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:

1. Implement the DataManipulator itself

2. Implement the ImmutableDataManipulator

When these steps are complete, the following must also be done:

3. Register the Key in the KeyRegistry

4. Implement the DataProcessor

5. Implement the ValueProcessor for each value being represented by the DataManipulator

6. Register everything in the SpongeSerializationRegistry

Bemærk

Make sure you follow our Si us plau, no envieu sol·licitud d’extracció de petits canvis menors de 20 línies. En lloc d’això, entri a #spongedev al IRC (irc.esper.net) <TODO> _ i el canviarem juntament amb els altres canvis menors..

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 DataManipulators 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 Manipulator you want 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 {

    public SpongeHealthData() {
        this(DataConstants.DEFAULT_HEALTH, DataConstants.DEFAULT_HEALTH);
    }

    public SpongeHealthData(double currentHealth, double maxHealth) {
        super(HealthData.class);
        checkArgument(currentHealth >= DataConstants.MINIMUM_HEALTH && currentHealth <= (double) Float.MAX_VALUE);
        checkArgument(maxHealth >= DataConstants.MINIMUM_HEALTH && maxHealth <= (double) Float.MAX_VALUE);
        this.currentHealth = currentHealth;
        this.maximumHealth = maxHealth;
        this.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.

Bemærk

Never use so-called magic values (arbitrary numbers, booleans etc) in your code. Instead, locate the org.spongepowered.common.data.util.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 health() and maxHealth(). Every call to those should yield a new Value.

public MutableBoundedValue<Double> health() {
    return SpongeValueFactory.boundedBuilder(Keys.HEALTH)
        .minimum(DataConstants.MINIMUM_HEALTH)
        .maximum(this.maximumHealth)
        .defaultValue(this.maximumHealth)
        .actualValue(this.currentHealth)
        .build();
}

Tip

Since Double is a Comparable, we do not need to explicitly specify a comparator.

If no current value is specified, calling get() on the Value returns the default value.

Copying and Serialization

The two methods copy() and 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 toContainer() is used for serialization purposes. Use a MemoryDataContainer 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 new MemoryDataContainer()
        .set(Keys.HEALTH, this.currentHealth)
        .set(Keys.MAX_HEALTH, this.maximumHealth);
}

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 mutable Value

Supplier and Consumer are functional interfaces, so Java 8 Lambdas can be used.

private void setCurrentHealthIfValid(double value) {
    if (value >= DataConstants.MINIMUM_HEALTH && value <= (double) Float.MAX_VALUE) {
        this.currentHealth = value;
    } else {
        throw new IllegalArgumentException("Invalid value for current health");
    }
}

private void setMaximumHealthIfValid(double value) {
    if (value >= DataConstants.MINIMUM_HEALTH && value <= (double) Float.MAX_VALUE) {
        this.maximumHealth = value;
    } else {
        throw new IllegalArgumentException("Invalid value for maximum health");
    }

}

private void registerGettersAndSetters() {
    registerFieldGetter(Keys.HEALTH, () -> SpongeHealthData.this.currentHealth);
    registerFieldSetter(Keys.HEALTH, SpongeHealthData.this::setCurrentHealthIfValid);
    registerKeyValue(Keys.HEALTH, SpongeHealthData.this::health);

    registerFieldGetter(Keys.MAX_HEALTH, () -> SpongeHealthData.this.maximumHealth);
    registerFieldSetter(Keys.MAX_HEALTH, SpongeHealthData.this::setMaximumHealthIfValid);
    registerKeyValue(Keys.MAX_HEALTH, SpongeHealthData.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 DataHolder``s 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.currentHealth = value if the first line has no 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 DataManipulators name with ImmutableSponge

• Inherit from ImmutableAbstractData instead

• Instead of registerGettersAndSetters(), the method is called registerGetters()

When creating ImmutableDataHolders or ImmutableValues, 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 KeyRegistry

