ViewModel testing in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a ViewModel instance in a test.

Android Kotlin

val viewModel = [1]()

Drag options to blanks, or click blank then click option'

ALiveData

BMyViewModel

CViewModelProvider

DMutableLiveData

Attempts:

3 left

💡 Hint

Common Mistakes

Using ViewModelProvider in unit tests without Android framework.

Trying to instantiate LiveData instead of ViewModel.

You create an instance of your ViewModel class directly in tests to test its logic.

2fill in blank

medium

Complete the code to observe LiveData value changes in a test.

Android Kotlin

viewModel.data.observeForever [1]

Drag options to blanks, or click blank then click option'

A{ value -> println(value) }

B(value) -> println(value)

CObserver { value -> println(value) }

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using Observer class syntax instead of lambda.

Passing 'this' which is not a valid observer.

In tests, observeForever takes a lambda to react to LiveData changes.

3fill in blank

hard

Fix the error in the test code to get LiveData value synchronously.

Android Kotlin

val value = viewModel.data.[1]()

Drag options to blanks, or click blank then click option'

ApostValue

BgetValue

Cobserve

DgetOrAwaitValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using getValue which returns null if no value is set.

Using observe which requires a lifecycle owner.

getOrAwaitValue is a helper function to get LiveData value in tests synchronously.

4fill in blank

hard

Fill both blanks to mock a repository and inject it into ViewModel for testing.

Android Kotlin

val mockRepo = [1]()
val viewModel = MyViewModel([2])

Drag options to blanks, or click blank then click option'

Amockk

BmockRepo

Crepository

Dmock

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the class name instead of the mock instance.

Not using a mocking library to create the mock.

Use mockk() to create a mock repository, then pass it to ViewModel constructor.

5fill in blank

hard

Fill all three blanks to verify a repository method call in ViewModel test.

Android Kotlin

verify(exactly = [1]) { mockRepo.[2]([3]) }

Drag options to blanks, or click blank then click option'

A1

BfetchData

Cany()

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 for exactly means no calls expected.

Not using any() causes verification to fail if argument differs.

Verify that fetchData was called exactly once with any argument.

Practice

1.

What is the main purpose of testing a ViewModel in Android development?

easy

A. To validate user input directly on the screen

B. To verify the data handling and business logic without UI involvement

C. To test the navigation between different screens

D. To check the layout and visual appearance of the app

Solution

1. Step 1: Understand ViewModel role

   The ViewModel holds and manages UI-related data but does not handle UI rendering.

2. Step 2: Identify testing focus

   Testing ViewModel focuses on data logic and state management, not UI or navigation.

3. Final Answer:

   To verify the data handling and business logic without UI involvement -> Option B

4. Quick Check:

   ViewModel testing = data logic check [OK]

Hint: ViewModel tests focus on data, not UI or navigation [OK]

Common Mistakes:

• Confusing ViewModel testing with UI testing
• Thinking ViewModel tests check screen layouts
• Assuming navigation is tested in ViewModel tests

2.

Which annotation is commonly used to set up a ViewModel instance before each test method runs?

easy

A. @Ignore

B. @Test

C. @After

D. @Before

Solution

1. Step 1: Recall JUnit lifecycle annotations

   @Before runs before each test method to prepare test environment.

2. Step 2: Match annotation to ViewModel setup

   Use @Before to initialize ViewModel before tests run.

3. Final Answer:

   @Before -> Option D

4. Quick Check:

   Setup before tests = @Before [OK]

Hint: Use @Before to prepare ViewModel before each test [OK]

Common Mistakes:

• Using @Test instead of @Before for setup
• Confusing @After with setup annotation
• Skipping setup annotation entirely

3.

Given the following test code snippet, what will be the value of result after running the test?

class MyViewModelTest {
  private lateinit var viewModel: MyViewModel

  @Before
  fun setup() {
    viewModel = MyViewModel()
  }

  @Test
  fun testIncrement() {
    viewModel.increment()
    val result = viewModel.counter
  }
}

Assuming counter starts at 0 and increment() adds 1.

medium

A. 1

B. 0

C. null

D. Throws an exception

Solution

1. Step 1: Understand initial state

   ViewModel's counter starts at 0 before increment.

2. Step 2: Apply increment method

   Calling increment() adds 1, so counter becomes 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Initial 0 + 1 increment = 1 [OK]

Hint: Increment adds 1 to counter starting at 0 [OK]

Common Mistakes:

• Assuming counter remains 0 after increment
• Thinking result is null without initialization
• Expecting an exception without error in code

4.

Identify the error in this ViewModel test code snippet:

class SampleViewModelTest {
  private lateinit var viewModel: SampleViewModel

  @Test
  fun testReset() {
    viewModel.reset()
    assertEquals(0, viewModel.counter)
  }
}

Assuming viewModel is not initialized anywhere else.

medium

A. Missing @Before setup to initialize viewModel before test

B. Incorrect assertEquals parameter order

C. reset() method does not exist in ViewModel

D. Test method missing @Test annotation

Solution

1. Step 1: Check viewModel initialization

   viewModel is declared but never initialized before test runs.

2. Step 2: Identify missing setup

   Without @Before method to initialize viewModel, calling reset() causes error.

3. Final Answer:

   Missing @Before setup to initialize viewModel before test -> Option A

4. Quick Check:

   Initialize viewModel before test = @Before needed [OK]

Hint: Always initialize ViewModel before tests with @Before [OK]

Common Mistakes:

• Forgetting to initialize viewModel before use
• Mixing assertEquals parameter order
• Assuming reset() method is missing without checking

5.

You want to test a ViewModel method that fetches data asynchronously and updates a LiveData property. Which approach is best to test this behavior?

hard

A. Mock the LiveData property and verify it was called

B. Call the method and immediately check LiveData value without waiting

C. Use a coroutine test rule to run suspend functions and observe LiveData changes synchronously

D. Test only the UI observing the LiveData, not the ViewModel method

Solution

1. Step 1: Understand asynchronous data fetching

   ViewModel uses coroutines to fetch data and update LiveData asynchronously.

2. Step 2: Use coroutine test tools

   Coroutine test rules allow running suspend functions in tests synchronously for reliable assertions.

3. Step 3: Observe LiveData changes properly

   Observing LiveData synchronously ensures test waits for updates before asserting values.

4. Final Answer:

   Use a coroutine test rule to run suspend functions and observe LiveData changes synchronously -> Option C

5. Quick Check:

   Async ViewModel test = coroutine test rule + LiveData observe [OK]

Hint: Use coroutine test rule to handle async LiveData updates [OK]

Common Mistakes:

• Checking LiveData immediately without waiting
• Mocking LiveData instead of testing real updates
• Testing UI instead of ViewModel logic