Main Thread processing issue - ios

Question

In my application i am using back ground thread for hitting multiple service and perform operation with core data. I have used main thread for back ground process ,Its working fine.
Here is my code
dispatch_queue_t main = dispatch_get_main_queue();
dispatch_async(main,
^{
[self backGroundCall];
});
-(void)backGroundCall
{
NSLog(#"Done");
if([CacheManager refreshDBforFirstTimeUseWithDelegate:self])
{
[[NSUserDefaults standardUserDefaults] setBool:YES forKey:#"IsDBInitialized"];
ContainerViewController *containerViewControllerInstance = [ContainerViewController getContainerInstance];
[containerViewControllerInstance setUserId:_userID];
[progressView setHidden:YES];
[self.view setUserInteractionEnabled:YES];
[self.navigationController setDelegate:containerViewControllerInstance];
[self.navigationController pushViewController:containerViewControllerInstance animated:YES];
}
}
once i initialize the data base , i need to navigate to the container view.During the initialization i will display one progress bar. That is working fine, when the entire background process is completed(app is in minimized state). During the background process if i come to the foreground progress bar is not showing at that time black screen is display instead of progress view . After the completion of the main threat container view all not display[if i comes to foreground of main thread process].
i need to show the progress bar, if i come back to the app in the middle of the main thread process. Please guide me to fix this issue.
Thanks.

Answer 1

dispatch_queue_t main = dispatch_get_main_queue();
dispatch_async(main,
^{
[self backGroundCall];
});
This is a bit misleading... You call the method backGroundCall, but you are actually doing this on the main thread. If you want to make some operation on a working thread, you can do this:
// Declare the queue
dispatch_queue_t workingQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(workingQueue,
^{
// My background job
dispatch_async(dispatch_get_main_queue(),
^{
// Update the UI
}
);
});