numberOfRowsInSection returning 0 in a threaded viewController - objective-c

Question

I have a viewController in an app that retrieves Data from JSON, parses it and populates in UITableView. I am using thread to load data so that app does not hang when it is retrieving data.
Problem:
numberOfRowsInSection returns 0 and UITableView does not get populated sometimes when app is started. While sometimes, everything works fine. It is all random :S
Possible Explanation:
The problem is, it seems like, sometimes numberOfRowsInSection is called before data is retrieved. numberOfRowsInSection returns the value of count of a NSMutableArray called 'subjects'. Objects in 'subjects' are added when loadData is called. So the numberOfRowsInSection should return the count of 'subjects' and it should not be called after 'subjects' is populated.
Sometimes when I start the app, numberOfRowsInSection is called after 'subjects' is populated and UITableView shows data but sometimes when I start the app, numberOfRowsInSection is called before 'subjects' is populated and UITableView shows no data.
Code:
Here is my code:
-(void)loadData:(id)sender
{
dispatch_queue_t getRemindersQueue=dispatch_queue_create("reminders JSON downloader with reload Button", NULL);
dispatch_async(getRemindersQueue, ^{
[self getReminders];
dispatch_async(dispatch_get_main_queue(), ^{
self.navigationItem.rightBarButtonItem=sender;
[self.tableView reloadData];
});
});
dispatch_release(getRemindersQueue);
}
-(void)getReminders
{
NSURL * aURL = [NSURL URLWithString: #"http://www.merrycode.com/apps/IELTS/RemindersJSON"];
NSURLRequest *request = [NSURLRequest requestWithURL:aURL cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:10.0];
NSError *responseError=nil;
// Perform request and get JSON back as a NSData object
NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:&responseError];
if(responseError)
{
dispatch_async(dispatch_get_main_queue(), ^{
UIAlertView *parsingError = [[UIAlertView alloc] initWithTitle:#"Network Error"
message:#"Can not reach the servers. Make sure you are connected to the internet."
delegate:nil
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[parsingError show];
});
return;
}
NSString *str = [[NSString alloc] initWithData:response encoding:NSUTF8StringEncoding];
NSLog(#" String of Reminders JSON: %#",str);
NSString *newStr= [self stringByRemovingControlCharacters:str];
response = [newStr dataUsingEncoding:NSUTF8StringEncoding];
NSError *jsonParsingError = nil;
NSArray *publicTimeline = [NSJSONSerialization JSONObjectWithData:response options:0 error:&jsonParsingError];
NSLog(#"%#", jsonParsingError);
NSLog(#" publicTimeline Array Count: %d", [publicTimeline count]);
if([publicTimeline count] == 0)
{
dispatch_async(dispatch_get_main_queue(), ^{
UIAlertView *parsingError = [[UIAlertView alloc] initWithTitle:#"Error Retriving Data"
message:#"There was an error reciving data from the server."
delegate:nil
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[parsingError show];
});
return;
}
NSDictionary *colleges;
for(int i=0; i<[publicTimeline count];i++)
{
colleges= [publicTimeline objectAtIndex:i];
NSLog(#"Reminders: %#", [colleges objectForKey:#"title"]);
[self.subjects addObject:[colleges objectForKey:#"title"]];
[self.dates addObject:[colleges objectForKey:#"date"]];
[self.description addObject:[colleges objectForKey:#"desc"]];
}
[self.subjectsInNSUserDefaults removeAllObjects];
[self.datesInNSUserDefaults removeAllObjects];
[self.descriptionInNSUserDefaults removeAllObjects];
[self.userDefaults setObject:self.subjects forKey:#"SUBJECTS"];
[self.userDefaults setObject:self.dates forKey:#"DATES"];
[self.userDefaults setObject:self.description forKey:#"DESCRIPTION"];
[self.userDefaults synchronize];
}
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
NSLog(#"Array Count in numberOfRowsInSection: %d",[self.subjects count]);
return [self.subjects count];
}

Answer 1

Couple of ideas...
You said this is a UIViewController (as opposed to UITableViewController). How are you setting up your UITableView and the delegate? If you are still setting up your UITableView or its delegates while this background thread is running, it is theoretically possible that the background thread could complete before you are done setting up your UITableView, which could create strange issues (and explain why this happens "randomly").
Also, have you checked to make sure your response object is populated with information about colleges in the cases where your UITableView isn't getting populated (and not some sort of other response, or no response at all)? I see where you are checking for response errors, but you seem to assume that if there isn't an error, it will be a response with information about colleges (which may or may not be a safe assumption).

Answer 2

If you're correct about the data retrieval being the problem, then I have also had this problem. My inelegant solution was just to set a timer to populate the table at a time when I knew the data would be loaded.