SEL firstMethodSelector = @selector(firstMethod);
SEL secondMethodSelector = @selector(secondMethod);
Method firstMethod = class_getInstanceMethod(self, firstMethodSelector);
Method secondMethod = class_getInstanceMethod(self, secondMethodSelector);
 
BOOL methodAdded = class_addMethod([self class],
                                   firstMethodSelector,
                                   method_getImplementation(secondMethod),
                                   method_getTypeEncoding(secondMethod));
  
if (methodAdded) {
class_replaceMethod([self class], 
                      secondMethodSelector, 
                      method_getImplementation(firstMethod),
                      method_getTypeEncoding(firstMethod));
} else {
  method_exchangeImplementations(firstMethod, secondMethod);
}