This class is download manager that download files from given url with caching data in disk of device for iOS

DownloadManager.swift

//
//  DownloadManager.swift
//  UrlCacheExample
//
//  Created by Coder ACJHP on 12.04.2019.
//  Copyright © 2019 Onur Işık. All rights reserved.
//

import UIKit

internal struct Gif {
    var name: String!
    var data: Data!
    var coverImage: UIImage!
}

class DownloadManager: NSObject {
    
    private let allowedDiskSize = 100 * 1024 * 1024
    private lazy var cache: URLCache = {
        return URLCache(memoryCapacity: 0, diskCapacity: allowedDiskSize, diskPath: "gifCache")
    }()
    
    private var urlStringList: [String]!
    typealias GetResultListCompletionHandler = (Array<Gif>) -> ()
    typealias DownloadCompletionHandler = (Result<Data,Error>) -> ()
    
    
    init(urlStringList: [String]) {
        self.urlStringList = urlStringList
    }
    
    public func getResultList(completionHandler: @escaping GetResultListCompletionHandler) {
        
        var dataList: Array<Gif> = []
        
        let dispatchGroup = DispatchGroup()
        
        for index in 0 ..< self.urlStringList.count {
            
            dispatchGroup.enter()
            self.downloadContent(fromUrlString: self.urlStringList[index], completionHandler: { (result) in
                
                switch result {
                case .success(let gifData):
                    
                    let fileName = self.getFileName(urlString: self.urlStringList[index])
                    dataList.append(Gif(name: fileName,
                                        data: gifData,
                                        coverImage: UIImage(named: "demoImage\(index)")))
                    dispatchGroup.leave()
                case .failure(let error):
                    debugPrint(error.localizedDescription)
                    dispatchGroup.leave()
                }
            })
        }
        
        dispatchGroup.wait()
        completionHandler(dataList)
    }
    
    private func downloadContent(fromUrlString: String, completionHandler: @escaping DownloadCompletionHandler) {
        
        guard let downloadUrl = URL(string: fromUrlString) else { return }
        let urlRequest = URLRequest(url: downloadUrl)
        
        if let cachedData = self.cache.cachedResponse(for: urlRequest) {
            print("Cached data in bytes:", cachedData.data)
            completionHandler(.success(cachedData.data))
            
        } else {
            
            createAndRetrieveURLSession().dataTask(with: urlRequest) { (data, response, error) in
                
                if let error = error {
                    completionHandler(.failure(error))
                } else {
                    
                    let cachedData = CachedURLResponse(response: response!, data: data!)
                    self.cache.storeCachedResponse(cachedData, for: urlRequest)
                    
                    completionHandler(.success(data!))
                }
            }.resume()
        }
    }
    
    private func createAndRetrieveURLSession() -> URLSession {
        let sessionConfiguration = URLSessionConfiguration.default
        sessionConfiguration.requestCachePolicy = .returnCacheDataElseLoad
        sessionConfiguration.urlCache = cache
        return URLSession(configuration: sessionConfiguration)
    }
    
    private func getFileName(urlString: String) -> String {
        guard let fileName =  NSURL(fileURLWithPath: urlString).deletingPathExtension?.lastPathComponent else {return "" }
        return fileName
    }
    
}