Archives: March 2012

Let’s start with two exotic strings (console output is in the code comments):

NSString* apples = NSGetFrenchWord();
NSString* oranges = NSGetFrenchWord();

NSLog(@"apples == '%@'", apples); 
//apples == 'café'
NSLog(@"oranges == '%@'", oranges); 
//oranges == 'café'

They look identical, but looks can be deceiving.

NSLog(@"isEqual? %@", [apples isEqual:oranges] ? @"YES" : @"NO");
//isEqual? NO
NSLog(@"[apples length] == %lu", [apples length]);
//[apples length] == 4
NSLog(@"[oranges length] == %lu", [oranges length]);
//[oranges length] == 5

GCD is a nice replacement for the old performSelectorInBackground:withObject: and performSelectorOnMainThread:withObject:waitUntilDone: methods and NSOperation. It’s also a nice supplement to NSThread.

However, I think it was over-hyped a little bit by Apple when it was first released. You probably have all these random deadlocks and race conditions and stuff whenever you use multiple threads, but GCD is soooooo good that you don’t have to worry about that anymore. Just use these magic blocks.

The problem is that concurrent programming is notoriously hard to get right. Maybe GCD helps to make it easier, but It doesn’t solve all of your problems. In fact, GCD and blocks introduce some problems of their own. This article will focus on some of those problems.