Suggestion for a feature to living entities "If player looked at mob" and "Mob looked at other entity"

You know how enderman get mad if you look at em, right? It'd be awesome if there was a feature where if you look at them you can add a procedure. This would be really helpful for alot of mods, and an extra feature for if a mob looks at you, you can add a procedure. There's probably something like that in already but that'd also be cool and helpful. Thanks!