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.

A word of caution: To be able to support transactions, ItemStorage.of assumes that each slot in the inventory is independent of other slots, and of other inventories. Do NOT use it if that is not the case for your inventory.

This section describes various ways to create and customize Storage<ItemVariant> instances.

The helpers described on the fluid implementation page can be used just fine for item transfer. However, screen handler interactions make heavy use of mutable ItemStacks. So if you are implementing an item-containing block with GUI slots, you should avoid SingleVariantStorage and look at the other solutions below instead.

FilteringStorage and CombinedStorage might still be useful to you.

A nice way to obtain a Storage<ItemVariant> instance is to create a SimpleInventory, and use InventoryStorage.of to create a wrapper around it.

Example:

public class MyBlockEntity extends BlockEntity {
	// ... [other code]
 
	// This is the internal inventory, it can be used directly, or to create Slots, etc...
	private final SimpleInventory inventory = new SimpleInventory(3) { // 3 here is the slot count
		@Override
		public int getMaxCountPerStack() {
			return 4; // Optional: customize the maximum count in each slot.
		}
 
		@Override
		public boolean isValid(int slot, ItemStack stack) {
			// Optional: restrict which stacks are allowed in which slot.
			if (slot == 1) {
				return stack.isOf(Items.COAL);
			}
			return true;
		}
 
		@Override
		public void markDirty() {
			// Don't forget to schedule the block entity for saving after a modification!
			MyBlockEntity.this.markDirty();
		}
	};
	// It's better for performance to store the result of InventoryStorage.of(...), since it can be a bit expensive.
	// It may also be nicer to use than the SimpleInventory in some cases.
	public final InventoryStorage inventoryWrapper = InventoryStorage.of(inventory, null);
 
	// Don't forget to implement fromNbt/toNbt.
 
	// Don't forget to expose inventoryWrapper through ItemStorage.SIDED.
 
	// [...] Other code omitted
}

This is a powerful implementation technique due to the low amount of code that is required, and it can be taken even further:

• Multiple SimpleInventory's can be created, for example if the block has a “main inventory” with a stack limit of 64, and an “upgrade inventory” with a stack limit of 1. They can be combined into one Storage<ItemVariant> with CombinedStorage.
• (Advanced) You can have your SimpleInventory subclass implement SidedInventory, and wrappers queried with a side via InventoryStorage.of(sidedSimpleInv, side) will check canInsert and canExtract.

Another (more complicated) option is to store a stack or a list of stacks somewhere, and also a SingleStackStorage. Please have a look at the javadoc if you wish to go down this path.

For compatibility with existing vanilla inventories, the Transfer API will automatically wrap any block entity implementing Inventory or its subinterface SidedInventory as a Storage<ItemVariant>. It will also wrap SidedInventory's returned by blocks implementing InventoryProvider if they return the same inventory multiple times. This means that implementing Inventory/SidedInventory on your block entity class will correctly allow other mods to use it. However, this approach is discouraged for the following reasons:

• Performance: InventoryStorage.of is called every time, and it can be an expensive function to call.
• Code size: this will require more code than using SimpleInventory as described above.
• Flexibility: this is less flexible than SimpleInventory.
• Correctness: correctly implementing Inventory is harder than reusing the implementation that already exists in Minecraft itself.