ExampleNetworking.java

package com.example.examplemod;

import net.fabricmc.fabric.api.client.networking.v1.ClientPlayNetworking;
import net.fabricmc.fabric.api.networking.v1.PayloadTypeRegistry;
import net.fabricmc.fabric.api.networking.v1.ServerPlayNetworking;
import net.minecraft.network.RegistryByteBuf;
import net.minecraft.network.codec.PacketCodec;
import net.minecraft.network.packet.CustomPayload;
import net.minecraft.util.Identifier;

/**
 * Networking switched to the CustomPayload record system in 1.20.5+ and is
 * unchanged through 1.21.11. Define payload, register codec + handler.
 */
public class ExampleNetworking {

    public static final Identifier PAYLOAD_ID = Identifier.of(ExampleMod.MOD_ID, "example_payload");

    // The payload itself - a simple record holding the data you want to send.
    public record ExamplePayload(String message, int amount) implements CustomPayload {
        public static final CustomPayload.Id<ExamplePayload> ID =
                new CustomPayload.Id<>(PAYLOAD_ID);

        public static final PacketCodec<RegistryByteBuf, ExamplePayload> CODEC =
                PacketCodec.tuple(
                        PacketCodec.STRING_UTF8, ExamplePayload::message,
                        PacketCodec.VAR_INT, ExamplePayload::amount,
                        ExamplePayload::new
                );

        @Override
        public Id<? extends CustomPayload> getId() {
            return ID;
        }
    }

    // Call once from ModInitializer#onInitialize() (common to client+server)
    public static void registerCommon() {
        PayloadTypeRegistry.playC2S().register(ExamplePayload.ID, ExamplePayload.CODEC);
        PayloadTypeRegistry.playS2C().register(ExamplePayload.ID, ExamplePayload.CODEC);

        // Server-side receiver
        ServerPlayNetworking.registerGlobalReceiver(ExamplePayload.ID, (payload, context) -> {
            context.player().sendMessage(
                    net.minecraft.text.Text.literal("Server got: " + payload.message() + " x" + payload.amount()),
                    false
            );
        });
    }

    // Call from ClientModInitializer#onInitializeClient()
    public static void registerClient() {
        ClientPlayNetworking.registerGlobalReceiver(ExamplePayload.ID, (payload, context) -> {
            // handle a packet sent from the server to this client
        });
    }

    // Example: sending a packet from client to server
    public static void sendExamplePacket(String message, int amount) {
        ClientPlayNetworking.send(new ExamplePayload(message, amount));
    }
}