Creating a mock UserDefaults

mockingUserDefaults.swift

import Testing
import Foundation
@testable import ExampleApp

class MockUserDefaults: UserDefaults {
    var persistedData: Data? = nil
    var persistedKey: String? = nil
    
    override func set(_ value: Any?, forKey defaultName: String) {
        persistedData = value as? Data
        persistedKey = defaultName
    }
    
    override func object(forKey defaultName: String) -> Any? {
        return persistedData
    }
}

@Suite("Some Service that DI's UserDefaults") struct SomeServiceTest {
  @MainActor @Test("Service restores default values from UserDefaults on init()") func doesSomethingWithUserDefaults() throws {
    // Setup - create something to store in UserDefaults and set it for the key that would be used
    let valueToStore = "bar"
    let someTestDataToStore = SomeModel(foo: valueToStore)
    let mockUserDefaults = MockUserDefaults()
    let testData = try? JSONEncoder().encode(someTestDataToStore)
    mockUserDefaults.set(testData, forKey: "whateverKeyIsUsed")
    
    // Init the service and verify that it pulls the value out
    let service = SomeServiceThatUsesUserDefaults(userDefaults: mockUserDefaults)
    #expect(service.foo == valueToStore)
  }
}