I have made an Item that has a Capability. I can use the capability without issues for the most part. The data persists on save/load world too. However, when I change the ItemStack (move it to another slot) when running dedicated server and client, the data just disappears. Here is the Item class:

public class LaptopItem extends Item {

    private static final ITextComponent containerName = new TranslationTextComponent("container.rubymod.laptop");

    public LaptopItem(Properties group) {
        super(group);
    }

    @Nullable
    @Override
    public ICapabilityProvider initCapabilities(ItemStack stack, @Nullable CompoundNBT nbt) {
        if (ComputerHolderProvider.CAPABILITY != null) {
            return new ComputerHolderProvider();
        }
        return super.initCapabilities(stack, nbt);
    }

    @Nullable
    @Override
    public CompoundNBT getShareTag(ItemStack stack) {
        CompoundNBT tag = new CompoundNBT();
        stack.getCapability(ComputerHolderProvider.CAPABILITY).ifPresent(cap -> {
            UUID computer = cap.getComputer();
            if (computer != null) {
                tag.putString("Computer", computer.toString());
            }
        });
        return tag;
    }

    @Override
    public void readShareTag(ItemStack stack, @Nullable CompoundNBT nbt) {
        stack.getCapability(ComputerHolderProvider.CAPABILITY).ifPresent(cap -> {
            if (nbt != null) {
                String uidString = nbt.getString("Computer");
                if (uidString.length() > 0) {
                    cap.setComputer(UUID.fromString(uidString));
                }
            }
        });
    }

    @Override
    public ActionResult<ItemStack> onItemRightClick(World worldIn, PlayerEntity player, Hand handIn) {
        if (worldIn.isRemote) {
            return ActionResult.resultSuccess(player.getHeldItem(handIn));
        } else {
            ItemStack stack = player.getHeldItem(handIn);
            stack.getCapability(ComputerHolderProvider.CAPABILITY).ifPresent(cap -> {
                UUID computer = cap.getComputer();
                INamedContainerProvider containerProvider = new SimpleNamedContainerProvider(
                        (windowId, inventory, _player) -> new ComputerContainer(windowId, inventory, computer),
                        containerName);
                NetworkHooks.openGui((ServerPlayerEntity) player, containerProvider,
                        packetBuffer -> packetBuffer.writeUniqueId(computer));
            });

            // TODO: add stats
            // player.addStat(Stats.INTERACT_WITH_LAPTOP);
            return ActionResult.resultSuccess(player.getHeldItem(handIn));
        }
    }

    @Override
    public void inventoryTick(ItemStack stack, World world, Entity entityIn, int itemSlot, boolean isSelected) {
        if (!world.isRemote) {
            stack.getCapability(ComputerHolderProvider.CAPABILITY).ifPresent(cap -> {
                world.getCapability(ComputerStorageProvider.CAPABILITY).ifPresent(cap2 -> {
                    Computer comp = cap2.getOrCreateComputer(cap.getComputer());
                    comp.clock();
                });
            });
        }
    }
}

I read in some other thread that syncing the data between client and server using getShareTag is supposed to fix this, however that didn't work for me. I suspect that something is wrong with the syncing code (it appears that the CompoundNBT in readShareTag is null.

How do I make sure Itemstack#getTag doesn't return null every time? How do I make my Item have a tag? I'm not sure what to use. Should I call getOrCreateTag somewhere?

Edited April 22, 20205 yr by deerangle
oopss

I think it's working now. Thanks for your help