HomeServiceSpy.swift

struct CardModel {}

class HomeService {
    
    static let shared = HomeService()
    private init() {}
    
    func fetchHomeCardModels(completion: @escaping (Result<[CardModel], Error>) -> Void) {
        // ...
    }
}

// Spy Object
class HomeServiceSpy: HomeService {
    
    var completion: ((Result<[CardModel], Error>) -> Void)?
    var fetchHomeCardModelsCalled = false
    
    override func fetchHomeCardModels(completion: @escaping (Result<[CardModel], Error>) -> Void) {
        fetchHomeCardModelsCalled = true
        self.completion = completion
    }
}

let viewController = SomeViewController()
viewController.homeService = HomeServiceSpy()