Implementing DataManipulators
This is a guide for contributors who want to help with Data API implementation by creating DataManipulators. An updated list of DataManipulators to be implemented can be found at SpongeCommon Issue #8.
To fully implement a DataManipulator these steps must be followed:
Implement the
DataManipulator
itselfImplement the ImmutableDataManipulator
When these steps are complete, the following must also be done:
Register the Key in the
KeyRegistryModule
Implement the
DataProcessor
Implement the
ValueProcessor
for each Value being represented by theDataManipulator
If the data applies to a block, several methods must also be mixed in to the block.
Note
Make sure you follow our Contribution Guidelines.
The following snippet shows the imports/paths for some classes in SpongeCommon that you will need:
import org.spongepowered.common.data.DataProcessor;
import org.spongepowered.common.data.ValueProcessor;
import org.spongepowered.common.data.manipulator.immutable.entity.ImmutableSpongeHealthData;
import org.spongepowered.common.data.manipulator.mutable.common.AbstractData;
import org.spongepowered.common.data.manipulator.mutable.entity.SpongeHealthData;
import org.spongepowered.common.data.processor.common.AbstractEntityDataProcessor;
import org.spongepowered.common.util.Constants;
import org.spongepowered.common.data.util.NbtDataUtil;
import org.spongepowered.common.registry.type.data.KeyRegistryModule;
1. Implement the DataManipulator
The naming convention for DataManipulator
implementations is the name of the interface prefixed with “Sponge”.
So to implement the HealthData interface, we create a class named SpongeHealthData
in the appropriate package.
For implementing the DataManipulator
first have it extend an appropriate abstract class from the
org.spongepowered.common.data.manipulator.mutable.common
package. The most generic there is AbstractData
but there are also abstractions that reduce boilerplate code even more for some special cases like
DataManipulator
s only containing a single value.
public class SpongeHealthData extends AbstractData<HealthData, ImmutableHealthData> implements HealthData {
[...]
}
There are two type arguments to the AbstractData class. The first is the interface implemented by this class, the
second is the interface implemented by the corresponding ImmutableDataManipulator
.
The Constructor
In most cases while implementing an abstract DataManipulator
you need to have two constructors:
One without arguments (no-args) which calls the second constructor with “default” values
The second constructor that takes all the values it supports.
The second constructor must
make a call to the
AbstractData
constructor, passing the class reference for the implemented interface.make sure the values passed are valid
call the
registerGettersAndSetters()
method
import static com.google.common.base.Preconditions.checkArgument;
public class SpongeHealthData extends AbstractData<HealthData, ImmutableHealthData> implements HealthData {
private double health;
private double maxHealth;
public SpongeHealthData() {
this(20D, 20D);
}
public SpongeHealthData(double health, double maxHealth) {
super(HealthData.class);
checkArgument(maxHealth > 0);
this.health = health;
this.maxHealth = maxHealth;
registerGettersAndSetters();
}
[...]
}
Since we know that both current health and maximum health are bounded values, we need to make sure no values
outside of these bounds can be passed. To achieve this, we use guava’s Preconditions
of which we import the
required methods statically.
Note
Never use so-called magic values (arbitrary numbers, booleans etc.) in your code. Instead, locate the
DataConstants
class and use a fitting constant - or create one, if necessary.
Accessors defined by the Interface
The interface we implement specifies some methods to access Value objects. For HealthData
, those are
HealthData#health() and HealthData#maxHealth(). Every call to those should yield a new Value
.
public MutableBoundedValue<Double> health() {
return SpongeValueFactory.boundedBuilder(Keys.HEALTH)
.minimum(0)
.maximum(this.maxHealth)
.defaultValue(this.maxHealth)
.actualValue(this.health)
.build();
}
Tip
Since Double
is a Comparable
, we do not need to explicitly specify a comparator.
If no current value is specified, calling BaseValue#get() on the Value
returns the default value.
Copying and Serialization
The two methods DataManipulator#copy() and DataManipulator#asImmutable() are not much work to implement. For both you just need to return a mutable or an immutable data manipulator respectively, containing the same data as the current instance.
The method DataSerializable#toContainer() is used for serialization purposes. Use
DataContainer#createNew() as the result and apply to it the values stored within this instance.
