How to programmatically change layout?

I'd like, in my code, to be able to trigger a layout instead of doing it in the UI.  This gives the flexibility to change layouts based on dynamic data (which isn't always usable in the UI). 

This would be the layouts equivalent of a page template suggestion in Drupal 7, I suppose.

Is there a preprocess function or something else I can use to change the layout that's used?