This article is part of a series on the Fabric Transfer API. Link to the home page of the series.

This tutorial aims to give an overview of the Fabric Item Transfer API. Please make sure that you read the various fluid tutorials before reading this one, since most concepts used in the Fluid Storage API apply to the Item Transfer API.

The core message is: The Item Transfer API is used exactly the same as the Fluid Transfer API, but with ItemVariant instead of FluidVariant.

Item variants represent the “type” of an item, i.e. the Minecraft `Item`, and also an optional NBT compound tag. They are immutable. The following examples are very similar to the FluidVariant ones, since they are the same concept. Reading them is recommended though since ItemVariant has a few nice functions to work with ItemStacks.

// Creating an item variant from an item, without an NBT tag.
ItemVariant ironVariant = ItemVariant.of(Items.IRON_INGOT);
ironVariant.getItem() // returns Items.IRON_INGOT
ironVariant.copyNbt() // returns a copy of the optional nbt tag, in this case null
// Creating an item variant from an item, with an NBT tag.
NbtCompound customTag = new NbtCompound();
customTag.putBoolean("magic", true);
ItemVariant magicIron = ItemVariant.of(Items.IRON_INGOT, customTag);
// Creating an item variant from an ItemStack
ItemStack specialStack = new ItemStack(Items.IRON_INGOT);
specialStack.setCustomName(new LiteralText("My Special Ingot"));
ItemVariant specialIngot = ItemVariant.of(specialStack);
// Further modifications to the ItemStack will not affect the ItemVariant since all the data is copied by of(...).
// Creating an item stack from an item variant
ItemStack size1 = specialIngot.toStack(); // no parameter -> count of the stack is 1
ItemStack anySize = specialIngot.toStack(32); // int parameter -> sets the count of the stack

Variants are always compared with .equals, NEVER WITH == !

ironVariant.equals(ironVariant); // returns true
ironVariant.equals(magicIron); // returns false
// You can easily test if a variant has some item:
ironVariant.isOf(Items.IRON_INGOT); // returns true
magicIron.isOf(Items.IRON_INGOT); // returns true
// The matches function can be used to compare an item variant with an item stack:
ironVariant.matches(specialStack); // returns false
specialIngot.matches(specialStack); // returns true

They can easily be serialized to and from NBT or network packets:

// NBT
NbtCompound compound = variant.toNbt();
ItemVariant variant = ItemVariant.fromNbt(compound);
// Network packets
variant.toPacket(buf);
ItemVariant variant = ItemVariant.fromPacket(buf);

Storage<ItemVariant> is the type used to represent item containers in the Transfer API. It is queried and exposed through the ItemStorage.SIDED lookup for access by other mods:

// Finding an instance
@Nullable
Storage<ItemVariant> maybeStorage = ItemStorage.SIDED.find(world, pos, side);
 
// To expose an instance, use one of the ItemStorage.SIDED.registerFor* functions

To learn how to use Storage, please refer to the Storage<FluidVariant> tutorial since the methods are exactly the same.

Sometimes, it is necessary to convert some Inventory to a Storage<ItemVariant>. For that purpose, InventoryStorage.of(inventory, side) can be used. InventoryStorage also allows recovering each slot separately through the getSlot and getSlots functions. The side @Nullable Direction parameter exists so that if a non-null side is specified and the inventory is a SidedInventory, canInsert and canExtract are checked before insertion and extraction respectively.

PlayerInventoryStorage serves a similar purpose, but for player inventories. Make sure to read its documentation, it contains very useful functions such as offerOrDrop.