A DataContainer is basically a map mapping DataQuerys to values. Since a Key always
contains a corresponding DataQuery
, just use those by passing the Key
directly.
public DataContainer toContainer() {
return super.toContainer()
.set(Keys.HEALTH, this.health)
.set(Keys.MAX_HEALTH, this.maxHealth);
}
registerGettersAndSetters()
A DataManipulator
also provides methods to get and set data using keys. The implementation for this is handled
by AbstractData
, but we must tell it which data it can access and how. Therefore, in the
registerGettersAndSetters()
method we need to do the following for each value:
register a Supplier to directly get the value
register a Consumer to directly set the value
register a
Supplier<Value>
to get the mutableValue
Supplier
and Consumer
are functional interfaces, so Java 8 Lambdas can be used.
private SpongeHealthData setCurrentHealthIfValid(double value) {
if (value >= 0 && value <= (double) Float.MAX_VALUE) {
this.health = value;
} else {
throw new IllegalArgumentException("Invalid value for current health");
}
return this;
}
private SpongeHealthData setMaximumHealthIfValid(double value) {
if (value >= 0 && value <= (double) Float.MAX_VALUE) {
this.maxHealth = value;
} else {
throw new IllegalArgumentException("Invalid value for maximum health");
}
return this;
}
private void registerGettersAndSetters() {
registerFieldGetter(Keys.HEALTH, () -> this.health);
registerFieldSetter(Keys.HEALTH, this::setCurrentHealthIfValid);
registerKeyValue(Keys.HEALTH, this::health);
registerFieldGetter(Keys.MAX_HEALTH, () -> this.maxHealth);
registerFieldSetter(Keys.MAX_HEALTH, this::setMaximumHealthIfValid);
registerKeyValue(Keys.MAX_HEALTH, this::maxHealth);
}
The Consumer
registered as field setter must perform the adequate checks to make sure the supplied value is valid.
This applies especially for DataHolders which won’t accept negative values. If a value is invalid, an
IllegalArgumentException
should be thrown.
Tip
The validity criteria for those setters are the same as for the respective Value
object, so you might delegate
the validity check to a call of this.health().set()
and just set this.health = value
if the first
line has not thrown an exception yet.
That’s it. The DataManipulator
should be done now.
2. Implement the ImmutableDataManipulator
Implementing the ImmutableDataManipulator is similar to implementing the mutable one.
The only differences are:
The class name is formed by prefixing the mutable
DataManipulator
s name withImmutableSponge
Inherit from
ImmutableAbstractData
insteadInstead of
registerGettersAndSetters()
, the method is calledregisterGetters()
When creating ImmutableDataHolder
s or ImmutableValue
s, check if it makes sense to use the
ImmutableDataCachingUtil
. For example, if you have WetData
which contains nothing more than a boolean, it
is more feasible to retain only two cached instances of ImmutableWetData
- one for each possible value. For
manipulators and values with many possible values (like SignData
) however, caching is proven to be too expensive.
Tip
You should declare the fields of an ImmutableDataManipulator
as final
in order to
prevent accidental changes.
3. Register the Key in the KeyRegistryModule
The next step is to register your Keys to the Keys. To do so, locate the
KeyRegistryModule
class and find the registerDefaults()
method.
There add a line to register (and create) your used keys.
import static org.spongepowered.api.data.DataQuery.of;
this.register("health", Key.builder()
.type(TypeTokens.BOUNDED_DOUBLE_VALUE_TOKEN)
.id("health")
.name("Health")
.query(of("Health"))
.build());
this.register("max_health", Key.builder()
.type(TypeTokens.BOUNDED_DOUBLE_VALUE_TOKEN)
.id("max_health")
.name("Max Health")
.query(of("MaxHealth"))
.build());
The register(Key)
method registers your Key
s for later use. The string used for the id should be the
corresponding constant name from the Keys
utility class in lowercase. The Key
itself is created by using the
Key.Builder provided by the Key#builder() method. You have to set a TypeToken
, an id
,
human readable name
and a DataQuery
.
The DataQuery
is used for serialization. It is created from the statically imported DataQuery.of()
method
accepting a string. This string should also be the constant name, stripped of underscores and capitalization changed to
upper camel case.
4. Implement the DataProcessors
Next up is the DataProcessor
. A DataProcessor
serves as a bridge between our DataManipulator
and
Minecraft’s objects. Whenever any data is requested from or offered to DataHolders
that exist in Vanilla
Minecraft, those calls end up being delegated to a DataProcessor
or a ValueProcessor
.
