ObjC Swizzlin'

gistfile1.m

// Every class using this inherits from the class defining these methods, 
// and provides a +uiKitSubclass method which returns the class object of
// the class which should be swizzled.


+ (void)subclassMethod:(SEL)selector
    withImplementation:(IMP)subclassedImplementation
{
  Method uiKitMethod = class_getInstanceMethod([self uiKitSubclass], selector);
  assert(uiKitMethod);
  BOOL methodAdded = 
      class_addMethod(
        [self uiKitSubclass], selector, subclassedImplementation,
        method_getTypeEncoding(uiKitMethod));

  // If the method wasn't added (because it already exists), we replace the
  // method and save the original implementation for later.
  if (!methodAdded) {
    Method method = class_getInstanceMethod([self uiKitSubclass], selector);

    IMP originalImplementation = 
        method_setImplementation(method, subclassedImplementation);

    objc_setAssociatedObject(
        [self uiKitSubclass], (void*)subclassedImplementation,
        [NSValue valueWithPointer:(void*)originalImplementation],
        OBJC_ASSOCIATION_RETAIN_NONATOMIC);
  }
}

+ (IMP)getOriginalImplementation:(IMP)subclassedImplementation
                     forSelector:(SEL)selector
                         andSelf:(id)aSelf
{
  // First check for a saved implementation.
  NSValue* originalImplementation =
      objc_getAssociatedObject(
          [aSelf class], (void*)subclassedImplementation);
  if (originalImplementation) {
    return (IMP)[originalImplementation pointerValue];
  } else {
    // Otherwise return super's implementation if it exists.
    Class superclass = class_getSuperclass([aSelf class]);
    Method superMethod = class_getInstanceMethod(superclass, selector);
    if (superMethod) {
      return method_getImplementation(superMethod);
    } else {
      return NULL;
    }
  }
}



// Then to call the original implementation inside the new implementation:
  IMP originalDealloc = 
      [UIComponentPlugin getOriginalImplementation:(IMP)dealloc
                                       forSelector:_cmd
                                           andSelf:self];
  originalDealloc(self, _cmd);