iOS Clean Architecture

This project demonstrates the implementation of a repository list using Clean Architecture, MVVM, and Combine in Swift.

Clean Architecture

Introduction

CleanArchitecture is an example application built to demonstrate the usage of Clean Architecture along with MVVM and Combine frameworks in Swift. The application fetches and displays a list of repositories from a remote API.

Installation

To install the necessary files using Swift Package Manager, follow these steps:

Open your Xcode project.
Select File > Add Packages...
Enter the URL of this repository: https://github.com/tuan188/CleanArchitecture
Select the appropriate package options and add the package to your project.

Alternatively, you can add the following dependency to your Package.swift file:

dependencies: [
    .package(url: "https://github.com/tuan188/CleanArchitecture", .upToNextMajor(from: "2.0.5"))   
]

Architecture

The architecture is structured into three main layers:

Data Layer: Responsible for data retrieval and manipulation: Gateway Implementations + API (Network) + Database
Domain Layer: Contains business logic and use cases: Entities + Use Cases + Gateway Protocols
UI/Presentation Layer: Manages user interface and user interactions: ViewModels + ViewControllers/Views + Navigator

Each layer has a clear responsibility and communicates with other layers via protocols and Combine publishers.

Dependency Direction

Domain Layer

The Domain Layer contains the application’s business logic and use cases.

Entities

Entities encapsulate enterprise-wide Critical Business Rules. An entity can be an object with methods, or it can be a set of data structures and functions. It doesn’t matter so long as the entities can be used by many different applications in the enterprise. — Clean Architecture: A Craftsman’s Guide to Software Structure and Design (Robert C. Martin)

Entities are simple data structures:

struct Repo {
    var id: Int?
    var name: String?
    var fullname: String?
    var urlString: String?
    var starCount: Int?
    var folkCount: Int?
    var owner: Owner?
    
    struct Owner: Decodable {
        var avatarUrl: String?
        
        private enum CodingKeys: String, CodingKey {
            case avatarUrl = "avatar_url"
        }
    }
}

Use Cases

The software in the use cases layer contains application-specific business rules. It encapsulates and implements all of the use cases of the system. These use cases orchestrate the flow of data to and from the entities, and direct those entities to use their Critical Business Rules to achieve the goals of the use case. — Clean Architecture: A Craftsman’s Guide to Software Structure and Design (Robert C. Martin)

UseCases are protocols which do one specific thing:

protocol GetRepoList {
    var repoGateway: RepoGatewayProtocol { get }
}

extension GetRepoList {
    func getRepos(dto: GetPageDto) -> AnyPublisher<PagingInfo<Repo>, Error> {
        repoGateway.getRepos(dto: dto)
    }
}

Gateway Protocols

Generally gateway is just another abstraction that will hide the actual implementation behind, similarly to the Facade Pattern. It could a Data Store (the Repository pattern), an API gateway, etc. Such as Database gateways will have methods to meet the demands of an application. However do not try to hide complex business rules behind such gateways. All queries to the database should relatively simple like CRUD operations, of course some filtering is also acceptable. - Source

protocol RepoGatewayProtocol {
    func getRepos(page: Int, perPage: Int) -> AnyPublisher<PagingInfo<Repo>, Error>
}

Note: For simplicity we put the Gateway protocols and implementations in the same files. In fact, Gateway protocols should be at the Domain Layer and implementations at the Data Layer.

Data Layer

The Data Layer is responsible for fetching data from the network and decoding it into usable models. It contains Gateway Implementations and one or many Data Stores. Gateways are responsible for coordinating data from different Data Stores. Data Store can be Remote or Local (for example persistent database). Data Layer depends only on the Domain Layer.

