detect changes to UIWebView's scroll view's contentSize

Question

I\'m trying to set a UIView at the bottom of the content of a UIScrollView, do to so I set the view\'s position to the scrollview\'s contentsize height. But my scrollview is

Answer 1

Your approach is half correct. You can surely override an existing method through a category, what you cannot do, though is accessing an ivar of the class.

In this case, what you need is method swizzling: you override setContentSize while at the same time keeping a reference to the original implementation of the method, so you can call it to set _contentSize value.

Here is the code that you could use, with comments:

@implementation UIScrollView (Height)

// -- this method is a generic swizzling workhorse
// -- it will swap one method impl with another and keep the old one
// under your own impl name
+ (void)swizzleMethod:(SEL)originalSel andMethod:(SEL)swizzledSel {

  Method original = class_getInstanceMethod(self, originalSel);
  Method swizzled = class_getInstanceMethod(self, swizzledSel);
  if (original && swizzled)
     method_exchangeImplementations(original, swizzled);
  else
    NSLog(@"Swizzling Fault: methods not found.");

}    

//-- this is called on the very moment when the categoty is loaded
//-- and it will ensure the swizzling is done; as you see, I am swapping
//-- setContentSize and setContentSizeSwizzled;
+ (void)load {
  [self swizzleMethod:@selector(setContentSize:) andMethod:@selector(setContentSizeSwizzled:)];
}

//-- this is my setContentSizeSwizzled implementation;
//-- I can still call the original implementation of the method
//-- which will be avaiable (*after swizzling*) as setContentSizeSwizzled
//-- this is a bit counterintuitive, but correct!
- (void)setContentSizeSwizzled:(CGSize)contentSize
{
  [self setContentSizeSwizzled:contentSize];
  [[NSNotificationCenter defaultCenter] postNotification:[NSNotification    notificationWithName:@"scrollViewContentSizeChanged" object:nil]];
}

@end

Hope it helps.