How do I know when Data finished downloading - swift3

Question

I would like to know when the function below has finished downloading the data.
I know I can use a completion handler, but this is in an NSObject and I'd like to know when to return the completion handler for this function, making sure the video is downloaded completely and ready to go, before the function returns the completion handler, and my View Controller resumes it's logic.
Thanks :)
func downloadVideo(identity:JSON_Video){
// use guard to make sure you have a valid url
let videoId = identity.video_id!
let videoString:String = "\(Constants.endPoint_video)\(videoId).mp4"
guard let videoURL = URL(string: videoString) else { return }
let library_url = self.findVideo(video: identity)
if self.findVideo(video: identity) != nil{
print("Video exists. No need to download")
print("Existing video \(library_url!)")
}else{
print("Video not found. Downloading now")
// Variables to input on request
let loginString = Constants.loginString
let loginData = loginString.data(using: String.Encoding.utf8)!
let base64LoginString = loginData.base64EncodedString()
// URL request
var urlRequest = URLRequest(url: videoURL)
urlRequest.httpMethod = "GET"
urlRequest.setValue("Basic \(base64LoginString)", forHTTPHeaderField: "Authorization")
// set up the session
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)
// make the request
let task = session.dataTask(with: urlRequest, completionHandler: { (data, response, error) in
// do stuff with response, data & error here
if let error = error{
print("Data Session Error: \(error.localizedDescription)")
print(response ?? "No response")
return
}
if let data = data {
print("Incoming video....")
print("Data: \(data.description)")
if let finalDatabaseURL = self.videosBaseUrl()?.appendingPathComponent("\(identity.video_id!).mp4"){
print("Copying from: \(videoURL)")
print("Copying to: \(finalDatabaseURL)")
do {
try
data.write(to: finalDatabaseURL)
print("Writing data to file")
// *****
// How do I know when data finished writing?
// Completion handler goes here ?
// *****
}catch{
print("Error writing data to file")
print(error.localizedDescription)
}
}
}
}
})
task.resume()
}
}