Gateway Implementations

struct RepoGateway: RepoGatewayProtocol {
    private struct GetReposResult: Decodable {
        var items = [Repo]()
    }
    
    func getRepos(page: Int, perPage: Int) -> AnyPublisher<PagingInfo<Repo>, Error> {
        APIServices.default
            .request(GitEndpoint.repos(page: page, perPage: perPage))
            .data(type: GetReposResult.self)
            .map { $0.items }
            .map { PagingInfo(page: page, items: $0) }
            .eraseToAnyPublisher()
    }
}

UI Layer

The UI Layer is responsible for presenting data to the user and handling user interactions.

ViewModel

ViewModel is the main point of MVVM application. The primary responsibility of the ViewModel is to provide data to the view, so that view can put that data on the screen.
It also allows the user to interact with data and change the data.
The other key responsibility of a ViewModel is to encapsulate the interaction logic for a view, but it does not mean that all of the logic of the application should go into ViewModel.
It should be able to handle the appropriate sequencing of calls to make the right thing happen based on user or any changes on the view.
ViewModel should also manage any navigation logic like deciding when it is time to navigate to a different view. Source

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

public protocol ViewModel {
    associatedtype Input
    associatedtype Output
    
    func transform(_ input: Input, cancelBag: CancelBag) -> Output
}

import Combine
import UIKit
import Factory

class ReposViewModel: GetRepoList, ShowRepoDetail {
    @Injected(\.repoGateway) 
    var repoGateway: RepoGatewayProtocol
    
    // MARK: - Use cases
    
    func getRepos(page: Int) -> AnyPublisher<PagingInfo<Repo>, Error> {
        return getRepos(page: page, perPage: 10)
    }
    
    // MARK: - Navigation
    
    func vm_showRepoDetail(repo: Repo) {
        showRepoDetail(repo: repo)
    }
}

// MARK: - ViewModel
extension ReposViewModel: ObservableObject, ViewModel {
    struct Input {
        let loadTrigger: AnyPublisher<Void, Never>
        let reloadTrigger: AnyPublisher<Void, Never>
        let loadMoreTrigger: AnyPublisher<Void, Never>
        let selectRepoTrigger: AnyPublisher<IndexPath, Never>
    }
    
    final class Output: ObservableObject {
        @Published var repos = [RepoItemViewModel]()
        @Published var isLoading = false
        @Published var isReloading = false
        @Published var isLoadingMore = false
        @Published var alert = AlertMessage()
        @Published var isEmpty = false
    }
    
    func transform(_ input: Input, cancelBag: CancelBag) -> Output {
        let output = Output()
        
        let config = PageFetchConfig(initialLoadTrigger: input.loadTrigger,
                                     reloadTrigger: input.reloadTrigger,
                                     loadMoreTrigger: input.loadMoreTrigger,
                                     fetchItems: getRepos)
        
        let (page, error, isLoading, isReloading, isLoadingMore) = fetchPage(config: config).destructured

        page
            .map { $0.items.map(RepoItemViewModel.init) }
            .assign(to: \.repos, on: output)
            .store(in: cancelBag)
        
        input.selectRepoTrigger
            .handleEvents(receiveOutput: { [unowned self] indexPath in
                let repo = config.pageSubject.value.items[indexPath.row]
                self.vm_showRepoDetail(repo: repo)
            })
            .sink()
            .store(in: cancelBag)
        
        error
            .receive(on: RunLoop.main)
            .map { AlertMessage(error: $0) }
            .assign(to: \.alert, on: output)
            .store(in: cancelBag)
        
        isLoading
            .assign(to: \.isLoading, on: output)
            .store(in: cancelBag)
        
        isReloading
            .assign(to: \.isReloading, on: output)
            .store(in: cancelBag)
        
        isLoadingMore
            .assign(to: \.isLoadingMore, on: output)
            .store(in: cancelBag)
        
        return output
    }
}

Dependency Injection

A ViewModel can be injected into a ViewController via property injection or initializer. Here is how the dependency injection is set up using Factory.

import Factory

extension Container {
    func reposViewController(navigationController: UINavigationController) -> Factory<ReposViewController> {
        Factory(self) {
            let vc = ReposViewController.instantiate()

CleanArchitecture

Install / Use

README