swift: nsurlsession downloading files - ios

Question

There is a tableViewController with 5 cells. When I click on a cell, the download starts, if the file is not found.
override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
if indexPath.row > 0 {
let path = NSSearchPathForDirectoriesInDomains(FileManager.SearchPathDirectory.documentDirectory, FileManager.SearchPathDomainMask.userDomainMask, true)
let documentDirectoryPath:String = path[0]
let fileManager = FileManager()
let destinationURLForFile = URL(fileURLWithPath: documentDirectoryPath.appendingFormat("/file%d.pdf",indexPath.row))
if fileManager.fileExists(atPath: destinationURLForFile.path){ self)
}else{
var downloadTask: URLSessionDownloadTask!
index = indexPath.row
let url = URL(string: "http://publications.gbdirect.co.uk/c_book/thecbook.pdf")!
downloadTask = backgroundSession.downloadTask(with: url)
downloadTask.resume()
}}
func urlSession(_ session: URLSession,
downloadTask: URLSessionDownloadTask,
didFinishDownloadingTo location: URL){
let path = NSSearchPathForDirectoriesInDomains(FileManager.SearchPathDirectory.documentDirectory, FileManager.SearchPathDomainMask.userDomainMask, true)
let documentDirectoryPath:String = path[0]
let fileManager = FileManager()
let destinationURLForFile = URL(fileURLWithPath: documentDirectoryPath.appendingFormat("/file%d.pdf",index))
do {
try fileManager.moveItem(at: location, to: destinationURLForFile)
}catch{
print("An error occurred while moving file to destination url")
}
}
The problem is that downloading one of the files stops if I download 2 files at a time. How to fix it?

Answer 1

A couple of thoughts:
A single, numeric index property is obviously insufficient to keep track of the multiple downloads that might be in progress. You need some structure to keep track of the correlation between downloads and their eventual file names in the Documents folder. It might be:
struct Download {
enum Status {
case notStarted
case started
case finished
case failed(Error?)
}
/// URL of resource on web server
let remoteURL: URL
/// URL of file in Documents folder
let localURL: URL
/// The status of the download
var status: Status
}
Now that you have a type to keep track of the state of an individual download, create an array of those Download objects:
var downloads = [Download]()
You might populate that in viewDidLoad, or something like that:
override func viewDidLoad() {
super.viewDidLoad()
// create the `Download` objects, e.g. I'll create one here
let remoteURL = URL(string: "http://publications.gbdirect.co.uk/c_book/thecbook.pdf")!
let fileURL = try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
.appendingPathComponent("file0.pdf")
let status: Download.Status
if try! fileURL.checkResourceIsReachable() {
status = .finished
} else {
status = .notStarted
}
downloads.append(Download(remoteURL: remoteURL, localURL: fileURL, status: status))
// since you're dealing with background session (e.g. tasks may have been previously
// scheduled), let's iterate through any pending tasks, updating status accordingly
backgroundSession.getAllTasks { tasks in
for task in tasks {
guard let index = self.downloads.index(where: { $0.remoteURL == task.originalRequest?.url }) else {
print("cannot find download for \(task.originalRequest?.url)")
return
}
self.downloads[index].status = .started
}
}
}
When the download is done, you can now just look up that download in our array of downloads in order to determine the file URL:
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL){
guard let index = downloads.index(where: { $0.remoteURL == downloadTask.originalRequest?.url }) else {
print("cannot find download for \(downloadTask.originalRequest?.url)")
return
}
do {
try FileManager.default.moveItem(at: location, to: downloads[index].localURL)
downloads[index].status = .finished
} catch {
print("An error occurred while moving file to destination url: \(error.localizedDescription)")
downloads[index].status = .failed(error)
}
}
It's worth noting that the logic that says "if the file doesn't exist, then start download" is, most likely, insufficient. Sure, if the file exists, then the download is done. But what if a download has been started already, but hasn't yet finished? You probably do not want to start a new download if a previously initiated download simply hasn't yet finished.
override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
if indexPath.row > 0 {
let index = indexPath.row - 1 // for some reason you're looking at indexes greater than zero, so let's adjust our index for a zero-based index within our array
switch downloads[index].status {
case .notStarted:
let downloadTask = backgroundSession.downloadTask(with: downloads[index].remoteURL)
downloads[index].status = .started
downloadTask.resume()
default:
break
}
}
}
Now, I don't want to get too lost in the details of these code snippets above, but rather I want to make sure you grok the basic concept, namely that you can't have a single numeric index, but rather you need some collection (an array or dictionary) to keep track of all of the various downloads that may be in progress at any given time.

Answer 2

You cannot download two files at a time if you are using single variables (index and downloadTask). Whenever the user selects the second cell, a new value for index is set, so using that value in urlSession:downloadTask:didFinishDownloadingTo: is a mistake when it is being called by the first task.
You need to keep that values in a collection, for example an array of tuples, keeping the index, the task and any other info about the file, for example the file path.