a4-1.swift

class PhotoListViewModel {
    let apiService: APIServiceProtocol

    init( apiService: APIServiceProtocol = APIService()) {
        self.apiService = apiService
    }

    func initFetch() {
        self.isLoading = true
        apiService.fetchPopularPhoto { [weak self] (success, photos, error) in
            self?.isLoading = false
            if let error = error {
                self?.alertMessage = error.rawValue
            } else {
                self?.processFetchedPhoto(photos: photos)
            }
        }
    }
}