Builder Pattern for creating instance of ViewModel with Dependency Injection

BuilderPattern.swift

// Dependency Inversion Principle: Define protocols for Service and Repository

protocol Service {
    func fetchData() -> String
}

protocol Repository {
    func saveData(data: String)
}

// Concrete implementations of Service and Repository

class ConcreteService: Service {
    func fetchData() -> String {
        return "Data from Service"
    }
}

class ConcreteRepository: Repository {
    func saveData(data: String) {
        print("Saving data: \(data)")
    }
}

// ViewModel with dependencies

class ViewModel {
    private let service: Service
    private let repository: Repository

    init(service: Service, repository: Repository) {
        self.service = service
        self.repository = repository
    }

    func processData() {
        let data = service.fetchData()
        repository.saveData(data: data)
    }
}

// Builder for ViewModel

class ViewModelBuilder {
    private var service: Service?
    private var repository: Repository?

    func setService(_ service: Service) -> ViewModelBuilder {
        self.service = service
        return self
    }

    func setRepository(_ repository: Repository) -> ViewModelBuilder {
        self.repository = repository
        return self
    }

    func build() -> ViewModel {
        guard let service = service, let repository = repository else {
            fatalError("Service and Repository must be set before building ViewModel.")
        }
        return ViewModel(service: service, repository: repository)
    }
}

// Usage

let viewModel = ViewModelBuilder()
    .setService(ConcreteService())
    .setRepository(ConcreteRepository())
    .build()

viewModel.processData()