Ability to be notified about copy progress

[eblis]

When uploading large files to WebDav servers it would be nice if we could have progress callbacks so we can be notified about the progress.

I think this could be achieved if the developer is able to specify a modified version of the RequestBody class.
An idea would be to be able to specify a RequestBodyFactory class or let the user call the private methods that take the RequestBody param(s).
Any other ideas welcome :)