Grand Central Dispatch

gcd-semaphore.m

// https://developer.apple.com/library/ios/documentation/userexperience/conceptual/LocationAwarenessPG/MapKit/MapKit.html

// Initialize the semaphore to 0 because there are no resources yet.
dispatch_semaphore_t snapshotSem = dispatch_semaphore_create(0);
	
// Get a global queue (it doesn't matter which one).
dispatch_queue_t queue = dispatch_get_global_queue(myQueuePriorityLevel, 0);
	
// Create variables to hold return values. Use the __block modifier because these
variables will be modified inside a block.
__block MKMapSnapshot *mapSnapshot = nil;
__block NSError *error = nil;
	
// Start the asynchronous snapshot-creation task.
[snapshotter startWithQueue:queue
          completionHandler:^(MKMapSnapshot *snapshot, NSError *e) {
              mapSnapshot = snapshot;
              error = e;
              // The dispatch_semaphore_signal function tells the semaphore that the async task is finished, which unblocks the main thread.
              dispatch_semaphore_signal(snapshotSem);
}];
			  
// On the main thread, use dispatch_semaphore_wait to wait for the snapshot task to complete.
dispatch_semaphore_wait(snapshotSem, DISPATCH_TIME_FOREVER);
	
if (error) { // Handle error. }

// Get the image from the newly created snapshot.
UIImage *image = mapSnapshot.image;