The Clean Architecture
This sample project is inspired on The Clean Architecture by Uncle Bob.
Its main goal is to follow the SOLID principles in order to keep the software well organised, testable, easy to maintain and more importantly easy to extend/change.
As explained in the posts above and in the book, databases, network clients and frameworks are abstracted in the upper layers (Entities, Data and Domain layers) and should not be imported/referenced into them, instead they should be injected in the App level as dependencies through the interfaces exposed by the upper layers.
Modules
In this sample app we have the following modules/layers:
DependencyInjector
Framework responsible for linking the Interfaces to the Factories. This should only the referenced/used in the App Module.
Entities
Top level module. This should not have access to any other module in the system and must be the simplest possible.
Data
Responsible for exposing a few Interfaces for external plugins implementations such as databases, memory caches, network, bluetooth, etc. This should not reference any concrete class related to these. It also contains the Repositories which are another abstraction for the use of the plugins mentioned above. This module can only access Entities layer, nothing else.
Domain
This module exposes UseCases to process business logic using the Command Pattern. The use cases must be simple and have only one responsibility. They usually communicate with Repositories from the Data layer through Interfaces. Ideally UseCases would return DataStructures instead of Entities to the lower layers, since these could contain UI-specific data which isn't part of the Entities layer.
App
In this layer we have all the code that's specific to the platform of development. In this sample project the platform is iOS, furthermore a concrete implementation of the CacheInterface exposed by the Data layer was implemented in the App layer using UserDefaults as the persistence choice, for example.
This layer may have ViewModels, Coordinators, ViewControllers, Views, third party frameworks and, finally, is also responsible for setting up the dependency graph for the project by linking all the interfaces to their respective concrete classes.
Tests
Even though I haven't written any tests yet, the whole project is completely testable, since all layers are being isolated by interfaces, as suggested by the SOLID principles.
