+ (void) load
{
	if ( dispatch_barrier_async == 0 )
		return;

	// switch in the dispatch_barrier_async() versions since they're available

	Method m1 = class_getInstanceMethod(self, @selector(handleEventsForObserver:usingBlock:));
	Method m2 = class_getInstanceMethod(self, @selector(_barrier_handleEventsForObserver:usingBlock:));

	if ( m1 != NULL && m2 != NULL )
		method_exchangeImplementations(m1, m2);

	m1 = class_getInstanceMethod(self, @selector(handleCancelEventsForObserver:usingBlock:));
	m2 = class_getInstanceMethod(self, @selector(_barrier_handleCancelEventsForObserver:usingBlock:));

	if ( m1 != NULL && m2 != NULL )
		method_exchangeImplementations(m1, m2);
}