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tutorial:inventory

Storing items in a block as an Inventory

Make sure you've made a block entity before reading this tutorial.

The standard way to store items in a BlockEntity is to make it an Inventory. This allows hoppers (or other mods) to insert and extract items from your BlockEntity without any extra work.

Implementing Inventory

Inventory is just an interface, which means the actual ItemStack state will need to be stored on your BlockEntity. A DefaultedList<ItemStack> can be used as an easy way to store these ItemStacks, as it can be set to default to ItemStack.Empty, which is the proper way of saying that there is no item in a slot. Implementing Inventory is fairly simple, but is tedious and prone to error, so we'll use a default implementation of it which only requires giving it a DefaultList<ItemStack> (copy this as a new file):

ImplementedInventory.java
/**
 * A simple {@code Inventory} implementation with only default methods + an item list getter.
 *
 * Originally by Juuz
 */
public interface ImplementedInventory extends Inventory {
    /**
     * Gets the item list of this inventory.
     * Must return the same instance every time it's called.
     */
    DefaultedList<ItemStack> getItems();
    // Creation
    /**
     * Creates an inventory from the item list.
     */
    static ImplementedInventory of(DefaultedList<ItemStack> items) {
        return () -> items;
    }
    /**
     * Creates a new inventory with the size.
     */
    static ImplementedInventory ofSize(int size) {
        return of(DefaultedList.ofSize(size, ItemStack.EMPTY));
    }
    // Inventory
    /**
     * Returns the inventory size.
     */
    @Override
    default int getInvSize() {
        return getItems().size();
    }
    /**
     * @return true if this inventory has only empty stacks, false otherwise
     */
    @Override
    default boolean isInvEmpty() {
        for (int i = 0; i < getInvSize(); i++) {
            ItemStack stack = getInvStack(i);
            if (!stack.isEmpty()) {
                return false;
            }
        }
        return true;
    }
    /**
     * Gets the item in the slot.
     */
    @Override
    default ItemStack getInvStack(int slot) {
        return getItems().get(slot);
    }
    /**
     * Takes a stack of the size from the slot.
     * <p>(default implementation) If there are less items in the slot than what are requested,
     * takes all items in that slot.
     */
    @Override
    default ItemStack takeInvStack(int slot, int count) {
        ItemStack result = Inventories.splitStack(getItems(), slot, count);
        if (!result.isEmpty()) {
            markDirty();
        }
        return result;
    }
    /**
     * Removes the current stack in the {@code slot} and returns it.
     */
    @Override
    default ItemStack removeInvStack(int slot) {
        return Inventories.removeStack(getItems(), slot);
    }
    /**
     * Replaces the current stack in the {@code slot} with the provided stack.
     * <p>If the stack is too big for this inventory ({@link Inventory#getInvMaxStackAmount()}),
     * it gets resized to this inventory's maximum amount.
     */
    @Override
    default void setInvStack(int slot, ItemStack stack) {
        getItems().set(slot, stack);
        if (stack.getCount() > getInvMaxStackAmount()) {
            stack.setCount(getInvMaxStackAmount());
        }
    }
    /**
     * Clears {@linkplain #getItems() the item list}}.
     */
    @Override
    default void clear() {
        getItems().clear();
    }
    @Override
    default void markDirty() {
        // Override if you want behavior.
    }
    @Override
    default boolean canPlayerUseInv(PlayerEntity player) {
        return true;
    }
}

Now in your BlockEntity Implement ImplementedInventory, and provide it with an instance of DefaultedList<ItemStack> items that stores the items. For this example we'll store a maximum of 2 items in the inventory:

public class DemoBlockEntity extends BlockEntity implements ImplementedInventory {
    private final DefaultedList<ItemStack> items = DefaultedList.ofSize(2, ItemStack.EMPTY);
 
    @Override
    public DefaultedList<ItemStack> getItems() {
        return items;
    }
    [...]
 