For your name, you should use the name of the DataManipulator
interface and append Processor
. Thus, for
HealthData
we create a HealthDataProcessor
.
In order to reduce boilerplate code, the DataProcessor
should inherit from the appropriate abstract class in
the org.spongepowered.common.data.processor.common
package. Since health can only be present on certain
entities, we can make use of the AbstractEntityDataProcessor
which is specifically targeted at Entities
based on net.minecraft.entity.Entity
. AbstractEntitySingleDataProcessor
would require less
implementation work, but cannot be used as HealthData
contains more than just one value.
public class HealthDataProcessor
extends AbstractEntityDataProcessor<EntityLivingBase, HealthData, ImmutableHealthData> {
public HealthDataProcessor() {
super(EntityLivingBase.class);
}
[...]
}
Depending on which abstraction you use, the methods you have to implement may differ greatly, depending on how much implementation work already could be done in the abstract class. Generally, the methods can be categorized.
Tip
It is possible to create multiple DataProcessor
s for the same data. If vastly different DataHolder
s
should be supported (for example both a TileEntity
and a matching ItemStack
), it may be beneficial to
create one processor for each type of DataHolder
in order to make full use of the provided abstractions.
Make sure you follow the package structure for items, tileentities and entities.
Validation Methods
Always return a boolean value. If any of the supports(target)
methods is called it should perform a general check if
the supplied target generally supports the kind of data handled by our DataProcessor
. Based on your level of
abstraction you might not have to implement it at all, if you have to just implement the most specific one, as the more
generic ones usually delegate to them.
For our HealthDataProcessor
supports()
is implemented by the AbstractEntityDataProcessor
. Per
default, it will return true if the supplied argument is an instance of the class specified when calling the
super()
constructor.
Instead, we are required to provide a doesDataExist()
method. Since the abstraction does not know how to
obtain the data, it leaves this function to be implemented. As the name says, the method should check if the data
already exists on the supported target. For the HealthDataProcessor
, this always returns true, since every
living entity always has health.
@Override
protected boolean doesDataExist(EntityLivingBase entity) {
return true;
}
Setter Methods
A setter method receives a DataHolder
of some sort and some data that should be applied to it, if possible.
The DataProcessor
interface defines a set()
method accepting a DataHolder
and a DataManipulator
which returns a DataTransactionResult
. Depending on the abstraction class used, some of the necessary
functionality might already be implemented.
In this case, the AbstractEntityDataProcessor
takes care of most of it and just requires a method to set
some values to return true
if it was successful and false
if it was not. All checks if the
DataHolder
supports the Data
is taken care of, the abstract class will just pass a Map mapping each
Key
from the DataManipulator
to its value and then construct a DataTransactionResult
depending on
whether the operation was successful or not.
@Override
protected boolean set(EntityLivingBase entity, Map<Key<?>, Object> keyValues) {
entity.getEntityAttribute(SharedMonsterAttributes.MAX_HEALTH)
.setBaseValue(((Double) keyValues.get(Keys.MAX_HEALTH)).floatValue());
float health = ((Double) keyValues.get(Keys.HEALTH)).floatValue();
entity.setHealth(health);
return true;
}
Tip
To understand DataTransactionResults, check the corresponding docs page and refer to the DataTransactionResult.Builder docs to create one.
Warning
Especially when working with ItemStacks it is likely that you will need to deal with
NBTTagCompound
s directly. Many NBT keys are already defined as constants in the NbtDataUtil
class.
If your required key is not there, you need to add it in order to avoid ‘magic values’ in the code.
Removal Method
The remove()
method attempts to remove data from the DataHolder
and returns a DataTransactionResult
.
Removal is not abstracted in any abstract DataProcessor
as the abstractions have no way of knowing if the data
is always present on a compatible DataHolder
(like WetData
or HealthData
) or if it may or may not be present
(like LoreData
). If the data is always present, remove()
must always fail. If it may or may not be present,
remove()
should remove it.
Since a living entity always has health, HealthData
is always present and removal therefore not supported.
Therefore we just return DataTransactionResult#failNoData().
@Override
public DataTransactionResult remove(DataHolder dataHolder) {
return DataTransactionResult.failNoData();
}
Getter Methods
Getter methods obtain data from a DataHolder
and return an optional DataManipulator
. The
DataProcessor
interface specifies the methods from()
and createFrom()
, the difference being that
from()
will return Optional.empty()
if the data holder is compatible, but currently does not contain the
data, while createFrom()
will provide a DataManipulator
holding default values in that case.
