An empty app to demonstrate race conditions in Objective-C and how to fix it with GCD.

Isolation.m

@interface LHLAppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;
@property (strong, nonatomic) NSNumber* counter; // will making this 'atomic' solve our problem?
@property (strong, nonatomic) dispatch_queue_t isolationQueue;

@property (strong, nonatomic) NSRecursiveLock* lock;

@end


@implementation LHLAppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window setRootViewController:[UIViewController new]];
    [self.window makeKeyAndVisible];
    
    self.isolationQueue = dispatch_queue_create("com.Lighthouse-Labs.isolationQueue", 0);
    self.lock = [NSRecursiveLock new];
    self.counter = @0;
    
    // let's get the default concurrent queue
    dispatch_queue_t q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    // let's create 4 blocks, each of which will increment our counter 1000 times
    void (^asyncBlock)() = ^{
        for (int i = 0 ; i < 1000; i++) {
            [self incrementCounter];
        }
        NSLog(@"Finished increments. Current count: %ld",
              (long)[self.counter integerValue]);
    };
    dispatch_async(q, asyncBlock);
    dispatch_async(q, asyncBlock);
    dispatch_async(q, asyncBlock);
    dispatch_async(q, asyncBlock);
    
    return YES;
}

-(void)incrementCounter
{
    [self incrementCounterWithIsolationQueue];
}

-(void)incrementCounterUnsafe 
{
    self.counter = @([self.counter integerValue] + 1);
}

-(void)incrementCounterWithIsolationQueue
{
    dispatch_async(self.isolationQueue, ^{
        self.counter = @([self.counter integerValue] + 1);
    });
}

-(void)incrementCounterWithLock
{
    [self.lock lock];
    self.counter = @([self.counter integerValue] + 1);
    [self.lock unlock];
}

-(void)incrementCounterWithSynchronized
{
    @synchronized(self) {
        self.counter = @([self.counter integerValue] + 1);
    }
}

@end