"We don't want the back-end code to do UI things like display a progress bar, however, we do want to be able to output progress information during slow calculations."

"I think the right way to abstract it is to use the progress_trace class from weblib.php. We need to add a progress_bar_progress_trace subclass. If the slow calculations take a progress_trace (defaulting to a null_progress_trace), then the calling code can decide what sort of output is appropriate."