Again, AbstractEntityDataProcessor
will provide most of the implementation for this and only requires a
method to get the actual values present on the DataHolder
. This method is only called after supports()
and doesDataExist()
both returned true, which means it is run under the assumption that the data is present.
Warning
If the data may not always exist on the target DataHolder
, e.g. if the remove()
function may be successful
(see above), it is imperative that you implement the doesDataExist()
method so that it returns true
if the data is present and false
if it is not.
@Override
protected Map<Key<?>, ?> getValues(EntityLivingBase entity) {
final double health = entity.getHealth();
final double maxHealth = entity.getMaxHealth();
return ImmutableMap.of(Keys.HEALTH, health, Keys.MAX_HEALTH, maxHealth);
}
Filler Methods
A filler method is different from a getter method in that it receives a DataManipulator
to fill with values.
These values either come from a DataHolder
or have to be deserialized from a DataContainer
. The method
returns Optional.empty()
if the DataHolder
is incompatible.
AbstractEntityDataProcessor
already handles filling from DataHolders
by creating a DataManipulator
from the holder and then merging it with the supplied manipulator, but the DataContainer
deserialization it
cannot provide.
@Override
public Optional<HealthData> fill(DataContainer container, HealthData healthData) {
if (!container.contains(Keys.MAX_HEALTH.getQuery()) || !container.contains(Keys.HEALTH.getQuery())) {
return Optional.empty();
}
healthData.set(Keys.MAX_HEALTH, getData(container, Keys.MAX_HEALTH));
healthData.set(Keys.HEALTH, getData(container, Keys.HEALTH));
return Optional.of(healthData);
}
The fill()
method is to return an Optional
of the altered healthData
, if and only if all required data could
be obtained from the DataContainer
.
Other Methods
Depending on the abstract superclass used, some other methods may be required. For instance,
AbstractEntityDataProcessor
needs to create DataManipulator
instances in various points. It can’t do this
since it knows neither the implementation class nor the constructor to use. Therefore it utilizes an abstract
function that has to be provided by the final implementation. This does nothing more than create a
DataManipulator
with default data.
If you implemented your DataManipulator
as recommended, you can just use the no-args constructor.
@Override
protected HealthData createManipulator() {
return new SpongeHealthData();
}
5. Implement the ValueProcessors
Not only a DataManipulator
may be offered to a DataHolder
, but also a keyed Value
on its own.
Therefore, you need to provide at least one ValueProcessor
for every Key
present in your
DataManipulator
. A ValueProcessor
is named after the constant name of its Key
in the Keys
class
in a fashion similar to its DataQuery
. The constant name is stripped of underscores, used in upper camel case
and then suffixed with ValueProcessor
.
A ValueProcessor
should always inherit from AbstractSpongeValueProcessor
, which already will handle a
portion of the supports()
checks based on the type of the DataHolder
. For Keys.HEALTH
, we’ll create
and construct HealthValueProcessor
as follows.
public class HealthValueProcessor
extends AbstractSpongeValueProcessor<EntityLivingBase, Double, MutableBoundedValue<Double>> {
public HealthValueProcessor() {
super(EntityLivingBase.class, Keys.HEALTH);
}
[...]
}
Now the AbstractSpongeValueProcessor
will relieve us of the necessity to check if the value is supported.
It is assumed to be supported if the target ValueContainer
is of the type EntityLivingBase
.
Tip
For a more fine-grained control over what EntityLivingBase
objects are supported, the
supports(EntityLivingBase)
method can be overridden.
Again, most work is done by the abstraction class. We just need to implement two helper methods for creating
a Value
and its immutable counterpart and three methods to get, set and remove data.
@Override
protected MutableBoundedValue<Double> constructValue(Double health) {
return SpongeValueFactory.boundedBuilder(Keys.HEALTH)
.minimum(0D)
.maximum(((Float) Float.MAX_VALUE).doubleValue())
.defaultValue(20D)
.actualValue(health)
.build();
}
@Override
protected ImmutableBoundedValue<Double> constructImmutableValue(Double value) {
return constructValue(value).asImmutable();
}
@Override
protected Optional<Double> getVal(EntityLivingBase container) {
return Optional.of((double) container.getHealth());
}
Since it is impossible for an EntityLivingBase
to not have health, this method will never return
Optional.empty()
.
