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Clean Architecture: Kotlin and Compose

By employing Clean Architecture, you can design applications with very low coupling and independent of technical implementation details, such as databases and frameworks. That way, the application becomes easy to maintain, flexible to change and intrinsically testable. Here I’ll show how I structure my applications. We’ll build an Android todo application using Jetpack Compose.

Let’s get started.

The package structure of the project takes on the following form:

├── Core
├── Data
├── Domain
└── Presentation

Let’s start with the Domain Layer.

The Domain layer describes WHAT your project/application does.

Let me explain.

Many applications are built and structured in the way of the programming pattern, (e.g MVC , MVVM, MVP). I think it’s better to structure the application in a way that explains what the application does just be looking at files and folders.

Using a building of a house analogy, you can quickly identify what a building would look like and its functionality by viewing the floor plan and elevation of the building

In the same way, the domain layer of our project should specify and describe WHAT our application does. In this folder, we would keep models, repository interfaces, and use cases.

├── Core
├── Data
├── Presentation
└── Domain
    ├── Model
    │   ├── Todo.kt
    │   └── User.kt
    ├── Repository
    │   ├── TodoRepository.kt
    │   └── UserRepository.kt
    └── UseCase
        ├── Todo
        │   ├── GetTodos.kt
        │   ├── GetTodo.kt
        │   ├── DeleteTodo.kt
        │   ├── UpdateTodo.kt
        │   └── CreateTodo.kt
        └── User
            ├── GetUsers.kt
            ├── GetUser.kt
            ├── DeleteUser.kt
            ├── UpdateUser.kt
            └── CreateUser.kt

1. Model: A model typically represents a real-world object that is related to the problem. In this folder, we would typically keep classes to represent objects. e.g. Todo, User, etc
2. Repository: Container for all repository interfaces. The repository is a central place to keep all model-specific operations. In this case, the Todo repository interface would describe repository methods. The actual repository implementation will be kept in the Data layer.
3. UseCases: Container to list all functionality of our application. e.g Get, Delete, Create , Update

The PRESENTATION layer will keep all of the consumer-related code as to HOW the application will interact with the outside world. The presentation layer can be WebForms, Command Line Interface, API Endpoints, etc. In this case, it would be the screens for a List of Todos and its accompanying view model.

├── Core
├── Data
├── Domain
└── Presentation
    └── Todo
        └── TodoList
            ├── TodoListViewModel.kt
            └── TodoListView.kt

The DATA layer will keep all the external dependency-related code as to HOW they are implemented:

├── Core
├── Domain
├── Presentation
├── Data
    ├── Repository
    │   ├── TodoRepositoryImpl.kt
    │   ├── TodoAPIDataSourceImpl.kt
    │   └── TodoDBDataSourceImpl.kt
    └── DataSource
        ├── API
        │   ├── TodoAPIDataSource.kt.
        │   └── Entity
        │       ├── TodoAPIEntity.kt
        │       └── UserAPIEntity.kt
        └── DB
            ├── TodoDBDataSource.kt.
            └── Entity
                ├── TodoDBEntity.kt
                └── UserDBEntity.kt

1. Repository: Repository implementations
2. DataSource: All data source interfaces and entities. An entity represents a single instance of your domain object saved into the database as a record. It has some attributes that we represent as columns in our DB tables or API endpoints. We can’t control how data is modeled on the external data source, so these entities are required to be mapped from entities to domain models in the implementations

and lastly, the CORE layer keep all the components that are common across all layers like constants or configs or dependency injection (which we won’t cover)

Our first task would be always to start with the domain models and data entities

Domain/Model/Todo.kt

Data/DataSource/API/Entity/TodoEntity.kt

Let’s now write an interface for the TodoDatasource. We need one to enforce how any data source (API, DB, etc) needs to behave.

interface TodoDataSource {    
    suspend fun getTodos(): List<Todo>
}

We have enough to write an implementation of this interface and we’ll call it TodoAPIImpl:

Data/Repository/TodoAPIImpl.kt

Note: this repository’s getTodos function returns a list of Todo. So, we have to map TodoAPIEntity -> Todo:

Before we write our TodoRepositoryImpl let’s write the interface for that in the Domain layer

Domain/Repository/TodoRepository.kt

Data/Repository/TodoRepositoryImpl.kt

We can now see that the TodoRepositoryImpl can take any datasource as a dependency, great for swapping out datasources.

Now that we have our todo repository, we can code up the GetTodos use case

Domain/UseCase/GetTodos.kt

and then in turn we can write our presentation’s view model and view

Presentation/TodoList/TodoListViewModel.kt

Presentation/TodoList/TodoListView.kt

MainActivity.kt

So to recap: