#import <objc/runtime.h>

// swap a class\'s instance method selectors, we do this to overload existing methods in category declarations
void swizzleMethodsForClass(Class c, SEL origMethodSel, SEL newMethodSel)
        {
        NSLog(@"swizzling %@ instance methods: %@ -> %@", NSStringFromClass(c),
                NSStringFromSelector(origMethodSel), NSStringFromSelector(newMethodSel));

        Method origMethod = class_getInstanceMethod(c, origMethodSel);
        Method newMethod = class_getInstanceMethod(c, newMethodSel);

        // check if method is inherited        from superclass
        if(class_addMethod(c, origMethodSel, method_getImplementation(newMethod), method_getTypeEncoding(newMethod)))
                class_replaceMethod(c, newMethodSel, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));

        // exchange un-subclassed method
        else
                method_exchangeImplementations(origMethod, newMethod);
        }
       

@interface UIDevice (SpoofUDID)

@end

#define UDID_TO_SPOOF        @"e0101010d38bde8e6740011211af315301010223"

@implementation UIDevice (SpoofUDID)

// swizzle this instance method for UIDevice class
- (NSString *) spoofUniqueIdentifier
        {
        static NSString *spoofUDID = UDID_TO_SPOOF;
        NSLog(@"spoofing %@ instead of %@", spoofUDID, [[UIDevice currentDevice]
spoofUniqueIdentifier]);
        return spoofUDID;
        }

@end

// call this from your app delegate
- (void) initUDID
        {
        NSString *UDID = [[UIDevice currentDevice] uniqueIdentifier];
        NSLog(@"this is my old udid: %@", UDID);

        swizzleMethodsForClass([UIDevice class], @selector(uniqueIdentifier), @selector(spoofUniqueIdentifier));

        NSString *UDID2 = [[UIDevice currentDevice] uniqueIdentifier];
        NSLog(@"this is my new udid: %@", UDID2);
        }