gcd_1.m

dispatch_queue_t myQueue = dispatch_queue_create("my queue",NULL);
//...
//...
//...
dispatch_async(imageQueue, ^{
        // This will run the method in a separate thread
        [self doSomeLongRunningTask];
    });


- (void) doSomeLongRunningTask {
    dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"We finished a heavy task!");
        });
}

/*
So, we create a dispatch queue handle for our separated thread,
than pass a block ^{} to dispatch_async that will be executed in a separate thread ( NOT in the main thread).
If you want to call a method in the main thread from another thread,
you call:  dispatch_async(dispatch_get_main_queue(), ^{ });
As you see, dispatch_get_main_queue() gives you the queue handle of the main thread.
For more information, check:
http://en.wikipedia.org/wiki/Grand_Central_Dispatch
*/