Repository testing with fakes in Android Kotlin - Build, Publish & Deploy

Build & Publish - Repository testing with fakes

Performance Impact

Using fakes in repository testing does not affect your app's runtime performance or battery life because these tests run separately from the app on your development machine. However, well-designed fakes help catch bugs early, preventing performance issues in the released app.

Optimization for Smooth Testing

Keep your fake implementations simple and focused on the data your tests need. Avoid complex logic or heavy operations in fakes to keep tests fast and reliable. Fast tests encourage frequent runs, improving code quality without slowing development.

App Bundle Size and Startup Time

Fakes are only used in testing and are not included in your app's release build. Therefore, they have no impact on your app's bundle size or startup time.

iOS vs Android Differences

On Android with Kotlin, repository testing with fakes is commonly done using JUnit and Kotlin classes. On iOS, similar testing uses Swift and XCTest with protocol-based fakes. Both platforms benefit from isolating data sources during tests, but the tools and languages differ.

Store Review Guidelines

Repository testing with fakes is a development practice and does not affect app store submissions directly. However, thorough testing helps ensure your app meets store quality guidelines by reducing crashes and bugs.

Self-Check Question

Your app takes 5 seconds to load this screen. What's likely wrong?

• The repository might be making real network or database calls during tests instead of using fakes, causing delays.
• Tests or app code may be blocking the main thread with heavy operations.

Key Result

Using fakes in repository testing improves code quality without impacting app performance or size, enabling faster development cycles and smoother app behavior in production.

Practice

1. What is the main purpose of using a fake repository in Android Kotlin testing?

easy

A. To replace the Android framework in tests

B. To improve UI animations during testing

C. To increase app size by adding more classes

D. To simulate data operations without accessing real databases or network

Solution

1. Step 1: Understand the role of a fake repository

   A fake repository mimics the real repository's behavior but uses in-memory data instead of real data sources.

2. Step 2: Identify the benefit in testing

   This allows tests to run quickly and reliably without depending on databases or network calls.

3. Final Answer:

   To simulate data operations without accessing real databases or network -> Option D

4. Quick Check:

   Fake repository = simulate data without real sources [OK]

Hint: Fakes replace real data sources to speed up tests [OK]

Common Mistakes:

• Thinking fakes improve UI animations
• Confusing fakes with Android framework replacement
• Assuming fakes increase app size

2. Which Kotlin syntax correctly declares a fake repository class implementing an interface UserRepository?

easy

A. class FakeUserRepository extends UserRepository { }

B. class FakeUserRepository implements UserRepository { }

C. class FakeUserRepository : UserRepository { }

D. class FakeUserRepository inherits UserRepository { }

Solution

1. Step 1: Recall Kotlin interface implementation syntax

   Kotlin uses a colon (:) to implement interfaces, not keywords like implements or extends.

2. Step 2: Match correct syntax

   class FakeUserRepository : UserRepository { } uses : correctly to implement UserRepository.

3. Final Answer:

   class FakeUserRepository : UserRepository { } -> Option C

4. Quick Check:

   Kotlin interface implementation uses ':' [OK]

Hint: Use ':' to implement interfaces in Kotlin [OK]

Common Mistakes:

• Using Java keywords like implements or extends in Kotlin
• Confusing inheritance with interface implementation
• Omitting the colon ':'

3. Given this fake repository code snippet, what will println(fakeRepo.getUserCount()) output?

interface UserRepository {
  fun getUserCount(): Int
}

class FakeUserRepository : UserRepository {
  private val users = mutableListOf("Alice", "Bob")
  override fun getUserCount() = users.size
}

val fakeRepo = FakeUserRepository()

medium

A. 2

B. 1

C. 0

D. Compilation error

Solution

1. Step 1: Analyze the users list initialization

   The users list contains two elements: "Alice" and "Bob".

2. Step 2: Understand getUserCount() behavior

   The method returns the size of the users list, which is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   users.size = 2 [OK]

Hint: Count elements in the list for size [OK]

Common Mistakes:

• Assuming empty list by default
• Confusing method return with list content
• Expecting compilation error due to interface

4. Identify the error in this fake repository implementation:

class FakeUserRepository : UserRepository {
  private val users = mutableListOf<String>()
  fun getUserCount() = users.size
}

medium

A. Missing override keyword for getUserCount()

B. users list should be immutable

C. Class should not implement UserRepository

D. No constructor defined

Solution

1. Step 1: Check interface method implementation

   The interface UserRepository requires getUserCount() to be overridden.

2. Step 2: Verify method declaration

   The method lacks the override keyword, causing a compilation error.

3. Final Answer:

   Missing override keyword for getUserCount() -> Option A

4. Quick Check:

   Interface methods need 'override' in Kotlin [OK]

Hint: Always use 'override' for interface methods [OK]

Common Mistakes:

• Forgetting 'override' keyword
• Thinking mutable list is wrong here
• Assuming constructor is mandatory

5. You want to test a repository method that fetches users filtered by minimum age. Your fake repository stores users as data class User(val name: String, val age: Int). Which fake repository method correctly returns users older than or equal to the given age?

hard

A. fun getUsersByMinAge(minAge: Int) = users.filter { it.age <= minAge }

B. fun getUsersByMinAge(minAge: Int) = users.filter { it.age >= minAge }

C. fun getUsersByMinAge(minAge: Int) = users.filter { it.age > minAge }

D. fun getUsersByMinAge(minAge: Int) = users.filter { it.name.length >= minAge }

Solution

1. Step 1: Understand filtering condition

   The requirement is to get users with age greater than or equal to minAge.

2. Step 2: Match correct filter predicate

   fun getUsersByMinAge(minAge: Int) = users.filter { it.age >= minAge } uses it.age >= minAge, which matches the requirement exactly.

3. Final Answer:

   fun getUsersByMinAge(minAge: Int) = users.filter { it.age >= minAge } -> Option B

4. Quick Check:

   Filter with '>= minAge' for correct users [OK]

Hint: Use '>=' to include minimum age users [OK]

Common Mistakes:

• Using '>' excludes users exactly at minAge
• Using '<=' returns younger users
• Filtering by name length instead of age