@Override
protected boolean set(EntityLivingBase container, Double value) {
if (value >= 0 && value <= (double) Float.MAX_VALUE) {
container.setHealth(value.floatValue());
return true;
}
return false;
}
The set()
method will return a boolean value indicating whether the value could successfully be set.
This implementation will reject values outside of the bounds used in our value construction methods above.
@Override
public DataTransactionResult removeFrom(ValueContainer<?> container) {
return DataTransactionResult.failNoData();
}
Since the data is guaranteed to be always present, attempts to remove it will just fail.
6. Register Processors
In order for Sponge to be able to use our manipulators and processors, we need to register them. This is done in the
DataRegistrar
class. In the setupSerialization()
method there are two large blocks of registrations to which we
add our processors.
DataProcessors
A DataProcessor
is registered alongside the interface and implementation classes of the DataManipulator
it
handles. For every pair of mutable / immutable DataManipulator
s at least one DataProcessor
must be registered.
DataUtil.registerDataProcessorAndImpl(HealthData.class, SpongeHealthData.class,
ImmutableHealthData.class, ImmutableSpongeHealthData.class,
new HealthDataProcessor());
ValueProcessors
Value processors are registered at the bottom of the very same function. For each Key
multiple processors
can be registered by subsequent calls of the registerValueProcessor()
method.
DataUtil.registerValueProcessor(Keys.HEALTH, new HealthValueProcessor());
DataUtil.registerValueProcessor(Keys.MAX_HEALTH, new MaxHealthValueProcessor());
Implementing Block Data
Block data is somewhat different from other types of data in that it is implemented by mixing in to the block itself.
There are several methods in org.spongepowered.mixin.core.block.MixinBlock
that must be overridden to implement
data for blocks.
@Mixin(BlockHorizontal.class)
public abstract class MixinBlockHorizontal extends MixinBlock {
[...]
}
supports()
should return true
if either the ImmutableDataManipulator
interface is assignable from the
Class
passed in as the argument, or the superclass supports it.
@Override
public boolean supports(Class<? extends ImmutableDataManipulator<?, ?>> immutable) {
return super.supports(immutable) || ImmutableDirectionalData.class.isAssignableFrom(immutable);
}
getStateWithData()
should return a new BlockState
with the data from the ImmutableDataManipulator
applied
to it. If the manipulator is not directly supported, the method should delegate to the superclass.
@Override
public Optional<BlockState> getStateWithData(IBlockState blockState, ImmutableDataManipulator<?, ?> manipulator) {
if (manipulator instanceof ImmutableDirectionalData) {
final Direction direction = ((ImmutableDirectionalData) manipulator).direction().get();
final EnumFacing facing = DirectionResolver.getFor(direction);
return Optional.of((BlockState) blockState.withProperty(BlockHorizontal.FACING, facing));
}
return super.getStateWithData(blockState, manipulator);
}
getStateWithValue()
is the equivalent of getStateWithData()
, but works with single Key
s.
@Override
public <E> Optional<BlockState> getStateWithValue(IBlockState blockState, Key<? extends BaseValue<E>> key, E value) {
if (key.equals(Keys.DIRECTION)) {
final Direction direction = (Direction) value;
final EnumFacing facing = DirectionResolver.getFor(direction);
return Optional.of((BlockState) blockState.withProperty(BlockHorizontal.FACING, facing));
}
return super.getStateWithValue(blockState, key, value);
}
Finally, getManipulators()
should return a list of all ImmutableDataManipulator
s the block supports, along with
the current values for the provided IBlockState
. It should include all ImmutableDataManipulator
s from the
superclass.
@Override
public List<ImmutableDataManipulator<?, ?>> getManipulators(IBlockState blockState) {
return ImmutableList.<ImmutableDataManipulator<?, ?>>builder()
.addAll(super.getManipulators(blockState))
.add(new ImmutableSpongeDirectionalData(DirectionResolver.getFor(blockState.getValue(BlockHorizontal.FACING))))
.build();
}
Further Information
With Data
being a rather abstract concept in Sponge, it is hard to give general directions on how to
acquire the needed data from the Minecraft classes itself. It may be helpful to take a look at already
implemented processors similar to the one you are working on to get a better understanding of how it should work.
If you are stuck or are unsure about certain aspects, go visit the #spongedev
IRC channel,
the #dev
channel on Discord, the forums, or open up an Issue on GitHub.
Be sure to check the Data Processor Implementation Checklist
for general contribution requirements.