I am making a server side mod in which I want to allow the player to open a chest (vanilla) and have several of the contents moving around. How do I make the contents update their location while the player has the chest GUI open?

Where do I change the slots?

I would use

// To do get the instance if the container that the player has open
player.openContainer
// Grab which block the player is looking at using MovingObjectPosition and the players look vectors.

If I recall correctly, the Container should have a reference to the Inventory, which should have a reference to the TileEntity in use, which should have a reference to the BlockPos.

 

But if that's not the case, then yes, you'd have to resort to a raytrace to fetch the current block, but that's not totally reliable in this case - what if another player or mob interposes itself between the player and the open chest while they are perusing, for example? Container will still be open, but now the MovingObjectPosition will probably contain the mob. Or a player might place blocks between them. Rare, certainly, but it may happen.

It does, it had a get method for the private field that I wan't aware of.