Block Entities

BlockEntities are like simplified Entities that are bound to a Block. They are used to store dynamic data, execute tick based tasks, and dynamic rendering. Some examples from vanilla Minecraft would be handling of inventories on chests, smelting logic on furnaces, or area effects on beacons. More advanced examples exist in mods, such as quarries, sorting machines, pipes, and displays.

note

BlockEntities aren't a solution for everything and they can cause lag when used wrongly. When possible, try to avoid them.

Registering

Block Entities are created and removed dynamically and as such are not registry objects on their own.

In order to create a BlockEntity, you need to extend the BlockEntity class. As such, another object is registered instead to easily create and refer to the type of the dynamic object. For a BlockEntity, these are known as BlockEntityTypes.

A BlockEntityType can be registered like any other registry object. To construct a BlockEntityType, its builder form can be used via BlockEntityType$Builder#of. This takes in two arguments: a BlockEntityType.BlockEntitySupplier which takes in a BlockPos and BlockState to create a new instance of the associated BlockEntity, and a varargs of Blocks which this BlockEntity can be attached to. Building the BlockEntityType is done by calling BlockEntityType$Builder#build. This takes in a Type which represents the type-safe reference used to refer to this registry object in a DataFixer. Since DataFixers are an optional system to use for mods, this can be passed as null.

// For some DeferredRegister<BlockEntityType<?>> REGISTER
public static final RegistryObject<BlockEntityType<MyBE>> MY_BE = REGISTER.register("mybe", () -> BlockEntityType.Builder.of(MyBE::new, validBlocks).build(null));

// In MyBE, a BlockEntity subclass
public MyBE(BlockPos pos, BlockState state) {
  super(MY_BE.get(), pos, state);
}

Creating a BlockEntity

To create a BlockEntity and attach it to a Block, the EntityBlock interface must be implemented on your Block subclass. The method EntityBlock#newBlockEntity(BlockPos, BlockState) must be implemented and return a new instance of your BlockEntity.

Storing Data within your BlockEntity

In order to save data, override the following two methods:

BlockEntity#saveAdditional(CompoundTag tag)

BlockEntity#load(CompoundTag tag)

These methods are called whenever the LevelChunk containing the BlockEntity gets loaded from/saved to a tag. Use them to read and write to the fields in your block entity class.

note

Whenever your data changes, you need to call BlockEntity#setChanged; otherwise, the LevelChunk containing your BlockEntity might be skipped while the level is saved.

danger

It is important that you call the super methods!

The tag names id, x, y, z, ForgeData and ForgeCaps are reserved by the super methods.

Ticking BlockEntities

If you need a ticking BlockEntity, for example to keep track of the progress during a smelting process, another method must be implemented and overridden within EntityBlock: EntityBlock#getTicker(Level, BlockState, BlockEntityType). This can implement different tickers depending on which logical side the user is on, or just implement one general ticker. In either case, a BlockEntityTicker must be returned. Since this is a functional interface, it can just take in a method representing the ticker instead:

// Inside some Block subclass
@Nullable
@Override
public <T extends BlockEntity> BlockEntityTicker<T> getTicker(Level level, BlockState state, BlockEntityType<T> type) {
  return type == MyBlockEntityTypes.MYBE.get() ? MyBlockEntity::tick : null;
}

// Inside MyBlockEntity
public static void tick(Level level, BlockPos pos, BlockState state, MyBlockEntity blockEntity) {
  // Do stuff
}

note

This method is called each tick; therefore, you should avoid having complicated calculations in here. If possible, you should make more complex calculations every X ticks. (The amount of ticks in a second may be lower then 20 (twenty) but won't be higher)

Synchronizing the Data to the Client

There are three ways of syncing data to the client: synchronizing on chunk load, on block updates, and with a custom network message.

Synchronizing on LevelChunk Load

For this you need to override

BlockEntity#getUpdateTag()

IForgeBlockEntity#handleUpdateTag(CompoundTag tag)

Again, this is pretty simple, the first method collects the data that should be sent to the client, while the second one processes that data. If your BlockEntity doesn't contain much data, you might be able to use the methods out of the Storing Data within your BlockEntity section.

caution

Synchronizing excessive/useless data for block entities can lead to network congestion. You should optimize your network usage by sending only the information the client needs when the client needs it. For instance, it is more often than not unnecessary to send the inventory of a block entity in the update tag, as this can be synchronized via its AbstractContainerMenu.

Synchronizing on Block Update

This method is a bit more complicated, but again you just need to override two or three methods. Here is a tiny example implementation of it:

@Override
public CompoundTag getUpdateTag() {
  CompoundTag tag = new CompoundTag();
  //Write your data into the tag
  return tag;
}

@Override
public Packet<ClientGamePacketListener> getUpdatePacket() {
  // Will get tag from #getUpdateTag
  return ClientboundBlockEntityDataPacket.create(this);
}

// Can override IForgeBlockEntity#onDataPacket. By default, this will defer to the #load.

The static constructors ClientboundBlockEntityDataPacket#create takes:

• The BlockEntity.
• An optional function to get the CompoundTag from the BlockEntity. By default, this uses BlockEntity#getUpdateTag.

Now, to send the packet, an update notification must be given on the server.

Level#sendBlockUpdated(BlockPos pos, BlockState oldState, BlockState newState, int flags)

The pos should be your BlockEntity's position. For oldState and newState, you can pass the current BlockState at that position. flags is a bitmask that should contain 2, which will sync the changes to the client. See Block for more info as well as the rest of the flags. The flag 2 is equivalent to Block#UPDATE_CLIENTS.

Synchronizing Using a Custom Network Message

This way of synchronizing is probably the most complicated but is usually the most optimized, as you can make sure that only the data you need to be synchronized is actually synchronized. You should first check out the Networking section and especially PayloadRegistrar before attempting this. Once you've created your custom network message, you can send it to all users that have the BlockEntity loaded with SimpleChannel#send(PacketDistributor$PacketTarget, MSG).

caution

It is important that you do safety checks, the BlockEntity might already be destroyed/replaced when the message arrives at the player! You should also check if the chunk is loaded (Level#hasChunkAt(BlockPos)).