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Accessing Capabilities on Players


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I'm trying to create a capability that reads a player's CompoundTag, and extracts some custom mod data from it to assemble an object, along with being given one of those mod objects and writing it to a Player's nbt. I've hopefully created the capability correctly, but I'm not sure how to go about having a `Player` read/write this data during a world save, and how I can go about retrieving the data from a given `Player` object on world join (I know I'd need to subscribe to the event, but I have no idea what I'd do from there). Additionally, this code results in "Invalid player data" whenever I join a world, and I don't know what's in here causes that (EDIT: Figured out this was due to my own bad serialization code. Does this mean that the NBT is automatically loaded on world join and saved on world leave?). My full repository can be found here: https://github.com/hammy3502/elementalism , and the relevant code snippets are below:


Object with Custom Mod Data (PlayerProgressionData.java):

public class PlayerProgressionData implements INBTSerializable<CompoundTag> {

    public static final String outerTag = "elementalism_data";

    public ProgressionInfo.ElementType type;
    public ProgressionInfo.ElementType type2;
    public List<ProgressionInfo> powersUnlocked;

    public PlayerProgressionData() {
        this.type = ProgressionInfo.ElementType.NONE;
        this.type2 = ProgressionInfo.ElementType.NONE;
        this.powersUnlocked = new ArrayList<>();

    public CompoundTag serializeNBT() {
        CompoundTag toRet = new CompoundTag();
        CompoundTag data = new CompoundTag();

        StringBuilder powersList = new StringBuilder();
        for (int i = 0; i < powersUnlocked.size(); i++) {
            if (i != powersUnlocked.size() - 1) {
        data.putString("unlockedPowers", powersList.toString());

        data.putString("element1", this.type.id);
        data.putString("element2", this.type2.id);

        toRet.put(outerTag, data);
        return toRet;

    public void deserializeNBT(CompoundTag nbt) {
        if (!nbt.contains(outerTag)) return;
        CompoundTag data = nbt.getCompound(outerTag);
        this.type = ProgressionInfo.ElementType.getElementById(data.getString("element1"));
        this.type2 = ProgressionInfo.ElementType.getElementById(data.getString("element2"));

        String[] powerIds = data.getString("unlockedPowers").split(",");
        for (String id : powerIds) {
            ProgressionInfo info = Objects.requireNonNull(ProgressionInfoInit.ALL_PROGRESSION_INFOS.get(id));


Capability Registration (Elementalism.java):

public void registerCapabilities(RegisterCapabilitiesEvent event) {


Capability Attaching (CommonSubscriber.java):

    public void attachCapabilities(AttachCapabilitiesEvent<Entity> event) {
        if (!(event.getObject() instanceof Player)) return;

        PlayerProgressionData progressionData = new PlayerProgressionData();
        LazyOptional<PlayerProgressionData> optionalProgressionData = LazyOptional.of(() -> progressionData);
        ICapabilityProvider provider = new ICapabilitySerializable<CompoundTag>() {

            public CompoundTag serializeNBT() {
                return progressionData.serializeNBT();

            public void deserializeNBT(CompoundTag nbt) {

            public <T> LazyOptional<T> getCapability(@Nonnull Capability<T> cap, @Nullable Direction side) {
                if (cap == DATA_STORAGE_CAPABILITY) {
                    return optionalProgressionData.cast();
                return LazyOptional.empty();

        event.addCapability(new ResourceLocation(Elementalism.MOD_ID, "data_storage"), provider);



Thank you so much for any help!

Edited by hammy3502
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