dispatch_queue not executing - ios

Question

i was undergoing stanford cs 193 p assignment5.The problem is that when i segue a data to destination view controller and use dispatch queue to fetch the data in viewDidLoad method.The dispatch_async does'nt execute in the destination view controller .Here is my code for View controller A and destination view controller.
view controller A code
-(void)viewDidLoad
{
[super viewDidLoad];
if(!self.places){
self.spinner.hidesWhenStopped = YES;
self.spinner.center = self.tableView.center;//
[self.view addSubview:self.spinner];
[self.spinner startAnimating];
dispatch_queue_t dispatchQueue = dispatch_queue_create("queue_top_places", NULL);
dispatch_async(dispatchQueue, ^{
self.places = [self getRecentPlacesFromFlicker];
// main queue to load table view data
dispatch_async(dispatch_get_main_queue(), ^{
// load table data
if(self.tableView.window){
[self.tableView reloadData];
[self.spinner stopAnimating];
}
});
});
dispatch_release(dispatchQueue);
}
// Uncomment the following line to preserve selection between presentations.
self.clearsSelectionOnViewWillAppear = YES;
}
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
if([segue.identifier isEqualToString:#"Show Recent Photo List"]){
int currentRow = self.tableView.indexPathForSelectedRow.row;
// set up photo list controller model
[segue.destinationViewController setPhotosList:[self.places objectAtIndex:currentRow]];
}
}
Here is my code for destination view controller
- (void)viewDidLoad
{
[super viewDidLoad];
// get the current top place name and fetch photos at that place from flicker
if ([self.photosList isKindOfClass:[NSDictionary class]])
{
UIActivityIndicatorView *spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
spinner.hidesWhenStopped = YES;
spinner.center = self.tableView.center;//
[self.view addSubview:spinner];
[spinner startAnimating];
dispatch_queue_t dispatchQueue1 = dispatch_queue_create("queue_top_50_photos", NULL);
dispatch_async(dispatchQueue1, ^{
self.photosList = [FlickrFetcher photosInPlace:self.photosList maxResults:50];
dispatch_async(dispatch_get_main_queue(), ^{
if (self.tableView.window ){
[self.tableView reloadData];
[spinner stopAnimating];
}
});
});
dispatch_release(dispatchQueue1);
}
// Uncomment the following line to preserve selection between presentations.
self.clearsSelectionOnViewWillAppear = YES;
self.title = #"50PhotoList";
}
The condition ([self.photosList isKindOfClass:[NSDictionary class]]) is satisfied checked with a debugger

Answer 1

I was just thinking about this, have you tried running this code in viewDidAppear instead of viewDidLoad? I'd give that a try.
Any reason why you create a new queue for each operation? What if you remove the dispatch_release instruction at the end? Maybe your async operation takes longer than the time it gets for your queue to get released.
Try to use a global queue instead of one that you create yourself, one you don't have to release when you're done.
Replace
dispatch_queue_t dispatchQueue1 = dispatch_queue_create("queue_top_50_photos", NULL);
with
dispatch_queue_t dispatchQueue1 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
and remove the dispatch_release(dispatchQueue1);

Answer 2

dispatch_async returns immediately and performs the task in background and executes rest of the code in the viewDidLoad or other viewDidAppear methods.
One solution would be to use dispatch_sync which waits for the task to execute and to nest it in dispatch_async like this
dispatch_queue_t dispatchQueue1 = dispatch_queue_create("queue_top_50_photos", NULL);
dispatch_async(dispatchQueue1, ^{
dispatch_queue_t dispatchSyncQueue = dispatch_queue_create("queue_top_50_photos", NULL);
dispatch_sync( dispatchSyncQueue,^){
self.photosList = [FlickrFetcher photosInPlace:self.photosList maxResults:50];
});
dispatch_sync(dispatch_get_main_queue(), ^{
if (self.tableView.window ){
[self.tableView reloadData];
[spinner stopAnimating];
}
});
dispatch_release(dispatchSyncQueue);
});
dispatch_release(dispatchQueue1);