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1. Method swizzling

   void Swizzle(Class class, SEL origSel, SEL newSel) {
       Method origMethod = class_getInstanceMethod(class, origSel);
       Method newMethod = class_getInstanceMethod(class, newSel);
   
       if(class_addMethod(class, origSel, method_getImplementation(newMethod), method_getTypeEncoding(newMethod))) {
           class_replaceMethod(class, newSel, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
       } else {
           method_exchangeImplementations(origMethod, newMethod);
       }
   }
   

   from Mike Ash

   static IMP original_initWithString_;
   
   @implementation NSURL (Ext)
   
   + (void)swizzleMethods { // call this once, early
       original_initWithString_ = method_getImplementation(class_getInstanceMethod([self class], @selector(initWithString:)));
       Swizzle([self class], @selector(initWithString:), @selector(my_initWithString:));
   }
   
   // -[NSURL urlWithString:] will now execute this method
   - (NSURL *)my_initWithString:(NSString *)s {
       NSLog(@"-- my_initWithString: %@", s);
       return original_initWithString_(self, @selector(initWithString:), s);
   }
   
   @end