I'm trying to create a stronghold-esque structure that only spawns once per world, but like the stronghold only spawns in certain biomes. Is there a way to run an event when the world is first created, so that I can pick the location of this structure?

Also, is there a way to run a method when the selected location is generated, so that the structure can be built?

Edited July 7, 20178 yr by laton95

AFAIK, there's no event that fires when the world is first *created. But once you generate a location, you already know the location has been generated, so the very existence of a non-null location should tell you "I've already done this, don't do it again." In other words, as long as you save the location to the world data and only generate a new one if the existing one is null, you can do this from anywhere, it doesn't have to be only when the world loads.

 

This page of the Forge docs may come in handy with this: https://mcforge.readthedocs.io/en/latest/datastorage/worldsaveddata/

But keep in mind that the structure can't be built in unloaded chunks, so you're better off *actually generating* the structure in a World Generator rather than when the world loads. So you can just have everything in the world generator and generate the location there when none exists.

Thanks for the help, but I need to generate the positions of the structures on world creation so that I can have an item that points to them (like ender eyes and vanilla strongholds).

 

Since I can't change the vanilla chunk generator to add a new structure, I have made a class that generates and tracks the positions. A new instance of this class is made on InitMapGenEvent, which works but the positions are recalculated each time the event is fired (On reopening a world usually). WorldSaveData is perfect to solve this problem, but I can't get it to work and I can't find a great tutorial. Can anyone explain how to save and load world NBT data?

 

What I have:

 

Class that extends worldSavedData.

Spoiler

public class WorldNBTHelper extends WorldSavedData {
    private static final String DATA_NAME = Reference.MOD_ID;

    // Required constructors
    public WorldNBTHelper() {
        super(DATA_NAME);
    }

    public WorldNBTHelper(String s) {
        super(s);
    }

    @Override
    public void readFromNBT(NBTTagCompound nbt) {
        // TODO Auto-generated method stub

    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound) {
        // TODO Auto-generated method stub
        return null;
    }
    
    public static WorldNBTHelper get(World world) {
          // The IS_GLOBAL constant is there for clarity, and should be simplified into the right branch.
          MapStorage storage = world.getMapStorage();
          WorldNBTHelper instance = (WorldNBTHelper) storage.getOrLoadData(WorldNBTHelper.class, DATA_NAME);

          if (instance == null) {
            instance = new WorldNBTHelper();
            storage.setData(DATA_NAME, instance);
          }
          return instance;
        }
}

My attempts to save and load data, results in a nullPointerException when the pause menu is opened, I guess when the game tries to save it.

Spoiler

WorldNBTHelper worldNBTHelper = WorldNBTHelper.get(world);
        MapStorage storage = world.getMapStorage();
        WorldSavedData data = storage.getOrLoadData(worldNBTHelper.getClass(), Reference.MOD_ID);
        if (data == null) {
            storage.setData(Reference.MOD_ID, worldNBTHelper);
            data = storage.getOrLoadData(worldNBTHelper.getClass(), Reference.MOD_ID);
        }
        NBTTagCompound nbt = new NBTTagCompound();
        data.readFromNBT(nbt);
        altarsGenerated  = nbt.getBoolean("altarsGenerated");
        
        if (!altarsGenerated) {
            LogHelper.info("Finding locations");
            nbt.setBoolean("altarsGenerated", true);
            data.writeToNBT(nbt);
            data.markDirty();
            //save locations to nbt
        } else {
            //read locations from nbt
        }

 

Edited July 10, 20178 yr by laton95

In WorldSavedData, are the method names for writeToNBT and readFromNBT swapped around? The implementation of write is supposed to return an NBT compound while read does not return anything. Surely a 'read' function should return the thing it is reading?

Edited July 11, 20178 yr by laton95

7 minutes ago, laton95 said:

In WorldSavedData, are the method names for writeToNBT and readFromNBT swapped around? The implementation of write is supposed to return an NBT compound while read does not return anything. Surely a 'read' function should return the thing it is reading?

No that is not how it is used, the write function returns a value because it takes what is saved to the NBTTagcompound and puts it into the file, the read should just take the data out of the NBTTagcompound and put it back into fields.

23 minutes ago, laton95 said:

In WorldSavedData, are the method names for writeToNBT and readFromNBT swapped around? The implementation of write is supposed to return an NBT compound while read does not return anything. Surely a 'read' function should return the thing it is reading?

The read methods take an NBT tag and read its contents into fields within the object reading it. It doesn't need to return anything. The write methods take the field values and write them into a supplied NBT tag container (often, but not always, a currently-empty NBT tag). It then returns that tag after the modifications have been made, so you can pass it around to other things to write to, or use it however you want.

So read updates the object's state from the contents of an NBT tag, while write stores the object's state into an NBT tag.

Edited July 11, 20178 yr by IceMetalPunk

Thanks, I've figured out WorldSaveData now. My problem now is that it only loads when I enter ungenerated chunks, since I am using the InitMapGen event. I am trying to move over to the WorldEvent.Load event but the world object I get from that event is Null, so I can't load NBT from it.

 

Can anyone recommend a good way to get a non-null world object when the server/world first loads?