A VERY simple Android app, a clean one!

Once upon a time I was reading about Clean Architecture! After reading some articles and papers and grasping the theory of clean architecture I said to myself now it's time to see clean architecture in action. I searched for apps that were implemented using clean architecture. I found some but I had a big problem with them: they were TOO complicated. So I decided to create a VERY simple app that just calls a web service to get a bunch of data items and displays them in a RecyclerView.

Structure

The app has 3 modules (layers):

1. presentation: Stuff related to handling Android views are defined in this module. Some notes about this module:

• It's an Android module
• It depends on data and domain modules.
• It uses MVP

2. data: This module is the data provider. Things like connecting to web services is done in this module. It depends on domain module.
3. domain: It's a pure java module that contains the business logic of the app. Use cases are defined in this module. This module knows nothing about other modules hence doesn't depend on aforementioned modules.

Workflow

The entry point of app is FeaturedCategoriesActivity. In method onCreate of FeaturedCategoriesActivity method getFeaturedCategories of FeaturedCategoriesPresenter is called to get the list of categories.

FeaturedCategoriesPresenterImpl, which implements FeaturedCategoriesPresenter, has two components: GetFeaturedCategoriesUseCase and CategoryModelDataMapper. presentation layer becomes dependent on domain layer because of GetFeaturedCategoriesUseCase. FeaturedCategoriesPresenterImpl doesn't know how categories are obtained. It just knows to ask GetFeaturedCategoriesUseCase for categories. GetFeaturedCategoriesUseCase returns a List<Category> but our view expects a List<CategoryModel>. Because of that we use CategoryModelDataMapper to map from List<Category> to List<CategoryModel>. In this way we have a better decoupling between presentation and domain layers.

We create a new instance of GetFeaturedCategoriesUseCase in presentation layer. In order to create this new instance we need an implementation of CategoryRepository (namely CategoryRepositoryImpl) which is defined in data layer. This cause presentation layer to depend on data layer.

CategoryRepositoryImpl is the actual place where connecting to web service is done. In this case I used Retrofit for handling REST API calls. CategoryRepositoryImpl obtains a List<CatagoryEntity> but GetFeaturedCategoriesUseCase of domain layer needs a List<Category>. A mapper is used to map from List<CatagoryEntity> to List<Category>.

In a nutshell the workflow of obtaining the list of categories is as follows:

1. FeaturedCategoriesActivity calls FeaturedCategoriesPresenter.getFeaturedCategories()
2. FeaturedCategoriesPresenter calls GetFeaturedCategoriesUseCase.execute()
3. GetFeaturedCategoriesUseCase calls CategoryRepository.getCategories()

Now that we have the list of categories (our data), CategoryRepository passes the data to GetFeaturedCategoriesUseCase, GetFeaturedCategoriesUseCase passes the data to FeaturedCategoriesPresenter and finally FeaturedCategoriesPresenter passes the data to FeaturedCategoriesActivity. It's now FeaturedCategoriesActivity's responsibility to display the data to user.

Disclaimer

I've done my best to follow the principles of Clean Architecture. If you find any inconsistency with Clean Architecture feel free to open an issue.

References

The Clean Architecture

Android architecture

Architecting Android...The clean way?