When the controller needs new data, control flows from the top down to the bottom (via the “load:more:” arrow/message).
When the raw response is received from the server, the data flows from the bottom up to the top.
SearchResultsModel acts as an abstraction of several different image search providers (in this case, Yahoo and Flickr).
SearchResult represents an individual search result, independent of the search provider.
TTTableItem represents the SearchResult in a manner suitable for use with TTTableViewController
PhotoItem represents the SearchResult in a manner suitable for use with TTPhotoViewController and TTThumbsViewController
URLModelResponse (not shown) is an abstraction of the HTTP response parser so that the SearchResultsModel implementation does not depend on the type of response format (e.g. JSON or XML). FlickrJSONResponse, et al are subclasses of URLModelResponse.