iOS Unit Testing Demo — XCTest + async/await

A complete guide to unit testing in iOS — testing ViewModels, Use Cases, and the Network layer using XCTest, Mocks, and async/await.

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What is Unit Testing?

A unit test verifies that one small piece of logic works correctly in complete isolation — no real network, no real database, no UI.

Without tests:             With tests:
"I think it works" ❓      "I know it works" ✅
Manual testing every time  Automated — runs in seconds
Fear of changing code      Confidence to refactor freely

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What is a Mock?

A Mock is a fake version of a dependency used only in tests.

// Real — hits the network
class UserRepository: UserRepositoryProtocol {
    func fetchUsers() async throws -> [User] {
        // real API call...
    }
}

// Mock — returns fake data instantly
class MockUserRepository: UserRepositoryProtocol {
    var usersToReturn: [User] = []
    var errorToThrow: Error? = nil

    func fetchUsers() async throws -> [User] {
        if let error = errorToThrow { throw error }
        return usersToReturn   // instant, predictable, no network
    }
}

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Test Structure — AAA Pattern

Every test follows the Arrange → Act → Assert pattern:

func test_loadUsers_onSuccess_populatesUsersArray() async {
    // ARRANGE — set up the scenario
    mockRepository.usersToReturn = [
        User(id: 1, name: "Ali", email: "ali@test.com")
    ]

    // ACT — run the function under test
    await sut.loadUsers()

    // ASSERT — verify the outcome
    XCTAssertEqual(sut.users.count, 1)
    XCTAssertNil(sut.errorMessage)
    XCTAssertFalse(sut.isLoading)
}

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Tests Included

UseCase Tests — FetchUsersUseCaseTests

Test	Verifies
test_execute_withValidUsers_returnsSortedUsers	Business rule: sorted A→Z
test_execute_withEmptyEmailUser_filtersThemOut	Business rule: no empty emails
test_execute_withEmptyList_returnsEmptyArray	Edge case: empty result
test_execute_whenRepositoryThrows_propagatesError	Error propagation
test_execute_callsRepositoryExactlyOnce	No duplicate calls
test_execute_withAllEmptyEmails_returnsEmptyArray	All filtered out

ViewModel Tests — UsersViewModelTests

Test	Verifies
test_loadUsers_onSuccess_populatesUsersArray	Happy path
test_loadUsers_onSuccess_clearsErrorMessage	Error cleared on retry
test_loadUsers_withEmptyResult_hasEmptyUsersArray	Empty state
test_loadUsers_isLoadingFalse_afterCompletion	Loading resets
test_loadUsers_onNoInternet_setsErrorMessage	Network error UI
test_loadUsers_onUnauthorized_setsCorrectError	401 error message
test_loadUsers_isLoadingFalse_evenAfterError	Loading resets on failure

Network Tests — NetworkClientTests

Test	Verifies
test_request_withValidJSON_decodesCorrectly	JSON decoding
test_request_withUsersListJSON_decodesAllUsers	Array decoding
test_request_withEmptyArray_returnsEmptyList	Empty array
test_request_withInvalidJSON_throwsDecodingError	Malformed JSON
test_request_whenUnauthorized_throwsUnauthorizedError	401 error
test_request_isCalledExactlyOnce	Call count tracking

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Project Structure

iOSUnitTesting/
├── Source/                          ← Production code
│   ├── Models/
│   │   └── User.swift
│   ├── Network/
│   │   ├── NetworkError.swift
│   │   └── NetworkClient.swift
│   ├── Repository/
│   │   ├── UserRepositoryProtocol.swift
│   │   └── UserRepository.swift
│   ├── UseCases/
│   │   └── FetchUsersUseCase.swift
│   └── ViewModel/
│       └── UsersViewModel.swift
│
└── Tests/                           ← Test code
    ├── Mocks/
    │   ├── MockUserRepository.swift  ← Fake repository
    │   └── MockNetworkClient.swift   ← Fake network + test data
    ├── UseCaseTests/
    │   └── FetchUsersUseCaseTests.swift
    ├── ViewModelTests/
    │   └── UsersViewModelTests.swift
    └── NetworkTests/
        └── NetworkClientTests.swift

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How to Run Tests

Xcode → Product → Test   (⌘ + U)

All tests should pass with green checkmarks ✅

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Key Principles

Principle	Description
Test in isolation	Each test focuses on one class only
No real network	Mocks replace all external dependencies
AAA Pattern	Arrange → Act → Assert, every test
One assertion per test	Clear failure messages
Protocol-based DI	Makes mocking possible

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Requirements

• iOS 17.0+
• Xcode 15.0+
• Swift 5.9+

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Author

Bilal Zafar — iOS Developer
GitHub • LinkedIn