For mobile developer intern applicants, Shopify posts a challenge: You’re asked to build an app that provides the user with a list of collections of products and allows them to look at any individual collection in more detail.
The challenge can be found here.
I aimed to build an extensible and testable architecture. Building on a data layer, which is modeled on the Shopify API, I use Fragments to display the individual screens.
Each Fragment possesses an accompanying ViewModel, which uses Retrofit and RxJava to fetch data. A ViewModel holds a ViewState, which encapsulates the current state of the view and is pushed to Fragment via a LiveData observer.
Interaction within a Fragment is performed reactively via Actions. Any event of interest — for instance a refresh by the user — is encapsulated as an Action. Performing this Action returns a Result, such as a fetched list of collections. The current ViewState and the Result are then reduced to create the next ViewState, which is then pushed to the Fragment.
I use Dagger 2 for dependency injection, making it easy to test individual components. For instance, by extracting string formatting functionality into a VariantsFormatter, classes like ProductsViewEntityMapper and DetailsViewModel can be tested with a MockVariantsFormatter without depending on a Context (and thus the Android framework). This enables us to write unit tests instead of instrumented tests.
The data flow described above makes it easy to extend the app with additional functionality. For instance, adding a database via Room to store collections and products requires only the following steps:
- Add the necessary Room annotations, DAOs and a database class. The latter is then injected via Dagger.
- In the ViewModel, subscribe to database changes and map them to a new Action subclass, such as Action.DatabaseChange(val collections: List).
- Merge all emitting sources via Observable.merge(pullToRefreshRelay, databaseChanges) and subscribe to it.
- Extend the processAction(action) method to handle Action.DatabaseChange.