Манипуляторы данных

Accessing and modifying data

A data manipulator represents a certain component and all of its data. It stores a representation of that data and can be offered to or created from data holders which possess a matching component. Again, let’s use an example. Once more we try to heal someone (or something).

Code Example: Healing with data manipulators

import org.spongepowered.api.data.DataHolder;
import org.spongepowered.api.data.DataTransactionResult;
import org.spongepowered.api.data.manipulator.mutable.entity.HealthData;
import org.spongepowered.api.data.value.mutable.MutableBoundedValue;

import java.util.Optional;

public static void heal(DataHolder target) {
    Optional<HealthData> healthOptional = target.get(HealthData.class);
    if (healthOptional.isPresent()) {
        HealthData healthData = healthOptional.get();

        double maxHealth = healthData.maxHealth().get();
        MutableBoundedValue<Double> currentHealth = healthData.health();
        currentHealth.set(maxHealth);
        healthData.set(currentHealth);

        target.offer(healthData);
    }
}

First, we need to check if our target has health data. We do so by first asking it to provide us with its health data by passing its class to the get() method. We get an Optional which we can use for our check. This Optional will be absent if either our target does not support HealthData or if it supports it but at the present moment does not hold any health data.

If the health data is present, it now contains a mutable copy of the data present on the data holder. We make our alterations and finally offer the changed data back to our target, where it is accepted (again, offer() will return a DataTransactionResult which we will disregard in this example and get back to at a later point).

As you can see, the results for health() and maxHealth() are again keyed values we obtain from the DataHolder. As the MutableBoundedValue we receive from calling health() again just contains a copy of the data, we first need to apply our changes back to the DataManipulator before we can offer the healthData back to our target.

Совет

Rule #1 of the Data API: Everything you receive is a copy. So whenever you change something, make sure that your change is propagated back to where the original value came from.

Another possible modification is fully removing a DataManipulator. This is done via the remove() method which accepts a class reference for the type of DataManipulator to remove. Some data cannot be removed and attempts to do so will always return a DataTransactionResult indicating failure. The following code attempts to remove a custom name from a given DataHolder. Again, the result of the transaction is discarded.

Code Example: Removing a custom display name

import org.spongepowered.api.data.manipulator.mutable.DisplayNameData;

public void removeName(DataHolder target) {
    target.remove(DisplayNameData.class);
}

DataManipulator vs. Keys

If you compared both of our healing examples, you may wonder „Why bother with data manipulators anyway, keys are so much easier“ and be right - for getting and setting single values. But the true merit of a data manipulator is that it contains all data pertaining to a certain component. Let us take a look at another example.

Code Example: Swapping two data holders“ health

public void swapHealth(DataHolder targetA, DataHolder targetB) {
    if (targetA.supports(HealthData.class) && targetB.supports(HealthData.class)) {
        HealthData healthA = targetA.getOrCreate(HealthData.class).get();
        HealthData healthB = targetB.getOrCreate(HealthData.class).get();
        targetA.offer(healthB);
        targetB.offer(healthA);
    }
}

First, we check if both targets support HealthData. If they do, we save the health of both in one variable each. We don’t need to bother with Optional this time since we verified that HealthData is supported and the getOrCreate() method ensures that even if no data is present, default values are generated.

Then we just offer the saved health data to the other target, thus switching their health status with each other.

This example done with Keys would be a bit longer and more complicated since we’d have to take care of each individual key by ourself. And if instead of health we swapped another data manipulator containing even more data (maybe InvisibilityData which even contains a list), we’d have a lot more work to do. But since the data holder itself takes care of all data pertaining to it, we could even modify the above function to swap arbitrary data between two holders.

Code Example: Swapping any data manipulator

import org.spongepowered.api.data.manipulator.DataManipulator;

public  <T extends DataManipulator<?,?>> void swapData(DataHolder targetA, DataHolder targetB, Class<T> dataClass) {
   if (targetA.supports(dataClass) && targetB.supports(dataClass)) {
       T dataA = targetA.getOrCreate(dataClass).get();
       T dataB = targetB.getOrCreate(dataClass).get();
       targetA.offer(dataB);
       targetB.offer(dataA);
   }
}

The ability to write a function that can just swap any data on a data holder with the same data on another data holder demonstrates the core design goal of the Data API: Maximum compatibility across the API.

Mutable vs. Immutable Data Manipulators

To every data manipulator, there is a matching ImmutableDataManipulator. For instance, both HealthData and ImmutableHealthData contain the same data, only the latter returns new instances when requesting modified data.

Conversion between mutable and immutable data manipulators is done via the asImmutable() and asMutable() methods, which each will return a copy of the data. The only way to obtain an immutable data manipulator from a data holder is obtaining a mutable one and then using asImmutable().

A possible use case for this would be a custom event fired when someone is healed. It should provide copies of the health data before and after, but event listeners should not be able to change them. Therefore, we can write our event to only provide ImmutableHealthData instances. That way, even if third party code gets to interact with our data, we can rest assured that it will not be changed.

Absent Data

As mentioned above, the get() method may return an empty Optional if one of the following is true:

  • The DataHolder does not support the given DataManipulator

  • The DataHolder does support the DataManipulator, but currently holds no data of that type

There is a big semantic difference between data not being present and the data consisting of default values. While the latter is always possible, there are cases where it is impossible for a DataHolder to support a type of data and then not hold it. Examples of those include:

  • HealthData is always present on every (vanilla) DataHolder that supports it

  • DisplayNameData is always present on a Player, but may be absent on other entities.