set) var keyPassedToStringForKey = ""

    func stringForKey(key: String) -> String {
        didCallStringForKey = true
        keyPassedToStringForKey = key
        return stringForKeyResult
    }
}

class LogOutputSpy: LogOutput {
    private(set) var loggedMessage = ""

    func logMessage(message: String) {
        loggedMessage = message
    }
}

//////////////////////////////////////////////////////////////////////

// UserDefaults implementation

class SystemUserDefaultsAdapter: UserDefaults {
    let systemUserDefaults: NSUserDefaults

    init(systemUserDefaults: NSUserDefaults) {
        self.systemUserDefaults = systemUserDefaults
    }

    func stringForKey(key: String) -> String {
        if let string = systemUserDefaults.stringForKey(key) {
            return string
        } else {
            fatalError("Could not find string for key: \(key)")
        }
    }
}

// LogOutput implementation

class SystemConsoleOutput: LogOutput {
    func logMessage(message: String) {
        print(message)
    }
}

//////////////////////////////////////////////////////////////////////

// Usage

NSUserDefaults.standardUserDefaults().setObject("john", forKey: "Username")

let userDefaults = SystemUserDefaultsAdapter(systemUserDefaults: NSUserDefaults.standardUserDefaults())
let preferences = Preferences(userDefaults: userDefaults)
let preferencesLogger = PreferencesLogger(preferences: preferences, output: SystemConsoleOutput())
preferencesLogger.log()

NSUserDefaults.standardUserDefaults().removeObjectForKey("Username")