Oleg Andreev

When you need to do CPU-heavy work, there is a nice pattern using GCD: jump to a background queue to do the work, and then back to caller’s queue to report the results. Do not forget to retain the caller’s queue because it may be deallocated while doing background work. Although compiler inserts retains for the ObjC objects referenced within blocks, queues are declared as opaque C structs (dispatch_queue_t), so they won’t be automatically retained.

dispatch_queue_t callerQueue = dispatch_get_current_queue();
dispatch_retain(callerQueue);

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
	
	// Do the work in the other thread...
	// Example: NSArray* items = parseJSON(data);
	
	dispatch_async(callerQueue, ^{
		
		// Report results to the caller...
		// Example: [self didParseItems:items];

		dispatch_release(callerQueue);
	});
});