public class BoxScreenHandler extends ScreenHandler {
    //We save the blockPos we got from the Server and provide a getter for it so the BoxScreen can read that information
    private BlockPos pos;
    private final Inventory inventory;

    //This constructor gets called on the client when the server wants it to open the screenHandler,
    //The client will call the super constructor with an empty Inventory and the screenHandler will automatically
    //sync this empty inventory with the inventory on the server

    //NEW: The constructor of the client now gets the PacketByteBuf we filled in the BlockEntity
    public BoxScreenHandler(int syncId, PlayerInventory playerInventory, PacketByteBuf buf) {
        this(syncId, playerInventory, new SimpleInventory(9));
        pos = buf.readBlockPos();
    }

    //This constructor gets called from the BlockEntity on the server, the server knows the inventory of the container
    //and can therefore directly provide it as an argument. This inventory will then be synced to the Client
    public BoxScreenHandler(int syncId, PlayerInventory playerInventory, Inventory inventory) {
        //[...]
        // See first Screenhandler Tutorial for the rest of the code
        
        //Why do we use BlockPos.ORIGIN here?
        //This is because the packetByteBuf with our blockPosition is only availible on the Client, so we need a placeholder
        //value here. This is not a problem however, as the Server version of the ScreenHandler does not really need this
        //information.
        pos = BlockPos.ORIGIN;

        [...]
    }

    //this getter will be used by our Screen class
    public BlockPos getPos() {
        return pos;
    }

    @Override
    public boolean canUse(PlayerEntity player) {
        return this.inventory.canPlayerUse(player);
    }

    // See Screenhandler Tutorial
    // Shift + Player Inv Slot
    @Override
    public ItemStack transferSlot(PlayerEntity player, int invSlot);
}