How to react to failed thumb image loading in TTThumbsViewController?

I'm currently using TTThumbsViewController in my project. I'm getting all the urls for TTPhotoVersionLarge and TTPhotoVersionThumbnail from the web so I can't tell that a url for the thumb images will work or not.

Currently the TTThumbsViewController will just display an empty image if the thumb url can't be loaded.

So I w开发者_运维问答ant to be notified if a thumb fails to load and do extra error handling when that happens like:

1. Try to load the url for TTPhotoVersionLarge
2. If that fails again display an error image (which is included in the bundle)

I have looked into the three20 code but can't find a proper place where I can implement this proper error handling.

---

The Three20 library is great, but I've found that it's often hard to change parts of their library such as this. For instance, TTThumbsViewController basically only allows you to set the dataSource and then takes care of the rest. If you want more control, perhaps look into code such as AQGridView: http://quatermain.tumblr.com/post/528737778/aqgridview-lives-for-my-ipad-dev-camp-hackathon

I've chosen to use that over Three20's equivalent because it gives you more control over what happens with your data.

Edit: In response to using TTThumbsViewController heavily, you may want to look at this method in TTTHumbsViewController.m:

- (NSString*)URLForPhoto:(id<TTPhoto>)photo {
  if ([photo respondsToSelector:@selector(URLValueWithName:)]) {
    return [photo URLValueWithName:@"TTPhotoViewController"];

  } else {
    return nil;
  }
}

It looks like you should be able to specify a different value there so long as you can find something to add to the if statement determining if the initial loading failed from the dataSource.