public class OpenCustomGuiPacket {
    public static final Identifier ID = new Identifier(TheClassicofMountainsandOceans.MOD_ID,"opengui");

    public static void sendToClient(ServerPlayerEntity player) {
        PacketByteBuf buf = new PacketByteBuf(Unpooled.buffer());
       
        ServerPlayNetworking.send(player, ID, buf);
    }

    public static void registerReceiver() {
        ClientPlayNetworking.registerGlobalReceiver(ID, (client, handler, buf, responseSender) -> {
            client.execute(() -> {
               
                MinecraftClient.getInstance().setScreen(new CustomScreen(
                        new CustomScreenHandler(0, MinecraftClient.getInstance().player.getInventory()),
                        MinecraftClient.getInstance().player.getInventory(),
                        MinecraftClient.getInstance().player.getName()
                ));
            });
        });
    }

registerReceiver這個方法是用來註冊一個接收器，當minecraft客戶端接收到特定訊息時可以進行某些操作。
registerGlobalReceiver用於註冊一個接收器，ID 是唯一標識符，用來區分不同的網路訊息。
當Minecraft客戶端接收到訊號的時候就會開啟全新的GUI。