Easy Dependency Injection for Swift

EasyDI.swift

protocol Repository {
    func fetchInt() -> Int
}

class RepositoryImpl: Repository {
    
    func fetchInt() -> Int {
        // 通信する
        let api = API()
        return api.fetch()
    }
}

class TestRepsisotry: Repository {
    
    func fetchInt() -> Int {
        // mockを返す
        return 5
    }
}

class UseCase {
    let repository: RepositoryImpl = RepositoryImpl()
    var testRepository: TestRepsisotry?
    
    func inject(testRepository: TestRepsisotry){
        self.testRepository = testRepository
    }
    
    func fetchString() -> String {
        let number: Int
        if let testRepository = testRepository {
            number = testRepository.fetchInt()
        } else {
            number = repository.fetchInt()
        }
        let nuberString = String(number)
        return nuberString
    }
}

class UseCaseTest {
    
    func testFetch(){
        let testRepo = TestRepsisotry()
        let useCase = UseCase()
        useCase.inject(testRepository: testRepo)
        let result = useCase.fetchString()
        assert(result == "5")
    }
}

struct API {
    func fetch() -> Int {
        return 5
    }
}