How do I add a custom NBT tag to an item?

[peak aspen]

I basically want that when the custom item gets used on a mob (or an entity), that entity is copied in some form in the item, and when the item gets used again (maybe while crouching, as an idea), an identical copy of the entity is spawned. Is there any way to do this?

[misty helm]

In what version? From 1.21 onward, nbt tags are no longer encouraged

But yes, you can. You can save the entities type and it's custom nbt to a component of your making and then simply create a new entity of the same type, initialize it with the saved data and call world#spawnEntity

[peak aspen]

oops, sorry. I'm in 1.20.4. So ok, I can, is there any documentation about it? the one I found is for 1.21 and uses components (If they are the preferred practice, is there someplace where they get explained?)

[misty helm]

It looks like item components are later than you are by a version. https://fabricmc.net/2024/04/19/1205.html

[misty helm]

There probably won't be too much documentation on what you want, but it shouldn't be that hard.

[peak aspen]

do you have any heads up? (or better, do they work like blockState proeprties?

public static final IntProperty CHARGES = IntProperty.of("charges", 0, 8);

    @Override
    protected void appendProperties(StateManager.Builder<Item, ItemStack> builder) {
        builder.add(CHARGES);
    }

would something like this work?

[misty helm]

Are you planning on staying on 1.20.4?

[peak aspen]

Would you suggest I backport to 1.20/ update to 1.20.6?

[misty helm]

Not necessarily, it depends on what version you want the mod to be playable on.

[peak aspen]

I kinda wanted 1.20.4, but the Big Version is now 1.20.1 no?

[misty helm]

Idk. If you want 1.20.4, do 1.20.4

In that case though, you just need an item with useOnEntity and useOnBlock methods

[misty helm]

An because no components, pick a namespace key and write to the nbt of the item stack

[peak aspen]

OK, I understand this

[peak aspen]

this piece I'm missing. how do I write to the nbt?

[misty helm]

You will create a new nbt compound, use the writeNBT method on the entity to save its data, remove it using remove and save the compound to your item

[peak aspen]

and for creating the compound? I have found the class at least

[misty helm]

Maybe not remove if you want to copy instead of move

[misty helm]

Use the constructor

[peak aspen]

ok so a map of strings, which I guess are the nbt identifiers, and NbtElements, which are ?

[misty helm]

It should be a parameterless constructor

[peak aspen]

and I can just put whatever in it with WriteNBT?

[misty helm]

Write nbt is the method on entity to save that entities data to that compound

You can use the methods on the compound to save arbitrary namespaced data

[peak aspen]

oooh ok I'm going to try

ok I have NbtCompound.putWhatever()

[misty helm]

EntityType#loadEntityWithPassengers may be helpful to you as well

[peak aspen]

@Override
    public ActionResult useOnEntity(ItemStack stack, PlayerEntity player, LivingEntity entity, Hand hand) {
        if (!player.getWorld().isClient()) {
            // Get or create NBT for the item
            NbtCompound itemNbt = stack.getOrCreateNbt();

            // Save the entity data into the NBT
            NbtCompound entityData = new NbtCompound();
            entity.saveNbt(entityData);  // Serialize the entity to NBT

            // Save entity's class name to reconstruct the exact entity later
            entityData.putString("EntityType", EntityType.getId(entity.getType()).toString());

            // Store the entity's NBT data inside the item's NBT under the "StoredEntity" key
            itemNbt.put("StoredEntity", entityData);

            // Apply the updated NBT back to the item
            stack.setNbt(itemNbt);

            // Feedback to the player
            player.sendMessage(Text.of("Entity data stored"), true);

            return ActionResult.SUCCESS;
        }
        return ActionResult.PASS;
    }

I'm really not getting the hang of this... this doesn't store any nbt in the item

[peak aspen]

.

[misty helm]

Sorry, I had classes.

[misty helm]

It doesn't look like you're calling writeNBT

[misty helm]

It's in Entity

[peak aspen]

I'm calling setNbt though?

[misty helm]

Yes

But write nbt tells the entity to save its data to the nbt. Without write nbt, you are just saving an empty compound without the entity data

SetNBT just saves the nbt you have to the itemstack

[peak aspen]

sorry, what should I do then? I'm getting confused

entity.saveNbt(entityData); // Serialize the entity to NBT

so here I should use writeNbt instead?

