Core Interfaces >=1.21.1 API changed in 26.1

BREAKING

In version 26.1, the IMoveableEntityBlock API has been completely redesigned from NBT-based clearData/setData to data-driven storeData/loadData. Choose the appropriate API based on your target version.

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Version Selection

Minecraft	API	Data Carrier	Documentation Section
1.21.1	clearData / setData	CompoundTag	1.21.1 API
26.1	storeData / loadData	ValueInput / ValueOutput	26.1 API current

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1.21.1 API: clearData / setData

1.21.1

IMoveableEntityBlock (1.21.1)

Interface that blocks must implement, extending EntityBlock.

public interface IMoveableEntityBlock extends EntityBlock {
    /**
     * Called when the block is about to be pushed. Returns the block entity data
     * that needs to be passed to the movement state.
     * @param level The world
     * @param pos   The current block position (before movement)
     * @return The NBT data to preserve; returns an empty CompoundTag by default
     */
    default CompoundTag clearData(Level level, BlockPos pos) {
        return new CompoundTag();
    }

    /**
     * Called when movement ends or the block entity is finally placed, writing
     * the previously extracted data to the destination position.
     * @param level The world
     * @param pos   The destination position
     * @param nbt   The data returned by clearData
     */
    default void setData(Level level, BlockPos pos, CompoundTag nbt) {
    }
}

clearData Implementation

Called when the block is about to be pushed by a piston. Should extract only the custom data from the block entity that needs to be preserved (rather than serializing the entire block entity), returning the NBT to be transferred:

@Override
public CompoundTag clearData(Level level, BlockPos pos) {
    BlockEntity be = level.getBlockEntity(pos);
    if (be instanceof MyBlockEntity myBE) {
        CompoundTag tag = new CompoundTag();
        tag.putInt("customValue", myBE.customValue);
        myBE.saveAdditional(tag, level.registryAccess());
        return tag;
    }
    return new CompoundTag();
}

setData Implementation

Called when movement ends and the block entity is created at the new position:

@Override
public void setData(Level level, BlockPos pos, CompoundTag nbt) {
    BlockEntity be = level.getBlockEntity(pos);
    if (be instanceof MyBlockEntity myBE) {
        myBE.customValue = nbt.getInt("customValue");
        myBE.loadAdditional(nbt, level.registryAccess());
        myBE.setChanged();
    }
}

Data Flow (1.21.1)

Movement start:
  BlockEntity (source position)
    → IMoveableEntityBlock.clearData() → CompoundTag
    → PistonBaseBlockMixin stores into anvillib$nbt (CompoundTag)
    → IPistonMovingBlockEntityExtension.setData(tag)
    → Temporarily stored in PistonMovingBlockEntity

Movement end:
  PistonMovingBlockEntity
    → IPistonMovingBlockEntityExtension.clearData() → CompoundTag
    → IMoveableEntityBlock.setData(pos, tag)
    → Written to the BlockEntity at the destination

IPistonMovingBlockEntityExtension (1.21.1)

public interface IPistonMovingBlockEntityExtension {
    @Nullable CompoundTag anvillib$clearData();
    void anvillib$setData(@Nullable CompoundTag nbt);
    @Nullable BlockState anvillib$getMoveState();
}

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26.1 API: storeData / loadData

26.1

In version 26.1, the API has migrated from manual NBT operations to NeoForge's native ValueInput/ValueOutput data-driven system.

IMoveableEntityBlock (26.1)

public interface IMoveableEntityBlock extends EntityBlock {
    /**
     * Called when the block is about to be pushed. Writes the block entity data
     * that needs to be preserved to the ValueOutput.
     * @param level   The world
     * @param pos     The current block position (before movement)
     * @param output  Data output (write data via ValueOutput instead of returning CompoundTag)
     */
    default void storeData(Level level, BlockPos pos, ValueOutput output) {
    }

    /**
     * Called when movement ends or the block entity is finally placed. Reads
     * previously saved data from the ValueInput.
     * @param level The world
     * @param pos   The destination position
     * @param input Data input (read data via ValueInput instead of receiving CompoundTag)
     */
    default void loadData(Level level, BlockPos pos, ValueInput input) {
    }
}

storeData Implementation

@Override
public void storeData(Level level, BlockPos pos, ValueOutput output) {
    BlockEntity be = level.getBlockEntity(pos);
    if (be instanceof MyBlockEntity myBE) {
        // Write data through ValueOutput acceptor methods
        // The specific implementation depends on NeoForge 26.1's ValueOutput API
    }
}

loadData Implementation

@Override
public void loadData(Level level, BlockPos pos, ValueInput input) {
    BlockEntity be = level.getBlockEntity(pos);
    if (be instanceof MyBlockEntity myBE) {
        // Read data via ValueInput and restore to the block entity
    }
}

Data Flow (26.1)

Movement start:
  BlockEntity (source position)
    → IMoveableEntityBlock.storeData(level, pos, output)
    → output writes to TagValueOutput
    → PistonBaseBlockMixin calls anvillib$nbt.buildResult()
    → Temporarily stored in PistonMovingBlockEntity

Movement end:
  PistonMovingBlockEntity
    → IPistonMovingBlockEntityExtension.clearData()
    → IMoveableEntityBlock.loadData(level, pos, input)
    → input reads data from TagValueInput
    → Restores the BlockEntity at the destination

Key Differences

Aspect	1.21.1	26.1
Data carrier	CompoundTag (manual NBT)	ValueInput / ValueOutput (data-driven)
Return style	clearData() → CompoundTag	storeData(..., ValueOutput) (write via output parameter)
Mixin temp field	CompoundTag anvillib$nbt	TagValueOutput anvillib$nbt
Error handling	No specific error reporting	Uses ProblemReporter.ScopedCollector
Save completeness	Must manually select fields to save	Can auto-serialize via saveWithFullMetadata

IPistonMovingBlockEntityExtension (26.1)

The interface signature is consistent with 1.21.1 (clearData/setData method names are preserved), but internal data types have changed from CompoundTag to the data-driven TagValueOutput:

public interface IPistonMovingBlockEntityExtension {
    // Method names unchanged; internal implementation uses TagValueOutput
    @Nullable TagValueOutput anvillib$clearData();
    void anvillib$setData(@Nullable TagValueOutput nbt);
    @Nullable BlockState anvillib$getMoveState();
}

Mod developers do not need to use this interface directly -- the Mixin layer handles data injection and extraction automatically.

AnvilLibMoveableEntityBlock

26.1

New helper class added in 26.1:

public class AnvilLibMoveableEntityBlock {
    public static final Logger LOGGER = ...;
}

Used by PistonBaseBlockMixin to provide unified logging.

Migration Checklist (1.21.1 → 26.1)

1. Replace clearData(Level, BlockPos) → CompoundTag with storeData(Level, BlockPos, ValueOutput)
2. Replace setData(Level, BlockPos, CompoundTag) with loadData(Level, BlockPos, ValueInput)
3. Remove manual NBT operations (CompoundTag.putInt(), etc.) and use the data-driven ValueOutput/ValueInput API instead
4. Check for compilation errors: CompoundTag is no longer used as a return/parameter type
5. If custom Mixins reference the anvillib$nbt field, update the field type