}

We're also gonna need to save the inventories to tag and load it from there. Inventories has helper methods that makes this very easy:

public class DemoBlockEntity extends BlockEntity implements ImplementedInventory {
    [...]
    @Override
    public void fromTag(CompoundTag tag) {
        super.fromTag(tag);
        Inventories.fromTag(tag,items);
    }
 
    @Override
    public CompoundTag toTag(CompoundTag tag) {
        Inventories.toTag(tag,items);
        return super.toTag(tag);
    }
}

Extracting and inserting from your inventory (or any inventory)

In our block class, we'll override the `activate` behavior to insert and extract items from our inventory. Note that this can be done to any Inventory instance, not just our own (so you could do the same thing to a chest block, for example). First we'll handle inserting into the inventory. The player will insert the item he is holding if he is holding one. It'll go into the first slot if it is empty, or to the second slot if the first one is empty, or if the second is empty too we'll print some information about the inventory. Note that we call copy() when inserting the ItemStack into the inventory so it doesn't get destroyed alongside the player's ItemStack.

public class ExampleBlock extends Block implements BlockEntityProvider {
    [...]
    @Override
    public boolean activate(BlockState blockState, World world, BlockPos blockPos, PlayerEntity player, Hand hand, BlockHitResult blockHitResult) {
        if (world.isClient) return true;
        Inventory blockEntity = (Inventory) world.getBlockEntity(blockPos);
 
 
        if (!player.getStackInHand(hand).isEmpty()) {
            // Check what is the first open slot and put an item from the player's hand there
            if (blockEntity.getInvStack(0).isEmpty()) {
                // Put the stack the player is holding into the inventory
                blockEntity.setInvStack(0, player.getStackInHand(hand).copy());
                // Remove the stack from the player's hand
                player.getStackInHand(hand).setCount(0);
            } else if (blockEntity.getInvStack(1).isEmpty()) {
                blockEntity.setInvStack(1, player.getStackInHand(hand).copy());
                player.getStackInHand(hand).setCount(0);
            } else {
                // If the inventory is full we'll print it's contents
                System.out.println("The first slot holds "
                        + blockEntity.getInvStack(0) + " and the second slot holds " + blockEntity.getInvStack(1));
            }
        } 
        return true;
    }
}

We'll have the opposite behavior when the player is not holding an item. We'll take the item from the second slot, and then the first one of the second is empty. If the first is empty as well we won't do anything.

public class ExampleBlock extends Block implements BlockEntityProvider {
    [...]
    @Override
    public boolean activate(BlockState blockState, World world, BlockPos blockPos, PlayerEntity player, Hand hand, BlockHitResult blockHitResult) {
        ...
        if (!player.getStackInHand(hand).isEmpty()) {
            ...
        } else {
            // If the player is not holding anything we'll get give him the items in the block entity one by one
 
             // Find the first slot that has an item and give it to the player
            if (!blockEntity.getInvStack(1).isEmpty()) {
                // Give the player the stack in the inventory
                player.inventory.offerOrDrop(world, blockEntity.getInvStack(1));
                // Remove the stack from the inventory
                blockEntity.removeInvStack(1);
            } else if (!blockEntity.getInvStack(0).isEmpty()) {
                player.inventory.offerOrDrop(world, blockEntity.getInvStack(0));
                blockEntity.removeInvStack(0);
            }
        }
 
        return true;
    }
}

Implementing SidedInventory

If you want to have different logic based on what side things (hopper or other mods) interact with your block you need to implement SidedInventory. If say you wanted to make it so you cannot insert from the upper side of the block, you would do this:

public class DemoBlockEntity extends BlockEntity implements ImplementedInventory, SidedInventory {
    [...]
    @Override
    public int[] getInvAvailableSlots(Direction var1) {
        // Just return an array of all slots
        int[] result = new int[getItems().size()];
        for (int i = 0; i < result.length; i++) {
            result[i] = i;
        }
 
        return result;
    }
 
    @Override
    public boolean canInsertInvStack(int slot, ItemStack stack, Direction direction) {
        return direction != Direction.UP;
    }
 
    @Override
    public boolean canExtractInvStack(int slot, ItemStack stack, Direction direction) {
        return true;
    }
}
tutorial/inventory.txt · Last modified: 2019/08/21 20:24 by fudge