I am looking for an event handler in SMAPI that indicates what item the player is currently holding. I basically want if(HeldItem == specific item && pressedKey == "Shift") { replace specific item with custom item }
No such event natively exists in SMAPI, EntoaroxFramework adds a event for when the held item changed, and you are welcome to re-use the code used to trigger that event if you dont want your mod to depend on EF.
EF passes both the previous and new item in its event, but if you are going to use the raw code you will need to copy the observer that triggers the event.