Can i redraw the UI inside an operator? I have a script which is working. At the end of the script, rendering is started. This is supposed to work as a template for people possibly unfamiliar with a freezing screen (blender is loading lots of data into cache from images used in the compositor). 
For that reason i want an "Info" in the top bar saying something like "This might take a while. Now rendering...".
So i would have this in my operator


draw message
redraw UI (to show message)
Run the rest of the script


Is this possible?
use report() in your operator to display reports on the top bar

self.report({'INFO'},"This ma take some time")
# rest of the code here