dispatch_async and block in iOS

Question

What this piece of code mean?

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{         TMBaseParser *parser=[[TMBaseParser alloc] init];         parser.delegate=self;         NSString *post =nil;         NSData *postData = [post dataUsingEncoding:NSUTF8StringEncoding];         [parser parseForServiceType:TMServiceCategories postdata:postData];     }); 

please explain it briefly.

Answer 1

The piece of code in

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  }); 

is run asynchronously on a background thread. This is done because parsing data may be a time consuming task and it could block the main thread which would stop all animations and the application wouldn't be responsive.

If you want to find out more, read Apple's documentation on Grand Central Dispatch and Dispatch Queue.