Unit testing ViewModels in iOS Swift - Mini App: Build & Ship

Build: CounterViewModel Test

This screen tests a simple CounterViewModel that increments and resets a count. It ensures the ViewModel logic works correctly without UI.

Target UI

-------------------------
| CounterViewModel Test |
|-----------------------|
| [Test Increment]      |
| [Test Reset]          |
|-----------------------|
| Results:              |
|                       |
|                       |
-------------------------

Create a CounterViewModel class with a count property starting at 0

Add increment() method to increase count by 1

Add reset() method to set count back to 0

Write unit tests to verify increment() and reset() behavior

Display test results in console or test output

Starter Code

iOS Swift

import Foundation

class CounterViewModel {
    // TODO: Add count property
    
    // TODO: Add increment() method
    
    // TODO: Add reset() method
}

// TODO: Write unit tests for CounterViewModel

Task 1

Task 2

Task 3

Task 4

Solution

iOS Swift

import Foundation
import XCTest

class CounterViewModel {
    private(set) var count: Int = 0
    
    func increment() {
        count += 1
    }
    
    func reset() {
        count = 0
    }
}

final class CounterViewModelTests: XCTestCase {
    var viewModel: CounterViewModel!
    
    override func setUp() {
        super.setUp()
        viewModel = CounterViewModel()
    }
    
    override func tearDown() {
        viewModel = nil
        super.tearDown()
    }
    
    func testIncrementIncreasesCount() {
        XCTAssertEqual(viewModel.count, 0, "Initial count should be 0")
        viewModel.increment()
        XCTAssertEqual(viewModel.count, 1, "Count should be 1 after one increment")
        viewModel.increment()
        XCTAssertEqual(viewModel.count, 2, "Count should be 2 after two increments")
    }
    
    func testResetSetsCountToZero() {
        viewModel.increment()
        XCTAssertEqual(viewModel.count, 1, "Count should be 1 before reset")
        viewModel.reset()
        XCTAssertEqual(viewModel.count, 0, "Count should be 0 after reset")
    }
}

We created a CounterViewModel class with a count property starting at 0. The increment() method adds 1 to count, and reset() sets it back to 0.

We then wrote unit tests using XCTest. The tests check that increment() correctly increases the count and that reset() sets it back to zero. This ensures the ViewModel logic works as expected without any UI.

Final Result

Completed Screen

-------------------------
| CounterViewModel Test |
|-----------------------|
| [Test Increment]      |
| [Test Reset]          |
|-----------------------|
| Results:              |
| All tests passed!     |
|                       |
-------------------------

Running tests shows 'All tests passed!' in the test console

If increment() or reset() fail, test output shows failure details

Stretch Goal

Add a test for multiple increments and verify count matches expected value

💡 Hint

Call increment() multiple times in a loop and check count equals the loop count

Practice

1. What is the main purpose of unit testing a ViewModel in iOS development?

easy

A. To verify the app's business logic without involving the user interface

B. To test the app's user interface elements visually

C. To check the app's network connectivity

D. To measure the app's performance speed

Solution

1. Step 1: Understand the role of ViewModel

   The ViewModel handles app logic and data preparation, separate from UI.

2. Step 2: Identify unit testing focus

   Unit testing targets logic correctness without UI involvement.

3. Final Answer:

   To verify the app's business logic without involving the user interface -> Option A

4. Quick Check:

   Unit testing ViewModel = test logic only [OK]

Hint: Unit tests check logic, not UI or performance [OK]

Common Mistakes:

• Confusing unit tests with UI tests
• Thinking unit tests check network or performance
• Assuming ViewModel tests require UI elements

2. Which XCTest method is used to prepare the environment before each test runs in a ViewModel test class?

easy

A. setUp()

B. runTest()

C. testExample()

D. tearDown()

Solution

1. Step 1: Recall XCTest lifecycle methods

   setUp() runs before each test to prepare needed objects.

2. Step 2: Differentiate from tearDown()

   tearDown() cleans up after each test, not before.

3. Final Answer:

   setUp() -> Option A

4. Quick Check:

   Prepare tests before running = setUp() [OK]

Hint: setUp() runs before each test to prepare environment [OK]

Common Mistakes:

• Using tearDown() to prepare tests instead of clean up
• Confusing testExample() as a setup method
• Assuming runTest() is a XCTest lifecycle method

3. Given this ViewModel test code snippet, what will be the value of result after the test runs?

class CounterViewModel {
  var count = 0
  func increment() { count += 1 }
}

func testIncrement() {
  let vm = CounterViewModel()
  vm.increment()
  let result = vm.count
}

medium

A. Compilation error

B. 0

C. nil

D. 1

Solution

1. Step 1: Analyze initial state

   CounterViewModel starts with count = 0.

2. Step 2: Apply increment method

   increment() adds 1 to count, so count becomes 1.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   Increment once = count 1 [OK]

Hint: Increment adds 1, so count becomes 1 [OK]

Common Mistakes:

• Assuming count stays 0 without increment
• Thinking result is nil because of missing return
• Expecting compilation error due to missing test annotation

4. Identify the error in this ViewModel unit test code:

class MyViewModel {
  var value = 0
  func updateValue() { value = 10 }
}

func testUpdateValue() {
  let vm = MyViewModel()
  vm.updateValue()
  XCTAssertEqual(vm.value, 0)
}

medium

A. updateValue() method is missing return type

B. XCTAssertEqual expects 10 but checks for 0

C. MyViewModel class is not marked as final

D. vm instance is not initialized properly

Solution

1. Step 1: Check updateValue() effect

   updateValue() sets value to 10, so vm.value should be 10.

2. Step 2: Compare XCTAssertEqual parameters

   Test expects vm.value to be 0, which is incorrect after updateValue().

3. Final Answer:

   XCTAssertEqual expects 10 but checks for 0 -> Option B

4. Quick Check:

   Test expected value must match updated value [OK]

Hint: Check expected vs actual values in XCTAssertEqual [OK]

Common Mistakes:

• Ignoring the expected value in XCTAssertEqual
• Thinking missing return type causes error here
• Assuming class must be final for testing

5. You want to test a ViewModel method that fetches user data asynchronously and updates a userName property. Which approach correctly tests this behavior using XCTest?

hard

A. Call the method, then immediately assert userName without waiting

B. Test only synchronous methods; skip async ones

C. Use XCTestExpectation to wait for the async update before asserting userName

D. Mock the ViewModel and do not call the real method

Solution

1. Step 1: Understand async testing need

   Async methods update properties after delay, so test must wait.

2. Step 2: Use XCTestExpectation

   Expectation waits for async completion before asserting userName value.

3. Final Answer:

   Use XCTestExpectation to wait for the async update before asserting userName -> Option C

4. Quick Check:

   Async test = use XCTestExpectation [OK]

Hint: Wait for async calls with XCTestExpectation before assertions [OK]

Common Mistakes:

• Asserting async results immediately without waiting
• Skipping async tests thinking they are too complex
• Mocking ViewModel without testing real async behavior