generic function for api service

Service.swift

//  Service.swift
//  Generics
//
//  Created by Yash Thaker on 25/05/20.
//  Copyright © 2020 YashThaker. All rights reserved.
//

/*
// ------------- How to use -------------
Service.shared.fetchUnsplashPhotos { result in
    switch result {
    case .failure(let error):
        print("Error: ", error)
    case .success(let unsplashResults):
        print(unsplashResults)
    }
}
*/

import UIKit
import Alamofire

protocol LocalizedDescriptionError: Error {
    var localizedDescription: String { get }
}

final class Service {
    
    static let shared = Service()
    
    private init() { }
    
    enum Result<Success, Failure: Error> {
        case success(Success)
        case failure(Failure)
    }
    
    enum ServiceError: LocalizedDescriptionError {
        case canNotCastResponseData
        
        var localizedDescription: String {
            switch self {
            case .canNotCastResponseData:
                return "Can not cast response data."
            }
        }
    }
    
    func fetchUnsplashPhotos(currentPage: Int = 1, completion: @escaping (Result<[Unsplash_Results], Error>) -> ()) {
        let urlString = "https://api.unsplash.com/photos/?"
        
        var parameters = [String: Any]()
        parameters["client_id"] = "a21c1d02e7f4850714142dc5664cb908af6568a31e9059562ca4b9b9a4a79f65"
        parameters["per_page"] = 60
        parameters["page"] = currentPage
        
        fetchGenericData(urlString: urlString, parameters: parameters, completion: completion)
    }
    
    private func fetchGenericData<T: Decodable>(urlString: String, parameters: [String: Any]? = nil, completion: @escaping (Result<T, Error>) -> ()) {
        AF.request(urlString, parameters: parameters).responseData { (response) in
            if let error = response.error {
                completion(.failure(error))
                return
            }
            guard let data = response.data else {
                completion(.failure(ServiceError.canNotCastResponseData))
                return
            }
            do {
                let model = try JSONDecoder().decode(T.self, from: data)
                completion(.success(model))
            } catch let jsonError {
                completion(.failure(jsonError))
                return
            }
        }
    }
    
}


// MARK:- Unsplash
struct Unsplash_Results : Decodable {
    let width : Int?
    let height : Int?
    let urls : Urls?
}

struct Urls : Decodable {
    let regular : String?
    let thumb : String?
}