FetchJSON.swift

private func fetchJSON() -> Promise<[Photo]> {
        return Promise { seal in
            Alamofire.request(photosURL, method: .get).validate().responseData { (data) in
                guard let data = data.result.value else {
                    seal.reject(PhotoError.ConvertToData)
                    return
                }
                guard let photos = try? JSONDecoder().decode([Photo].self, from: data) else {
                    seal.reject(PhotoError.PhotoDecoding)
                    return
                }
                seal.fulfill(photos)
            }
        }
    }