gistfile1.m

// NOTE:  GameCenter does not guarantee that callback blocks will be execute on the main thread. 
// As such, your application needs to be very careful in how it handles references to view
// controllers.  If a view controller is referenced in a block that executes on a secondary queue,
// that view controller may be released (and dealloc'd) outside the main queue.  This is true
// even if the actual block is scheduled on the main thread.  In concrete terms, this code
// snippet is not safe, even though viewController is dispatching to the main queue:
//
//	[object doSomethingWithCallback:  ^()
//	{
//		dispatch_async(dispatch_get_main_queue(), ^(void)
//		{
//			[viewController doSomething];
//		});
//	}];
//
// UIKit view controllers should only be accessed on the main thread, so the snippet above may
// lead to subtle and hard to trace bugs.  Many solutions to this problem exist.  In this sample,
// I'm bottlenecking everything through  "callDelegateOnMainThread" which calls "callDelegate". 
// Because "callDelegate" is the only method to access the delegate, I can ensure that delegate
// is not visible in any of my block callbacks.

- (void)callSelf:(SEL)selector withArg:(id)arg error:(NSError*)err
{
	assert([NSThread isMainThread]);
	if([self respondsToSelector: selector])
	{
		if(arg != NULL)
		{
			[self performSelector: selector withObject: arg withObject: err];
		}
		else
		{
			[self performSelector: selector withObject: err];
		}
	}
	else
	{
		NSLog(@"Missed Method");
	}
}