Progress indicators that moving perfectly and steadily are probably time based, but I'd have thought they've always been work based. Example: you know you have to copy 20GB across the network; update the progress bar with 'percent of data complete' periodically (on a timer, in the byte copying loop, whatever.)
I've seen dual progress bars: one for "overall progress" (each one of five tasks fills this bar 20%) and one for the current task (maybe downloading, maybe unpacking, maybe copying, maybe waiting for hardware to reply...)
I've written progress bars that combine all of the above. Each sub-task had a weight though, and adaptive interpolation was used to animate the progress bar between chunky updates from less granular tasks based on how far they usually increment over which time interval.
I've seen dual progress bars: one for "overall progress" (each one of five tasks fills this bar 20%) and one for the current task (maybe downloading, maybe unpacking, maybe copying, maybe waiting for hardware to reply...)