Simple AppleScript progress bar demo for Yosemite

progressBar.applescript

set progress description to "A simple progress indicator"
set progress additional description to "Preparing…"
set progress total steps to -1
 
delay 5
 
set progress total steps to 100
repeat with i from 1 to 100
	try
		set progress additional description to "I am on step " & i
		set progress completed steps to i
		delay 0.2
	on error thisErr
		display alert thisErr
		exit repeat
	end try
end repeat

@RobLewis, it looks from my experimentation that the -1 is what makes it show the progress indicator as intdeterminate. When running in the Script Editor, this makes it a spinny thingy (instead of a pie), and when it's an Application, it shows the progress bar filled solid when total steps is at -1 (instead of showing one at 0%). I think it's what you would do when you're doing some kind of startup work (especially when you don't know how long it'll take) before you enter your main processing loop.