Mobserve adds one very simple thing - new advancement trigger for when the player is looking at a certain mob.

The trigger for it is "mobserve:observe_entity", and it's condition parameters are:

entity (an entity predicate seen in kill an entity or similar triggers)
distance (the maximal distance a player's eye can be away from the hitbox of the entity) - OPTIONAL, default value is 32.

A little bit of technical details:

The mod itself considers the "observing" trigger true only when the bounding box of an entity intersects with player's look.

The point of intersection is then taken to check for the distance between eye (camera) position.

Permissions:

You can use this mod in your modpack.
You are not allowed to repost this mod to other sites without providing a CurseForge download link and my permission ( Zeitheron on Discord)
You are not allowed to include this (or any other mods of DragonForge team) in your own mod. Building addons, however, is allowed, but a friendly PM would be good.