iOS First Launch Helper

LaunchCounter.swift

import Foundation

/// Registers how many times an app has been launched, using UserDefaults as storage.
class LaunchCounter {

    private var defaults: UserDefaults
    private var defaultsKey = "launchCount"

    /// Initialise, optionally with a custom UserDefaults instance
    init(defaults: UserDefaults = .standard) {
        self.defaults = defaults
    }

    /// The number of times the app has launched
    var launchCount: Int {
        get { return defaults.integer(forKey: defaultsKey) }
        set { defaults.set(newValue, forKey: defaultsKey) }
    }

    /// True if this is the first time the app has launched
    var isFirstLaunch: Bool {
        return launchCount == 1
    }

    /// Register an app launch. Call before any other methods, and call only once.
    func registerLaunch() {
        launchCount += 1
    }

}

LaunchCounterTests.swift

import XCTest
@testable import App

class LaunchCounterTests: XCTestCase {

    let launchCounter = LaunchCounter(defaults: MockUserDefaults())

    func testLaunchCounter() {
        XCTAssert(launchCounter.launchCount == 0, "Identify never launched")
        XCTAssert(launchCounter.isFirstLaunch == false, "Don't identify never launched as first")

        launchCounter.registerLaunch()
        XCTAssert(launchCounter.isFirstLaunch == true, "Identify first launch")

        launchCounter.registerLaunch()
        XCTAssert(launchCounter.launchCount == 2, "Identify second launch")
        XCTAssert(launchCounter.isFirstLaunch == false, "Don't identify second launch as first")

        let randomCount = Int(arc4random_uniform(100) + 1)
        (0..<randomCount).forEach { _ in launchCounter.registerLaunch() }

        XCTAssert(launchCounter.launchCount == 2 + randomCount, "Identify Nth launch")
        XCTAssert(launchCounter.isFirstLaunch == false, "Don't identify Nth launch as first")
    }

}

MockUserDefaults.swift

import Foundation

class MockUserDefaults: UserDefaults {

    var storage: [String:Any] = [:]

    override func object(forKey defaultName: String) -> Any? {
        return storage[defaultName]
    }

    override func set(_ value: Any?, forKey defaultName: String) {
        storage[defaultName] = value
    }

}