title: Download a Audio file using SessionManager in Alamofire 4 in Swift 3
body: <p>In alomofire 3.5 the following code worked well</p>
  
  <pre><code>self.sessionManager.download(.GET, AppConstants.musicFileURL + musicFilename, destination: { (url, response) -&gt; NSURL in
          let fileManager = NSFileManager.defaultManager()
          let directoryURL = fileManager.URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)[0]
          let fileUrl = directoryURL.URLByAppendingPathComponent(musicFilename)
          return fileUrl
        })
      .progress { bytesRead, totalBytesRead, totalBytesExpectedToRead in
        self.percentageDownloaded[exhibitId]![artworkId] = (Double(totalBytesRead) ,Double(totalBytesExpectedToRead))
        let meanPercentageDone = self.calculatePercentageDoneForExhibit(exhibitId, artworkArray: self.artworkArray)
        let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        appDelegate.notifyDownloadProgress(meanPercentageDone)
      }
      .response { _, _, _, _ in
  
        NSLog("Completed downloading audio file,%@ , for artwork %@", musicFilename, artworkId)
        if DataManager.saveMusicDownloadComplete(artworkId, exhibitId: exhibitId){
          let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
          appDelegate.notifyMusicDownloadsComplete(exhibitId)
          self.percentageDownloaded[exhibitId] = [String: (Double, Double)?]()
          NSLog("when all music files have been downloaded")
        }
    }
  }
  </code></pre>
  
  <p>How to migrate above code to Alamofire 4. Not able to see any comments in Migration document.</p>