For example the following would block the http://www.apple.com site and all secure web sites using the HTTPS scheme:

- (BOOL)shouldBlockURL:(NSURL*)url
{
  if ([[url host] isEqualToString:@"www.apple.com"]) {
    return YES;
  } else if ([[url scheme] isEqualToString:@"https"]) {
    return YES;
  }
  return NO;
}

For more complex filtering you would have an array of URLs or URL parts you would check against in a loop, so the filter manager class would also need methods to load and maintain the list of filters (URLs or URL components) etc. This is why I’ve assumed to do this in a special “filter manager” class. But depending of our requirements, you can do this differently as well.

Does this still work on iOS 6.0?

To avoid this you either have to implement your own HTTP protocol handler (by implementing the methods of the NSURLProtocol class) so you can intercept every single network request of UIWebView, or you only need to make sure that you never use the “reload” method of UIWebView.

But depending of the task you want to achieve, it can be nevertheless be a good idea to show these “missing image” icons to make it obvious that you’ve blocked something. At least if the filter is a feature the user can control (like for example in iCab Mobile). This way the user can see that there’s something blocked, and if it is obvious that something important is missing on the page, the user has the chance to suspect an over-blocking filter and can do something to fix this. But he has to see that something is blocked to get suspicious.