Clean architecture with RxSwift

Contibutions are welcome and highly appreciated!!

You can do this by:

• opening an issue to discuss the current solution, ask a question, propose your solution etc. (also English is not my native language so if you think that something can be corrected please open a PR 😊)
• opening a PR if you want to fix bugs or improve something

High level overview

Domain

The Domain is basically what is your App about and what it can do (Entities, UseCase etc.) It does not depend on UIKit or any persistence framework, and it doesn't have implementations apart from entities

Platform

The Platform is a concrete implementation of the Domain in a specific platform like iOS. It does hide all implementation details. For example Database implementation whether it is CoreData, Realm, SQLite etc.

Application

Application is responsible for delivering information to the user and handling user input. It can be implemented with any delivery pattern e.g (MVVM, MVC, MVP). This is the place for your UIViews and UIViewControllers. As you will see from the example app, ViewControllers are completely independent of the Platform. The only responsibility of a view controller is to "bind" the UI to the Domain to make things happen. In fact, in the current example we are using the same view controller for Realm and CoreData.

Detail overview

To enforce modularity, Domain, Platform and Application are separate targets in the App, which allows us to take advantage of the internal access layer in Swift to prevent exposing of types that we don't want to expose.

Domain

Entities are implemented as Swift value types

public struct Post {
    public let uid: String
    public let createDate: Date
    public let updateDate: Date
    public let title: String
    public let content: String
}

UseCases are protocols which do one specific thing:

public protocol AllPostsUseCase {
    func posts() -> Observable<[Post]>
}

public protocol SavePostUseCase {
    func save(post: Post) -> Observable<Void>
}

UseCaseProvider is a service locator. In the current example, it helps to hide the concrete implementation of use cases.

Platform

In some cases, we can't use Swift structs for our domain objects because of DB framework requirements (e.g. CoreData, Realm).

final class CDPost: NSManagedObject {
    @NSManaged public var uid: String?
    @NSManaged public var title: String?
    @NSManaged public var content: String?
    @NSManaged public var createDate: NSDate?
    @NSManaged public var updateDate: NSDate?
}

final class RMPost: Object {
    dynamic var uid: String = ""
    dynamic var createDate: NSDate = NSDate()
    dynamic var updateDate: NSDate = NSDate()
    dynamic var title: String = ""
    dynamic var content: String = ""
}

The Platform also contains concrete implementations of your use cases, repositories or any services that are defined in the Domain.

final class SavePostUseCase: Domain.SavePostUseCase {
    private let repository: AbstractRepository<Post>

    init(repository: AbstractRepository<Post>) {
        self.repository = repository
    }

    func save(post: Post) -> Observable<Void> {
        return repository.save(entity: post)
    }
}

final class Repository<T: CoreDataRepresentable>: AbstractRepository<T> where T == T.CoreDataType.DomainType {
    private let context: NSManagedObjectContext
    private let scheduler: ContextScheduler

    init(context: NSManagedObjectContext) {
        self.context = context
        self.scheduler = ContextScheduler(context: context)
    }

    override func query(with predicate: NSPredicate? = nil,
                        sortDescriptors: [NSSortDescriptor]? = nil) -> Observable<[T]> {
        let request = T.CoreDataType.fetchRequest()
        request.predicate = predicate
        request.sortDescriptors = sortDescriptors
        return context.rx.entities(fetchRequest: request)
            .mapToDomain()
            .subscribeOn(scheduler)
    }

    override func save(entity: T) -> Observable<Void> {
        return entity.sync(in: context)
            .mapToVoid()
            .flatMapLatest(context.rx.save)
            .subscribeOn(scheduler)
    }
}

As you can see, concrete implementations are internal, because we don't want to expose our dependecies. The only thing that is exposed in the current example from the Platform is a concrete implementation of the UseCaseProvider.

public final class UseCaseProvider: Domain.UseCaseProvider {
    private let coreDataStack = CoreDataStack()
    private let postRepository: Repository<Post>

    public init() {
        postRepository = Repository<Post>(context: coreDataStack.context)
    }

    public func getAllPostsUseCase() -> AllPostsUseCase {
        return CDAllPostsUseCase(repository: postRepository)
    }

    public func getCreatePostUseCase() -> SavePostUseCase {
        return CDSavePostUseCase(repository: postRepository)
    }
}

Application

In the current example, Application is implemented with the MVVM pattern and heavy use of RxSwift, which makes binding very easy.

Where the ViewModel performs pure transformation of a user Input to the Output

protocol ViewModelType {
    associatedtype Input
    associatedtype Output
    
    func transform(input: Input) -> Output
}

final class PostsViewModel: ViewModelType {
    struct Input {
        let trigger: Driver<Void>
        let createPostTrigger: Driver<Void>
        let selection: Driver<IndexPath>
    }
    struct Output {
        let fetching: Driver<Bool>
        let posts: Driver<[Post]>
        let createPost: Driver<Void>
        let selectedPost: Driver<Post>
        let error: Driver<Error>
    }
    
    private let useCase: AllPostsUseCase
    private let navigator: PostsNavigator
    
    init(useCase: AllPostsUseCase, navigator: PostsNavigator) {
        self.useCase = useCase
        self.navigator = navigator
    }
    
    func transform(input: Input) -> Output {
       ......
    }

A ViewModel can be injected into a ViewController via property injection or initializer. In the current example, this is done by Navigator.

protocol PostsNavigator {
    func toCreatePost()
    func toPost(_ post: Post)
    func toPosts()
}

class DefaultPostsNavigator: PostsNavigator {
    private let storyBoard: UIStoryboard
    private let navigationController: UINavigationController
    private let services: ServiceLocator
    
    init(services: ServiceLocator,
         navigationController: UINavigationController,
         storyBoard: UIStoryboard) {
        self.services = services
        self.navigationController = navigationController
        self.storyBoard = storyBoard
    }
    
    func toPosts() {
        let vc = storyBoard.instantiateViewController(ofType: PostsViewController.self)
        vc.viewModel = PostsViewModel(useCase: services.getAllPostsUseCase(),
                                      navigator: self)
        navigationController.pushViewController(vc, animated: true)
    }
    ....
}

class PostsViewController: UIViewController {
    private let disposeBag = DisposeBag()
    
    var viewModel: PostsViewModel!
    
    ...
}

Example

The example app is Post/TODOs app which uses Realm, CoreData and Network at the same time as a proof of concept that the Application level is not dependant on the Platform level implementation details.

CoreData	Realm	Network

TODO:

• add tests
• add MVP example
• Redux example??

Links

• RxSwift
• RxSwift Book
• Robert C Martin - Clean Architecture and Design
• Cycle.js
• ViewModel in Rx world

Any questions?

• ping me on Twitter