Domain layer

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The domain layer is an optional layer that sits between the UI layer and the data layer.

When it is included, the optional domain layer provides dependencies to
    the UI layer and depends on the data layer.
Figure 1. The domain layer's role in app architecture.

The domain layer is responsible for encapsulating complex business logic, or simple business logic that is reused by multiple ViewModels. This layer is optional because not all apps will have these requirements. You should only use it when needed-for example, to handle complexity or favor reusability.

A domain layer provides the following benefits:

  • It avoids code duplication.
  • It improves readability in classes that use domain layer classes.
  • It improves the testability of the app.
  • It avoids large classes by allowing you to split responsibilities.

To keep these classes simple and lightweight, each use case should only have responsibility over a single functionality, and they should not contain mutable data. You should instead handle mutable data in your UI or data layers.

Naming conventions in this guide

In this guide, use cases are named after the single action they're responsible for. The convention is as follows:

verb in present tense + noun/what (optional) + UseCase.

For example: FormatDateUseCase, LogOutUserUseCase, GetLatestNewsWithAuthorsUseCase, or MakeLoginRequestUseCase.

Dependencies

In a typical app architecture, use case classes fit between ViewModels from the UI layer and repositories from the data layer. This means that use case classes usually depend on repository classes, and they communicate with the UI layer the same way repositories do—using either callbacks (for Java) or coroutines (for Kotlin). To learn more about this, see the data layer page.

For example, in your app, you might have a use case class that fetches data from a news repository and an author repository, and combines them:

class GetLatestNewsWithAuthorsUseCase(
  private val newsRepository: NewsRepository,
  private val authorsRepository: AuthorsRepository
) { /* ... */ }

Because use cases contain reusable logic, they can also be used by other use cases. It's normal to have multiple levels of use cases in the domain layer. For example, the use case defined in the example below can make use of the FormatDateUseCase use case if multiple classes from the UI layer rely on time zones to display the proper message on the screen:

class GetLatestNewsWithAuthorsUseCase(
  private val newsRepository: NewsRepository,
  private val authorsRepository: AuthorsRepository,
  private val formatDateUseCase: FormatDateUseCase
) { /* ... */ }
GetLatestNewsWithAuthorsUseCase depends on repository classes from the
    data layer, but it also depends on FormatDataUseCase, another use case class
    that is also in the domain layer.
Figure 2. Example dependency graph for a use case that depends on other use cases.

Call use cases in Kotlin

In Kotlin, you can make use case class instances callable as functions by defining the invoke() function with the operator modifier. See the following example:

class FormatDateUseCase(userRepository: UserRepository) {

    private val formatter = SimpleDateFormat(
        userRepository.getPreferredDateFormat(),
        userRepository.getPreferredLocale()
    )

    operator fun invoke(date: Date): String {
        return formatter.format(date)
    }
}

In this example, the invoke() method in FormatDateUseCase allows you to call instances of the class as if they were functions. The invoke() method is not restricted to any specific signature—it can take any number of parameters and return any type. You can also overload invoke() with different signatures in your class. You'd call the use case from the example above as follows:

class MyViewModel(formatDateUseCase: FormatDateUseCase) : ViewModel() {
    init {
        val today = Calendar.getInstance()
        val todaysDate = formatDateUseCase(today)
        /* ... */
    }
}

To learn more about the invoke() operator, see the Kotlin docs.

Lifecycle

Use cases don't have their own lifecycle. Instead, they're scoped to the class that uses them. This means that you can call use cases from classes in the UI layer, from services, or from the Application class itself. Because use cases shouldn't contain mutable data, you should create a new instance of a use case class every time you pass it as a dependency.

Threading

Use cases from the domain layer must be main-safe; in other words, they must be safe to call from the main thread. If use case classes perform long-running blocking operations, they are responsible for moving that logic to the appropriate thread. However, before doing that, check if those blocking operations would be better placed in other layers of the hierarchy. Typically, complex computations happen in the data layer to encourage reusability or caching. For example, a resource-intensive operation on a big list is better placed in the data layer than in the domain layer if the result needs to be cached to reuse it on multiple screens of the app.

The following example shows a use case that performs its work on a background thread:

class MyUseCase(
    private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {

    suspend operator fun invoke(...) = withContext(defaultDispatcher) {
        // Long-running blocking operations happen on a background thread.
    }
}

Common tasks

This section describes how to perform common domain layer tasks.

Reusable simple business logic

You should encapsulate repeatable business logic present in the UI layer in a use case class. This makes it easier to apply any changes everywhere the logic is used. It also allows you to test the logic in isolation.

Consider the FormatDateUseCase example described earlier. If your business requirements regarding date formatting change in the future, you only need to change code in one centralized place.

Combine repositories

In a news app, you might have NewsRepository and AuthorsRepository classes that handle news and author data operations respectively. The Article class that NewsRepository exposes only contains the name of the author, but you want to display more information about the author on the screen. Author information can be obtained from the AuthorsRepository.

GetLatestNewsWithAuthorsUseCase depends on two different repository
    classes from the data layer: NewsRepository and AuthorsRepository.
Figure 3. Dependency graph for a use case that combines data from multiple repositories.

Because the logic involves multiple repositories and can become complex, you create a GetLatestNewsWithAuthorsUseCase class to abstract the logic out of the ViewModel and make it more readable. This also makes the logic easier to test in isolation, and reusable in different parts of the app.

/**
 * This use case fetches the latest news and the associated author.
 */
class GetLatestNewsWithAuthorsUseCase(
  private val newsRepository: NewsRepository,
  private val authorsRepository: AuthorsRepository,
  private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {
    suspend operator fun invoke(): List<ArticleWithAuthor> =
        withContext(defaultDispatcher) {
            val news = newsRepository.fetchLatestNews()
            val result: MutableList<ArticleWithAuthor> = mutableListOf()
            // This is not parallelized, the use case is linearly slow.
            for (article in news) {
                // The repository exposes suspend functions
                val author = authorsRepository.getAuthor(article.authorId)
                result.add(ArticleWithAuthor(article, author))
            }
            result
        }
}

The logic maps all items in the news list; so even though the data layer is main-safe, this work shouldn't block the main thread because you don't know how many items it'll process. That's why the use case moves the work to a background thread using the default dispatcher.

Other consumers

Apart from the UI layer, the domain layer can be reused by other classes such as services and the Application class. Furthermore, if other platforms such as TV or Wear share codebase with the mobile app, their UI layer can also reuse use cases to get all the aforementioned benefits of the domain layer.

Testing

General testing guidance applies when testing the domain layer. For other UI tests, developers typically use fake repositories, and it's good practice to use fake repositories when testing the domain layer as well.