Example of GCD

gistfile1.m

- (void)writeData:(NSData *)data toOutputStream:(NSOutputStream *)stream {	
	dispatch_async(dispatch_get_current_queue(), ^(void){
		BOOL sent = NO;
		
		do {
			if (outputStreamReady) {
				outputStreamReady = NO;
				
				uint8_t *readBytes = (uint8_t *)[data bytes];
				int data_len = [data length];
				uint8_t buf[data_len];
				(void)memcpy(buf, readBytes, data_len);
				[stream write:(const uint8_t *)buf maxLength:data_len];
				
				sent = YES;
			}
		} while(!sent);
	});
}

This function is from a chat program I'm writing.

The outputStreamReady var is defined on the instance level and changed based on the delegate methods from the NSOutputStream object connected to the server. I was having a problem where in some cases the user would type something to be sent out over the chat but the output stream was not ready to send more data so the message would just be lost.