Is a view list generated with each step for each mob, or is it only calculated when the code requests one?

I assume that a master list gets updated each time any mob moves, and that the contents of my view are taken out of that.

I am interested in doing my own collision detection, and would like to know a bit more about view, if anyone would be so kind as to inform me.

Im pretty sure that its calculated when the code calls it.
But Im not expert, I just use the procs, I dont write them, well do write them, but not the built in ones, unless I want a custom version of that proc, but that isnt often... My head hurts.