The next step is to register your Keys to the KeyRegistry. To do so, locate the org.spongepowered.common.data.key.KeyRegistry class and find the static generateKeyMap() function. There add a line to register (and create) your used keys.

keyMap.put("health"), makeSingleKey(Double.class, MutableBoundedValue.class, of("Health")));
keyMap.put("max_health", makeSingleKey(Double.class, MutableBoundedValue.class, of("MaxHealth")));

The keyMap maps strings to Keys. The string used should be the corresponding constant name from the Keys utility class in lowercase. The Key itself is created by one of the static methods provided by KeyFactory, in most cases makeSingleKey. makeSingleKey requires first a class reference for the underlying data, which in our case is a »Double«, then a class reference for the Value type used. The third argument is the DataQuery 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 DataProcessors for the same data. If vastly different DataHolders 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 the method is called supports() it should perform a general check if the supplied target generally supports the kind of data handled by our DataProcessor.

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.

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.

protected boolean set(EntityLivingBase entity, Map<Key<?>, Object> keyValues) {
    entity.getEntityAttribute(SharedMonsterAttributes.maxHealth)
        .setBaseValue(((Double) keyValues.get(Keys.MAX_HEALTH)).floatValue());
    entity.setHealth(((Double) keyValues.get(Keys.HEALTH)).floatValue());
    return true;
}

Tip

To understand DataTransactionResult s, check the corresponding docs page and refer to the DataTransactionResult.Builder docs to create one.

Advarsel

Especially when working with ItemStacks it is likely that you will need to deal with NBTTagCompounds directly. Many NBT keys are already defined as constants in the org.spongepowered.common.data.util.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. In such cases the doesDataExist() method should be overridden.

Since a living entity always has health, HealthData is always present and removal therefore not supported. Therefore we just return failNoData() and do not override the doesDataExist() method.

public DataTransactionResult remove(DataHolder dataHolder) {
    return DataTransactionBuilder.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.

Advarsel

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 override the doesDataExist() method so that it returns true if the data is present and false if it is not.

protected Map<Key<?>, ?> getValues(EntityLivingBase entity) {
    final double health = entity.getHealth();
    final double maxHealth = entity.getMaxHealth();
    return ImmutableMap.<Key<?>, Object>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 can not provide.

public Optional<HealthData> fill(DataContainer container, HealthData healthData) {
    final Optional<Double> health = container.getDouble(Keys.HEALTH.getQuery());
    final Optional<Double> maxHealth = container.getDouble(Keys.MAX_HEALTH.getQuery());
    if (health.isPresent() && maxHealth.isPresent()) {
        healthData.set(Keys.HEALTH, health.get());
        healthData.set(Keys.MAX_HEALTH, maxHealth.get());
        return Optional.of(healthData);
    }
    return Optional.empty();
}

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.

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.

protected MutableBoundedValue<Double> constructValue(Double value) {
    return SpongeValueFactory.boundedBuilder(Keys.HEALTH)
        .minimum(DataConstants.MINIMUM_HEALTH)
        .maximum((double) Float.MAX_VALUE)
        .defaultValue(DataConstants.DEFAULT_HEALTH)
        .actualValue(value)
        .build();
}

protected ImmutableValue<Double> constructImmutableValue(Double value) {
    return constructValue(value).asImmutable();
}

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().

protected boolean set(EntityLivingBase container, Double value) {
    if (value >= DataConstants.MINIMUM_HEALTH && 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.

public DataTransactionResult removeFrom(ValueContainer<?> container) {
    return DataTransactionBuilder.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 org.spongepowered.common.data.SpongeSerializationRegistry 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 DataManipulators at least one DataProcessor must be registered.

dataRegistry.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.

dataRegistry.registerValueProcessor(Keys.HEALTH, new HealthValueProcessor());
dataRegistry.registerValueProcessor(Keys.MAX_HEALTH, new MaxHealthValueProcessor());

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 forums, or open up an Issue on GitHub. Be sure to check the Data Processor Implementation Checklist for general contribution requirements.