[1.8] Affecting behavior when Items are picked up or sat down in Inventory

I may be going mad, but I swear when I first dipped my toe into modding, somewhere back in the not-easily-remembered past, that there were functions in Item for changing behavior on an item being picked up or set down in an inventory, but now I'm not seeing anything even remotely similar.

Does anyone know how you could achieve this in 1.8 or 1.7.10?  Say like you wanted to have two items that could be combines by dropping one on the other in the inventory.