How to test and mock property wrappers in Swift? - ios

Question

Let's say I have a very common use case for a property wrapper using UserDefaults.
#propertyWrapper
struct DefaultsStorage<Value> {
private let key: String
private let storage: UserDefaults
var wrappedValue: Value? {
get {
guard let value = storage.value(forKey: key) as? Value else {
return nil
}
return value
}
nonmutating set {
storage.setValue(newValue, forKey: key)
}
}
init(key: String, storage: UserDefaults = .standard) {
self.key = key
self.storage = storage
}
}
I am now declaring an object that would hold all my values stored in UserDefaults.
struct UserDefaultsStorage {
#DefaultsStorage(key: "userName")
var userName: String?
}
Now when I want to use it somewhere, let's say in a view model, I would have something like this.
final class ViewModel {
func getUserName() -> String? {
UserDefaultsStorage().userName
}
}
Few questions arise here.
It seems that I am obliged to use .standard user defaults in this case. How to test that view model using other/mocked instance of UserDefaults?
How to test that property wrapper using other/mocked instance of UserDefaults? Do I have to create a new type that is a clean copy of the above's DefaultsStorage, pass mocked UserDefaults and test that object?
struct TestUserDefaultsStorage {
#DefaultsStorage(key: "userName", storage: UserDefaults(suiteName: #file)!)
var userName: String?
}

Answer 1

As #mat already mentioned in the comments, you need a protocol to mock UserDefaults dependency. Something like this will do:
protocol UserDefaultsStorage {
func value(forKey key: String) -> Any?
func setValue(_ value: Any?, forKey key: String)
}
extension UserDefaults: UserDefaultsStorage {}
Then you can change your DefaultsStorage propertyWrapper to use a UserDefaultsStorage reference instead of UserDefaults:
#propertyWrapper
struct DefaultsStorage<Value> {
private let key: String
private let storage: UserDefaultsStorage
var wrappedValue: Value? {
get {
return storage.value(forKey: key) as? Value
}
nonmutating set {
storage.setValue(newValue, forKey: key)
}
}
init(key: String, storage: UserDefaultsStorage = UserDefaults.standard) {
self.key = key
self.storage = storage
}
}
After that a mock UserDefaultsStorage might look like this:
class UserDefaultsStorageMock: UserDefaultsStorage {
var values: [String: Any]
init(values: [String: Any] = [:]) {
self.values = values
}
func value(forKey key: String) -> Any? {
return values[key]
}
func setValue(_ value: Any?, forKey key: String) {
values[key] = value
}
}
And to test DefaultsStorage, pass an instance of UserDefaultsStorageMock as its storage parameter:
import XCTest
class DefaultsStorageTests: XCTestCase {
class TestUserDefaultsStorage {
#DefaultsStorage(
key: "userName",
storage: UserDefaultsStorageMock(values: ["userName": "TestUsername"])
)
var userName: String?
}
func test_userName() {
let testUserDefaultsStorage = TestUserDefaultsStorage()
XCTAssertEqual(testUserDefaultsStorage.userName, "TestUsername")
}
}

Answer 2

This might not be the best solution, however, I haven't figured out a way to inject UserDefaults that use property wrappers into a ViewModel. If there is such an option, then gcharita's proposal to use another protocol would be a good one to implement.
I used the same UserDefaults in the test class as in the ViewModel. I save the original values before each test and restore them after each test.
class ViewModelTests: XCTestCase {
private lazy var userDefaults = newUserDefaults()
private var preTestsInitialValues: PreTestsInitialValues!
override func setUpWithError() throws {
savePreTestUserDefaults()
}
override func tearDownWithError() throws {
restoreUserDefaults()
}
private func newUserDefaults() -> UserDefaults.Type {
return UserDefaults.self
}
private func savePreTestUserDefaults() {
preTestsInitialValues = PreTestsInitialValues(userName: userDefaults.userName)
}
private func restoreUserDefaults() {
userDefaults.userName = preTestsInitialValues.userName
}
func testUsername() throws {
//"inject" User Defaults with the desired values
let username = "No one"
userDefaults.userName = username
let viewModel = ViewModel()
let usernameFromViewModel = viewModel.getUserName()
XCTAssertEqual(username, usernameFromViewModel)
}
}
struct PreTestsInitialValues {
let userName: String?
}