Clean Architecture layers in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the Presentation Layer in Clean Architecture?

The Presentation Layer handles the user interface and user interaction. It displays data and sends user actions to the next layer without knowing business logic details.

beginner

Which layer contains the business rules and application logic in Clean Architecture?

The Domain Layer contains business rules and application logic. It is independent of UI and data sources, focusing on what the app does.

beginner

What role does the Data Layer play in Clean Architecture?

The Data Layer manages data sources like databases, network, or cache. It provides data to the Domain Layer and hides details of data fetching or storage.

intermediate

Why is it important that the Domain Layer is independent of other layers?

Independence allows the Domain Layer to be reusable and testable. Changes in UI or data sources don’t affect business rules, making the app easier to maintain.

intermediate

How do layers communicate in Clean Architecture?

Layers communicate through interfaces or abstractions. Higher layers depend on lower layers’ interfaces, not their implementations, ensuring loose coupling.

Which layer in Clean Architecture is responsible for fetching data from a remote server?

A Presentation Layer

B Domain Layer

C Data Layer

D UI Layer

The Data Layer handles data fetching from sources like remote servers or databases.

What does the Domain Layer mainly contain?

A Business rules and logic

B User interface code

C Network calls

D Database schemas

The Domain Layer contains the core business rules and application logic.

Why should the Domain Layer be independent of the Data Layer?

A To allow UI changes without affecting business logic

B To make the app slower

C To mix UI and data code

D To avoid testing

Independence allows business logic to remain stable even if data sources or UI change.

Which layer directly interacts with the user in Clean Architecture?

A Data Layer

B Domain Layer

C Repository Layer

D Presentation Layer

The Presentation Layer handles user interaction and displays information.

How do layers in Clean Architecture communicate?

A Directly accessing each other's code

B Through interfaces or abstractions

C By sharing variables globally

D Using UI events only

Using interfaces keeps layers loosely coupled and easier to maintain.

Explain the responsibilities of each layer in Clean Architecture and why this separation helps in app development.

Think about who talks to the user, who holds the app’s rules, and who manages data.

Describe how communication between layers happens in Clean Architecture and why it is designed that way.

Imagine layers as friends who only talk through letters, not by visiting each other’s homes.

Practice

1. Which layer in Clean Architecture is responsible for business rules and logic?
domain, data, or presentation?

easy

A. Presentation layer

B. Domain layer

C. Data layer

D. UI layer

Solution

1. Step 1: Understand the role of each layer

   The domain layer contains business rules and logic, independent of UI or data sources.

2. Step 2: Match the layer to its responsibility

   Since business logic is in the domain layer, it is the correct answer.

3. Final Answer:

   Domain layer -> Option B

4. Quick Check:

   Business logic = Domain layer [OK]

Hint: Business rules live in domain layer, not UI or data [OK]

Common Mistakes:

• Confusing data layer with domain layer
• Thinking UI handles business logic
• Mixing presentation with domain

2. Which Kotlin syntax correctly defines a repository interface in the domain layer?

?

easy

A. interface UserRepository { fun getUser(id: Int): User }

B. class UserRepository { fun getUser(id: Int): User }

C. fun UserRepository() = User()

D. val UserRepository = object { fun getUser() = User() }

Solution

1. Step 1: Identify interface syntax in Kotlin

   Interfaces use the keyword interface followed by function signatures without bodies.

2. Step 2: Check each option

   interface UserRepository { fun getUser(id: Int): User } correctly declares an interface with a function signature. Others define classes or objects incorrectly for an interface.

3. Final Answer:

   interface UserRepository { fun getUser(id: Int): User } -> Option A

4. Quick Check:

   Kotlin interface syntax = interface UserRepository { fun getUser(id: Int): User } [OK]

Hint: Interfaces use 'interface' keyword with function signatures [OK]

Common Mistakes:

• Using class instead of interface for repository
• Defining function bodies inside interface
• Using val or fun incorrectly for interface

3. Given this code snippet in the data layer:

class UserRepositoryImpl(private val api: UserApi) : UserRepository {
  override fun getUser(id: Int): User {
    return api.fetchUser(id)
  }
}

What does getUser(5) return?

medium

A. Null because fetchUser returns nothing

B. An error because UserRepositoryImpl lacks a constructor

C. A User object fetched from the API with id 5

D. A compile-time syntax error

Solution

1. Step 1: Understand class and method behavior

   UserRepositoryImpl implements UserRepository and calls api.fetchUser(id) to get a User.

2. Step 2: Analyze getUser(5) call

   Calling getUser(5) returns the User object fetched by api.fetchUser(5), assuming api works correctly.

3. Final Answer:

   A User object fetched from the API with id 5 -> Option C

4. Quick Check:

   Method calls API and returns User [OK]

Hint: Implementation calls API method and returns result [OK]

Common Mistakes:

• Assuming missing constructor causes error
• Thinking fetchUser returns null by default
• Confusing syntax errors with runtime behavior

4. Identify the error in this presentation layer code snippet:

class UserViewModel(private val repo: UserRepository) {
  fun loadUser(id: Int) {
    val user = repo.getUser(id)
    println(user.name)
  }
}

medium

A. UserViewModel should not call repository directly

B. User object might be null causing a crash

C. No error; code is correct

D. UserRepository interface is not implemented

Solution

1. Step 1: Recall Clean Architecture dependencies

   The presentation layer (ViewModel) should depend on use cases in the domain layer, not directly on repository interfaces.

2. Step 2: Identify the violation

   Directly injecting and calling UserRepository in ViewModel bypasses use cases, mixing concerns and violating layer separation.

3. Final Answer:

   UserViewModel should not call repository directly -> Option A

4. Quick Check:

   ViewModel -> UseCase -> Repository [OK]

Hint: ViewModels call UseCases, not Repositories directly [OK]

Common Mistakes:

• Focusing on null safety (type system prevents null here)
• Thinking no architecture error in the code
• Assuming missing interface implementation

5. You want to add caching to your app using Clean Architecture. Where should the cache logic be placed?
Options:
A) In the domain layer
B) In the data layer
C) In the presentation layer
D) In the UI components

hard

A. Domain layer to keep business rules

B. UI components for faster display

C. Presentation layer to update UI directly

D. Data layer to manage data sources including cache

Solution

1. Step 1: Understand caching responsibility

   Caching is a data management concern, related to how data is stored and retrieved.

2. Step 2: Match caching to Clean Architecture layer

   The data layer handles data sources like network, database, and cache, so caching logic belongs here.

3. Final Answer:

   Data layer to manage data sources including cache -> Option D

4. Quick Check:

   Cache logic = Data layer [OK]

Hint: Cache belongs in data layer managing data sources [OK]

Common Mistakes:

• Putting cache in domain layer (business logic)
• Adding cache in UI or presentation layers
• Mixing UI concerns with data management