minecraft-mod-tutorials/player-read-write-nbt.java

package org.example.examplemod.mixin;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.UUID;
import net.minecraft.entity.player.PlayerEntity;
import net.minecraft.nbt.NbtCompound;
import net.minecraft.nbt.NbtElement;
import net.minecraft.nbt.NbtList;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.Unique;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;

@Mixin(PlayerEntity.class)
public abstract class PlayerEntityMixin {
    @Unique
    public List<MyObject> myList = new ArrayList<>();

    // I recommend you read writeMyDataToNbt first before trying to understand readCustomDataFromNbt
    @Inject(method = "readCustomDataFromNbt", at = @At("TAIL"))
    public void readMyDataFromNbt(NbtCompound playerNbt, CallbackInfo ci) {
        // read our serialized list
        NbtList myListNbt = playerNbt.getList("MyList", NbtElement.COMPOUND_TYPE);

        // iterate over its items
        for (int i = 0; i < myListNbt.size(); i++) {
            // construct the object we need to deserialize
            MyObject myObject = new MyObject();
            // read the serialized object
            NbtCompound myObjectNbt = myListNbt.getCompound(i);

            // read properties that are supported and write them to our object
            myObject.mySupportedObject = myObjectNbt.getInt("MySupportedObject");
            // if mySupportedList was int[], we could've just used getIntArray, but since it's a List<Integer>, we have to do something else
            Arrays.stream(myObjectNbt.getIntArray("MySupportedList")).forEach(myInteger -> myObject.mySupportedList.add(myInteger));

            // read properties that aren't supported
            MyAnotherObject myAnotherObject = new MyAnotherObject();
            NbtCompound myNotSupportedObjectNbt = myObjectNbt.getCompound("MyNotSupportedObject");
            myAnotherObject.myUuid = myNotSupportedObjectNbt.getUuid("MyUUID");

            // write them to our object
            myObject.myNotSupportedObject = myAnotherObject;

            // finally, add our constructed object to our list
            myList.add(myObject);
        }
    }

    @Inject(method = "writeCustomDataToNbt", at = @At("TAIL"))
    public void writeMyDataToNbt(NbtCompound playerNbt, CallbackInfo ci) {
        // create a new NBT record for our list
        NbtCompound myListNbt = new NbtCompound();

        // you can use a regular for-loop if you want, there's no difference. FabricMC wiki uses Iterable#forEach so that's why it's used here
        myList.forEach(myObject -> {
            // create a new NBT record for our object
            NbtCompound myObjectNbt = new NbtCompound();
            // write properties that are supported
            myObjectNbt.putInt("MySupportedObject", myObject.mySupportedObject);
            myObjectNbt.putIntArray("MySupportedList", myObject.mySupportedList);

            // write myNotSupportedObject, which doesn't have a method for reading/writing in NbtCompound
            NbtCompound myNotSupportedObjectNbt = new NbtCompound();
            myNotSupportedObjectNbt.putUuid("MyUUID", myObject.myNotSupportedObject.myUuid);

            // you can nest NBT records infinitely
            myObjectNbt.put("MyNotSupportedObject", myNotSupportedObjectNbt);

            // put our newly created record into the list record
            myListNbt.put("MyObject", myObjectNbt);
        });

        // finally, put our list record into the player's NBT
        playerNbt.put("MyList", myListNbt);
    }
}

// these classes are in the same file for simplicity sake. you should make them public and put them in their own files
class MyObject {
    // the NbtCompound class has methods to write and read this object - putInt and getInt
    public int mySupportedObject = 0;
    // there are even some supported lists! this one uses putIntArray
    public List<Integer> mySupportedList = new ArrayList<>();
    // if there's no method in NbtCompound to handle your class,
    // you'll have to write their properties separately and, when reading, construct the objects using the written properties
    // just how are we currently doing with this object
    public MyAnotherObject myNotSupportedObject;
}

class MyAnotherObject {
    public UUID myUuid;
}