You can use GCD for thread substitutions. It needs blocks.

Example of use:

 - (void)doTimeConsumingOperation:(id)operation {  
    dispatch_queue_t queue;  
    queue = dispatch_queue_create("com.example.operation",NULL);  
    dispatch_async(queue,^{  
       [operation doOperation]; // Main op.  
    });  
    dispatch_release(queue)  
 }

You can use dispatch_sync instead dispatch_async for locked code execution.

Another sample of code is this way to perform a delayed selector on an object:

 - (void)doLaterOperation:(id)operation {  
      dispatch_time_t delay;  
      delay = dispatch_time(DISPATCH_TIME_NOW, 50000 /* nanoseconds */);  
      dispatch_after(delay,^{  
           [operation doOperation]; // Main op.  
      });  
      dispatch_release(delay);
 }

Finally other fine use of GCD is for creating safe singleton objects:

 +(MyClass *) sharedInstance {  
      static dispatch_once_t predicate;  
      static MyClass *shared = nil;  
      dispatch_once(&predicate, ^{  
          shared = [[MyClass alloc] init];  
      });  
      return shared;  
 }

With dispatch_once the code only will be executed one and only one time.

Available in iOS 4.0 and later.
Available in Mac OS X v10.6 and later.
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