Alamofire-based downloader used to download file

Downloader.swift

//
//  Downloader.swift
//
//  Created by Natata on 2017/9/15.
//  Copyright © 2017 Natata. All rights reserved.
//

import Foundation
import Alamofire

protocol DownloaderDelegate {
    func updateProgress(completed: Double)
}

class Downloader {
    
    let urlString: String
    let fileName: String
    
    var delegate: DownloaderDelegate?
    var completion: ((String) -> Void)?
    var downloadRequest: DownloadRequest?
    var destination: DownloadRequest.DownloadFileDestination?
    var isPaused: Bool = false
    var resumeData: Data?
    
    init(urlString: String, fileName: String, completion: ((String) -> Void)?) {
        self.urlString = urlString
        self.fileName = fileName
        self.completion = completion
        self.destination = {_, _ in
            let documentsURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
            let destinationURL = documentsURL.appendingPathComponent(fileName, isDirectory: false)
            return (destinationURL, [.removePreviousFile, .createIntermediateDirectories])
        }
    }
    
    func resume() {
        self.isPaused = false
        
        if let resumeData = resumeData {
            print("redownload from resume data")
            downloadRequest = Alamofire.download(resumingWith: resumeData, to: destination)
        } else if let downloadRequest = downloadRequest {
            print("resume")
            downloadRequest.resume()
        } else {
            downloadRequest = Alamofire.download(self.urlString, to: destination)
        }
        setDownloadRequest()
    }
    
    func pause() {
        self.isPaused = true
        if let downloadRequest = downloadRequest {
            print("paused")
            downloadRequest.suspend()
        }
    }
    
    func cancel() {
        self.isPaused = true
        if let downloadRequest = downloadRequest {
            print("cancel")
            downloadRequest.cancel()
            resumeData = downloadRequest.resumeData
        }
    }
    
    func setDownloadRequest() {
        downloadRequest?.downloadProgress { progress in
            self.delegate?.updateProgress(completed: progress.fractionCompleted)
        }.response { response in
            print("[Download] complete response: \(response)")
            if response.error == nil,
                let path = response.destinationURL?.path {
                self.completion?(path)
            } else {
                print("[Download] error happended: \(response.error?.localizedDescription))")
                self.resumeData = response.resumeData
            }
        }
    }
}

let downloader = Downloader(urlString: "https://httpbin.org/image/png", fileName: "pig.png") { (filePath) in
    print(filePath)
}