Having issue with multiple file downloads - ios

Question

Currently I'm implementing a file download application.
In my application server there are around 2500 Resource files, I need to download those files from server to my document directory.
My Code:
#implementation DownloadManager
{
NSURLSession *session;
BOOL downloading;
}
#pragma mark - NSURLSessionDownloadDelegate
// Handle download completion from the task
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
NSInteger index = [self assetDownloadIndexForDownloadTask:downloadTask];
if (index < 0)
{
return;
}
DownloadHelper *movieDownload = _assetsToDownload[index];
// Copy temporary file
NSError * error;
[[NSFileManager defaultManager] copyItemAtURL:location toURL:[NSURL fileURLWithPath:[movieDownload localPath]] error:&error];
downloading = NO;
}
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes
{
// Required delegate method
}
// Handle task completion
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
if (error)
NSLog(#"Task %# failed: %#", task, error);
NSLog(#"Task %# Success: %#", task, error);
if ([_assetsToDownload count])
{
[_assetsToDownload removeObjectAtIndex:0];
}
downloading = NO;
if ([_assetsToDownload count])
{
[self downloadFiles];
}
else
{
[self downloadAssets];
}
}
// Handle progress update from the task
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
NSInteger index = [self assetDownloadIndexForDownloadTask:downloadTask];
if (index < 0) return;
// DownloadHelper *movieDownload = _assetsToDownload[index];
double progress = (double) (totalBytesWritten/1024) / (double) (totalBytesExpectedToWrite/1024);
dispatch_async(dispatch_get_main_queue(), ^{
// Showing progress
});
}
#pragma mark - Movie Download Handling & UI
// Helper method to get the index of a Asset from the array based on downloadTask.
- (NSInteger)assetDownloadIndexForDownloadTask:(NSURLSessionDownloadTask *)downloadTask
{
NSInteger foundIndex = -1;
NSInteger index = 0;
for (DownloadHelper *asset in _assetsToDownload)
{
if (asset.downloadTask == downloadTask)
{
foundIndex = index;
break;
}
index++;
}
return foundIndex;
}
- (void)addAssetDownload
{
DownloadInfo *info = nil;
NSString *assetFolder = nil;
for (int index = 0; index<[_assets count]; index++)
{
info = [_assets objectAtIndex:index];
NSURL *url = [NSURL URLWithString:info.assetURL];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSessionDownloadTask *downloadTask = [session downloadTaskWithRequest:request];
DownloadHelper *assetDownload = [[DownloadHelper alloc] initWithURL:url downloadTask:downloadTask];
assetDownload.assetName = info.assetName;
if (info.categoryId == 1)
{
assetFolder = [self getImagePath:info.assetName];
}
else if (info.categoryId == 2)
{
assetFolder = [self getVideoPath:info.assetName];
}
else if (info.categoryId == 3)
{
//assetFolder = [self getDBPath:info.assetName];
}
else
{
assetFolder = [self filePath:info.assetName];
}
assetDownload.assetFolder = assetFolder;
[_assetsToDownload addObject:assetDownload];
}
}
// Initialize the download, session and tasks
- (void)initialize
{
for (DTEDownloadHelper *movieDownload in _assetsToDownload)
{
// Cancel each task
NSURLSessionDownloadTask *downloadTask = movieDownload.downloadTask;
[downloadTask cancel];
}
// Cancel all tasks and invalidate the session (also releasing the delegate)
[session invalidateAndCancel];
session = nil;
_assetsToDownload = [[NSMutableArray alloc] init];
// Create a session configuration passing in the session ID
NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfiguration:#"DTEDownloadBackground"];
sessionConfiguration.discretionary = YES;
session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];
[self addAssetDownload];
// Reset the UI
downloading = NO;
[self downloadFiles];
}
// Download handler
- (void)downloadFiles
{
if ([_assetsToDownload count] > 0)
{
// Acquire the appropriate downloadTask and respond appropriately to the user's selection
NSURLSessionDownloadTask * downloadTask = [_assetsToDownload[0] downloadTask];
if (downloadTask.state == NSURLSessionTaskStateCompleted)
{
// Download is complete. Play movie.
// NSURL *movieURL = [NSURL fileURLWithPath:[_assetsToDownload[0] localPath]];
}
else if (downloadTask.state == NSURLSessionTaskStateSuspended)
{
// If suspended and not already downloading, resume transfer.
if (!downloading)
{
[self showHUD:[NSString stringWithFormat:#"Downloading %#",[_assetsToDownload[0] assetName]]];
[downloadTask resume];
downloading = YES;
}
}
else if (downloadTask.state == NSURLSessionTaskStateRunning)
{
// If already downloading, pause the transfer.
[downloadTask suspend];
downloading = NO;
}
}
}
- (void)downloadAssets
{
_assets = [self retreiveAssets]; // Getting the resource details from the database
if (![_assets count])
{
// Hide progress
}
[self addAssetDownload];
[self downloadFiles];
}
#end
Issue :
Sometimes it downloads the first file and stops there, on next time onwards it is not downloading anything. I couldn't find the issue till now, I wasted almost a day because of this issue. Please help me to find the issue. Thanks in advance.

Answer 1

When using background sessions, old download requests can persist from session to session. Have you tried checking for old, outstanding background tasks with getTasksWithCompletionHandler? I had a bear of a time until I realized that when my app starts, it can get backlogged behind old background requests. And if you have any invalid requests sitting in that background session, everything can get a little backed up.
Also, is your app delegate handling the handleEventsForBackgroundURLSession method, re-instantiating the background session and saving that completionHandler that is passed to your app? And is the delegate of your NSURLSession calling that completion handler (presumably in URLSessionDidFinishEventsForBackgroundURLSession: method)? You want to make sure you clean up these background sessions. I don't see any this method in your code snippet, but perhaps you omitted it for the sake of brevity.
A discussion of this can be found in the Background Transfer Considerations section of the URL Loading System Programming Guide: Using NSURLSession guide. Also example of this is shown about 40 minutes into the WWDC 2013 What’s New in Foundation Networking video.

Answer 2

Using NSURLSessionDownloadTask was a mess to me. So finally I implemented a custom download manager using NSOperationQueue and blocks.
I have added this library to GitHub.