[misty helm]

entity.writeNbt(compound)
Itemstack.setNbt(compound)

You have most of it right, you're just not quite there

[peak aspen]

if (!player.getWorld().isClient()) {
            // Get or create NBT for the item
            NbtCompound itemNbt = stack.getOrCreateNbt();

            // Save the entity data into the NBT
            NbtCompound entityData = new NbtCompound();

            //added this
            entity.writeNbt(entityData);
            entity.saveNbt(entityData);  // Serialize the entity to NBT

            // Save entity's class name to reconstruct the exact entity later
            entityData.putString("EntityType", EntityType.getId(entity.getType()).toString());

            // Store the entity's NBT data inside the item's NBT under the "StoredEntity" key
            itemNbt.put("StoredEntity", entityData);

            
            

            // Apply the updated NBT back to the item
            stack.setNbt(itemNbt);

            // Feedback to the player
            player.sendMessage(Text.of("Entity data stored"), true);
            
        }
        return ActionResult.SUCCESS;```
like this?

[peak aspen]

?

[misty helm]

The saveNbt call should not be nessesary, writeNbt does it if needed.

[misty helm]

You shouldn't need to save the entity type if you use this when recreating

But that looks good

[peak aspen]

Ok, so I think I get it in this case, but what do all those functions actually do? like write put save set? is it all in the docs?

[misty helm]

Most of it is not documented, unfortunately. You can learn these things by reading mc code

The fabric + yarn javadocs can also be a good resource

[peak aspen]

nope, still doesn't work...

@Override
    public ActionResult useOnEntity(ItemStack stack, PlayerEntity player, LivingEntity entity, Hand hand) {
        if (!player.getWorld().isClient()) {
            // Get or create NBT for the item
            NbtCompound itemNbt = stack.getOrCreateNbt();

            // Save the entity data into the NBT
            NbtCompound entityData = new NbtCompound();

            //added this
            entity.writeNbt(entityData);
            entity.saveNbt(entityData);  // Serialize the entity to NBT

            // Save entity's class name to reconstruct the exact entity later
            entityData.putString("EntityType", EntityType.getId(entity.getType()).toString());

            // Store the entity's NBT data inside the item's NBT under the "StoredEntity" key
            itemNbt.put("StoredEntity", entityData);

            // Apply the updated NBT back to the item
            stack.setNbt(itemNbt);

            // Feedback to the player
            player.sendMessage(Text.of("Entity data stored"), true);

        }
        return ActionResult.SUCCESS;
    }

    

reposting the full code for clarity

[peak aspen]

:(

[misty helm]

What's wrong?

[peak aspen]

doesn't store the nbt in the item :(

[misty helm]

You may need to set the item in the players hand, it's been a bug in Minecraft for a while

[peak aspen]

ok, it stores the nbt now (thanks), but now there's an issue with spawning the clone

[Server thread/WARN] (Minecraft) UUID of added entity already exists: CowEntity['Cow'/10, l='ServerLevel[New World]', x=-18.50, y=56.00, z=-25.50]

this is very funny I'm going to throw myself off a cliff

@Override
    public ActionResult useOnBlock(ItemUsageContext context) {
        ItemStack stack = context.getStack();
        PlayerEntity player = context.getPlayer();

        if (!context.getWorld().isClient()) {
            // Get the NBT from the item
            NbtCompound itemNbt = stack.getNbt();

            if (itemNbt != null && itemNbt.contains("StoredEntity")) {
                // Retrieve the stored entity data
                NbtCompound entityData = itemNbt.getCompound("StoredEntity");
                String entityTypeString = entityData.getString("EntityType");
                EntityType<?> entityType = Registries.ENTITY_TYPE.get(new Identifier(entityTypeString));
                if (entityType != null) {
                    Entity clonedEntity = entityType.create(context.getWorld());
                    if (clonedEntity != null) {
                        clonedEntity.readNbt(entityData);
                        clonedEntity.refreshPositionAndAngles(
                                context.getBlockPos().getX() + 0.5,
                                context.getBlockPos().getY() + 1,
                                context.getBlockPos().getZ() + 0.5,
                                player.getYaw(),
                                player.getPitch()
                        );
                        context.getWorld().spawnEntity(clonedEntity);
                    }
                } else {
                    player.sendMessage(Text.of("Failed to spawn entity"), true);
                }
            } else {
                player.sendMessage(Text.of("No entity stored"), true);
            }
        }
        return ActionResult.SUCCESS;
    }```
code

[misty helm]

You may wish to look into how the uuid is stored and where it comes from for new entities. It shouldn't be a problem to assign a new one to the clone

Are you using an llm

[peak aspen]

Well, I found a gpt for fabric, and since I'm new, I figured at least it should know about some things. Usually it gets a lot things wrong though :/

[misty helm]

Yeah, we have a warning tag cause they tend to be wrong more than right

Do you have strong java knowledge?

[peak aspen]

Also that is not really my strong suit, but as long as I'm working with things I know in fabric, I get around fine enough