Android MVI

Feature module structure

Each feature module contains non-layer components and 3 layers with distinct set of responsibilities.

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Presentation layer

This layer is closest to what the user sees on the screen. The presentationlayer is a mix of MVVM(Jetpack ViewModelused to preserve data across activity restart) and MVI (actionsmodify the common stateof the view and then new state is edited to a view via LiveData to be rendered).

Components:

View (Fragment) - presents data on the screen and pass user interactions to View Model. Views are hard to test, so they should be as simple as possible.

ViewModel- dispatches (through LiveData) state changes to the view and deals with user interactions (these view models are not simply POJO classes).

ViewState - common state for a single view

NavManager- singleton that facilitates handling all navigation events inside NavHostActivity (instead of separately, inside each view)

Domain layer

This is the core layer of the application. Notice that the domainlayer is independent of any other layers. This allows to make domain models and business logic independent from other layers. In other words, changes in other layers will have no effect on domain layer eg. changing database (data layer) or screen UI (presentationlayer) ideally will not result in any code change withing domain layer.

Components:

UseCase - contains business logic

DomainModel - defies the core structure of the data that will be used within the application. This is the source of truth for application data.

Repository interface- required to keep the domainlayer independent from the data layer (Dependency inversion).

Data layer

Manages application data and exposes these data sources as repositories to the domain layer. Typical responsibilities of this layer would be to retrieve data from the internet and optionally cache this data locally.

Components:

Repositoryis exposing data to the domainlayer. Depending on application structure and quality of the external APIs repository can also merge, filter, and transform the data. The intention of these operations is to create high-quality data source for the domain layer, not to perform any business logic (domainlayer use case responsibility).

Mapper- maps data modelto domain model(to keep domainlayer independent from the data layer).

RetrofitService - defines a set of API endpoints.

DataModel - defines the structure of the data retrieved from the network and contains annotations, so Retrofit (Moshi) understands how to parse this network data (XML, JSON, Binary...) this data into objects.