如何使用异步数据

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;
     }