example of using apple's method_exchangeImplementations from objective-c runtime

gistfile1.txt

- (void)atest1
{
    NSLog(@"this is test 1");
}

- (void)atest2
{
    NSLog(@"this is test 2");
    [self atest2];
}

- (void)testExchange
{
    Method originalMethod = class_getInstanceMethod([self class], @selector(atest1));
    Method categoryMethod = class_getInstanceMethod([self class], @selector(atest2));
    method_exchangeImplementations(originalMethod, categoryMethod);
    
    [self atest1];
    
    NSLog(@"The log should show \"this is test 2\" and then \"this is test 1\"");
}