How to use asynchronous data - ios

Question

I'm trying to fetch some data from a remote file. I'm creating an _items array inside the asynchronous request block, which is actually full of data, but when calling the _items anywhere else in my controller is still empty.
I'm not sure but i'm guessing that as the data are loaded asyncronously, when calling _items is still empty. So how how can i use the array with data, after the async task is completed?
- (IBAction)fetchEvent
{
_items = [[NSMutableArray alloc] initWithCapacity:5];
NSString *link = [NSString stringWithFormat:#"url...?id=%#", self.eId];
NSURL *url = [NSURL URLWithString:link];
NSURLRequest *request = [NSMutableURLRequest requestWithURL:url];
[NSURLConnection sendAsynchronousRequest:request
queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse *response,
NSData *data, NSError *connectionError)
{
if (data.length > 0 && connectionError == nil)
{
NSDictionary *match = [NSJSONSerialization JSONObjectWithData:data
options:0
error:NULL];
NSArray *odds = [match objectForKey:#"odds"];
OddsItem *item;
for ( NSDictionary *books in odds ) {
item = [[OddsItem alloc] init];
item.oddsBook = [odds objectForKey:#"book"];
[_items addObject:item];
}
}
}];
}
- (void)viewDidLoad
{
[super viewDidLoad];
[self fetchEvent];
}

Answer 1

mutable data is dangerous in multi thread situation. NSArray is better.
#property (nonatomic, copy) NSArray *items;
block can capture outside variables but can not modify it directly. In completion block try this
if (data.length > 0 && connectionError == nil)
{
NSDictionary *match = [NSJSONSerialization JSONObjectWithData:data
options:0
error:NULL];
NSArray *odds = [match objectForKey:#"odds"];
NSMutableArray *items = [[NSMutableArray alloc] initWithCapacity:5];
for ( NSDictionary *books in odds ) {
OddsItem *item = [[OddsItem alloc] init];
item.oddsBook = [odds objectForKey:#"book"];
[items addObject:item];
}
self.items = items;
}