I needed a quick spike to demonstrate queueing N asynchronous tasks on a GCD queue. So here it is, just in case you ever need to, you know, asynchronously execute N muppets in batches of M. Well, you never know.

gistfile1.m

//
//  AppDelegate.m
//  GCD_SPIKE_2
//
//  Created by Steve Trewick on 29/05/2013.
//  Copyright (c) 2013 Enigmatic Ape. All rights reserved.
//

#import "AppDelegate.h"
#import <dispatch/dispatch.h>


@implementation AppDelegate {

    dispatch_queue_t   _q;
    dispatch_group_t   _group;
    NSMutableArray   * _wokkas;
    
}



-(void) moar_muppets  {
    
    for( int i = 0; i < 3; i++ ) {
        
        if( _wokkas.count == 0 ) {
            break;
        }
        
        NSString * wokka = [_wokkas lastObject];
        [_wokkas removeLastObject];
        
        dispatch_group_async(_group, _q, ^{
            NSLog(@"%@ (%i)",wokka, i);
        });
    }

    dispatch_group_notify(_group, _q, ^{
        [self group_done];
    });
}

-(void) group_done {

   if( _wokkas.count == 0) {
        NSLog(@"We'll see you next time on the muppet show");
    }
    else {
        NSLog(@"MOAR!");
        [self moar_muppets];
    }
}


- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    
    
    _group   = dispatch_group_create();
    _q       = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
    
    
    _wokkas = [[NSMutableArray alloc] init];
    for( int i = 0; i < 13; i ++ ) {
        [_wokkas addObject:@"wokka" ];
    }
        
    [self moar_muppets];
    
}

@end