How to Create and Render a Client-Side World

[zkhssb]

Description

I am developing a fabric-mod for version 1.20.4, and I want to make the TitleScreen more engaging by rendering players in a client-side world (similar to Minecraft: Dungeons):

My general idea is:
Create a world that exists only on the client side

• Load an empty world somehow
• Load the buildings within the TitleScreen area into the empty world
• Spawn the player entity at a certain location
• Render the camera in front of the player entity
• Create a custom Screen and implement all the buttons from the TitleScreen

Then I tried to create a world and have the client load it after the MinecraftClient.onFinishedLoading method is called:

@Mixin(MinecraftClient.class)
public class MinecraftClientMixin {
    @Inject(method = "onFinishedLoading", at = @At(value = "RETURN", target = "Lnet/minecraft/client/MinecraftClient;collectLoadTimes(Lnet/minecraft/client/MinecraftClient$LoadingContext;)V"))
    private void onFinishedLoading(CallbackInfo ci) {
        ClientEvents.INIT_FINISHED.invoker().onInitFinished(MinecraftClient.getInstance());
    }
}

    @Override
    public void onInitializeClient() {
        ClientEvents.INIT_FINISHED.register((client) -> {
            FakeWorldUtil.createAndJoin();
        });
    }

public class FakeWorldUtil {
    @Getter
    private static ClientBackgroundWorld fakeWorld;

    public static void createAndJoin() {
        fakeWorld = ClientBackgroundWorld.create();
        var client = MinecraftClient.getInstance();

        client.joinWorld(fakeWorld);
        if(client.player == null){
            client.player = client.interactionManager.createPlayer(fakeWorld, new StatHandler(), new ClientRecipeBook());
            if(client.getServer() != null){
                client.getServer().setLocalPlayerUuid(client.player.getUuid());
            }
        }

        client.debugRenderer.reset();
        client.player.init();


    }
}

public class ClientBackgroundWorld extends ClientWorld {
    public ClientBackgroundWorld(ClientPlayNetworkHandler networkHandler, Properties properties, RegistryKey<World> registryRef, RegistryEntry<DimensionType> dimensionTypeEntry, int loadDistance, int simulationDistance, Supplier<Profiler> profiler, WorldRenderer worldRenderer, boolean debugWorld, long seed) {
        super(networkHandler, properties, registryRef, dimensionTypeEntry, loadDistance, simulationDistance, profiler, worldRenderer, debugWorld, seed);
    }

    public static ClientBackgroundWorld create() {
        Properties properties = new Properties(
                Difficulty.HARD,
                false,
                true 
        );
        var minecraftClient = MinecraftClient.getInstance();
        //var entry = minecraftClient.getNetworkHandler().getRegistryManager().get(RegistryKeys.DIMENSION_TYPE).entryOf(DimensionTypes.OVERWORLD);
        return new ClientBackgroundWorld(minecraftClient.getNetworkHandler(), properties, World.OVERWORLD, null, 0, 0, minecraftClient::getProfiler, minecraftClient.worldRenderer, false, 0);
    }
}

But I ran into trouble... networkHandler is not initialized at this point, so I can't create a world, and the final result of the code is NullPointerException

The call to ClientBackgroundWorld threw an exception when calling its superclass method, and I think my approach to creating the world is entirely wrong ;-;

So... I don't know how to create a world that exists only on the client side and how to load the structures.

I seek the assistance of experienced developers to provide some guidance!