Get entity attack target

I think it would be very useful to have a parameter that checks if an <event/source entity> is currently attacking anything/aggressive (true or false), which could be useful for particle effects or animations.
Also a selector for the entity the <event/source entity> is aggressive towards, so that it may be selected for (for example) potion effects or homing projectiles.

I've seen neutral mobs do this in code, but I don't understand exactly how it works just yet, so maybe you can turn it into a feature :D