Inside nsurlsession and background transfer - nsurlsession

Question

I am working in project, where I need to download large JSON response in background (Even if app crashes).
I also want to understand internals of background transfer service, i.e. what iOS is doing internally and how. So I have saved call stack in UserDefaults.
there were 2 scenarios.
1) Without app crash.
(
Download task created with taskidentifier : 1,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 1,
didCompleteWithError->success for taskIdentifier 1,
)
2) When I crash app manually using null pointer dereference after downloading started, results were weird.
(
Download task created with taskidentifier : 1,
Download task created with taskidentifier : 2,
handleEventsForBackgroundURLSession,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 1,
didCompleteWithError->success for taskIdentifier 1,
URLSessionDidFinishEventsForBackgroundURLSession,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 2,
didCompleteWithError->success for taskIdentifier 2
)
I was only creating 1 task.
Does iOS automatically creates second task ?
What is happening here.
Here is my code
#import "KiviSyncLogin.h"
#import "AppDelegate.h"
#implementation KiviSyncLogin
-(instancetype)init{
self = [super init];
return self;
}
-(void)startSync{
if(self.session != nil){
NSLog(#"session is not nil");
return;
}
self.session = [self setUpSession]; // init session with once token
NSLog(#"This is session : %#",self.session);
[self pullDataFromServerWithSession:self.session];
}
-(NSURLSession *)setUpSession{
static dispatch_once_t onceToken;
static NSURLSession *session = nil;
dispatch_once(&onceToken, ^{
NSURLSessionConfiguration *config = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:#"com.kivi.login.sync"];
config.HTTPMaximumConnectionsPerHost = 1;
session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];
});
return session;
}
-(void)pullDataFromServerWithSession:(NSURLSession *)session{
NSString *serverUrlString = #"apiurl";
NSMutableDictionary *mydic = [[NSMutableDictionary alloc] init];
[mydic setObject:#"email" forKey:#"email"];
[mydic setObject:#"password" forKey:#"password"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:serverUrlString]];
request.HTTPMethod = #"POST";
request.HTTPBody = [[self encodedString:mydic] dataUsingEncoding:NSUTF8StringEncoding];
self.downloadTask = [self.session downloadTaskWithRequest:request];
NSLog(#"Download task %# created with identifier : %ld", self.downloadTask,self.downloadTask.taskIdentifier);
[self.downloadTask resume];
}
/* common delegate Start */
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location{
NSLog(#"file downloaded");
NSFileManager *fileManager = [NSFileManager defaultManager];
NSArray *URLs = [fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask];
NSURL *documentsDirectory = [URLs objectAtIndex:0];
NSURL *originalURL = [[downloadTask originalRequest] URL];
NSURL *destinationURL = [documentsDirectory URLByAppendingPathComponent:[originalURL lastPathComponent]];
NSError *errorCopy;
[fileManager removeItemAtURL:destinationURL error:NULL];
BOOL success = [fileManager copyItemAtURL:location toURL:destinationURL error:&errorCopy];
if (success){
/* Store data in database */
}
else{
NSLog(#"Error during the copy: %#", [errorCopy localizedDescription]);
}
}
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
if(task.taskIdentifier == self.downloadTask.taskIdentifier){
if(error){
NSLog(#"Download task completed with error : %#",error);
/* Show error that data is not downloaded */
[self windUpSession];
}else{
NSLog(#"Download task completed successfully");
}
}
}
-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session
{
NSLog(#"Session %# URL Session Did Finish Events For Background URL Session\n", session);
dispatch_async(dispatch_get_main_queue(), ^{
AppDelegate *appDelegate = (AppDelegate *)[UIApplication sharedApplication].delegate;
[session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
if ([downloadTasks count] == 0 && [uploadTasks count] == 0) {
if (appDelegate.backgroundTransferCompletionHandler != nil) {
NSLog(#"I have completion handler");
void(^completionHandler)(void) = appDelegate.backgroundTransferCompletionHandler;
appDelegate.backgroundTransferCompletionHandler = nil;
[self windUpSession];
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
completionHandler();
}];
}
}
}];
});
}
- (void)windUpSession{
//[self.session invalidateAndCancel];
self.session = nil;
self.downloadTask = nil;
}
- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error{
if(error == nil){
NSLog(#"Session is invalidated successfully");
}else{
NSLog(#"Error while invalidating session");
}
}
/* Common Delegates End */
/* Helper methods */
-(NSString *)encodedString:(NSDictionary *)mydic{
NSString *params = #"";//[NSString stringWithFormat:#"email=%#&password=%#",email,token];
int i = 0;
for (NSString *key in [mydic allKeys]) {
NSString *stringToBeAppended;
if(i++ == 0)
stringToBeAppended = [NSString stringWithFormat:#"%#=%#",key,mydic[key]];
else
stringToBeAppended = [NSString stringWithFormat:#"&%#=%#",key,mydic[key]];
params = [params stringByAppendingString:stringToBeAppended];
}
return params;
}
#end
If I add [self.session finishTasksAndInvalidate]; while creating download task, it will create 2nd task but do not execute it as session is invalidated.
in this case call stack is (If I carsh app manually.)
(
Download task created with taskidentifier : 1,
Download task created with taskidentifier : 2,
handleEventsForBackgroundURLSession,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 1,
didCompleteWithError->success for taskIdentifier 1,
URLSessionDidFinishEventsForBackgroundURLSession
)
Edit 1
I have also noticed that when my app crashes, viewDidLoad method of viewController from where I am crashing app and from which I am initializing above class and start fetching is being called.