Mapping IBlockStates to IBakedModels. ISmartBlockModel.

This topic is just a few notes about how Minecraft uses IBlockStates to retrieve the corresponding IBakedModel, to help you understand the vanilla code a bit more (which is pretty complicated)

When rendering a block, the renderer needs to retrieve the correct IBakedModel for the given IBlockState of the block.  Broadly speaking, this occurs in two decoupled steps:

1.  The IBlockState is used to look up a ModelResourceLocation using BlockStateMapper.
2.  The ModelResourceLocation is then used to retrieve the IBakedModel using ModelManager.

After retrieving the IBakedModel, Forge checks if this is an ISmartBlockModel.  If so, it is customized using IExtendedBlockState information for the specific Block position being rendered.

Some miscellaneous notes

1. ModelResourceLocation is a tag for the model, of the form “mymodid:granite#normal”.  By default, the ModelLoader will look for it in
   assets/mymodid/blockstates/granite.json:
   

   {
       "variants": {
           "normal": { "model": "granite_model" }
       }
   }

   In this example, it will create an IBakedModel from granite_model.json.
2. It’s usually not necessary to provide an appropriate BlockStateMapping, because the default uses your block’s registered name and its properties, which are generally what you need.  If you do need a custom mapper, use ModelLoader.setCustomStatemapper().  BlockModel.registerAllBlocks() has good examples how to ignore some of a block’s properties, for example cactus model is the same for all AGE values.
3. By default, ModelLoader registers a ModelResourceLocation for every entry in the BlockStateMapper.
4. You can use ModelBakeEvent to insert custom models into the ModelManager; alternatively you can register an ICustomerModelLoader with ModelLoaderRegistry, which bypasses the vanilla ModelLoader for your chosen blocks– for example see B3DLoader.
5. The ISmartBlockModel instance stored in the ModelManager is the “default”.  When called, ISmartBlockModel.handleBlockState() then returns a new instance to suit the IExtendedBlockState.  You shouldn't modify the "default" in handleBlockState because block rendering is multithreaded - you need to return a new instance, or alternatively an immutable one that you have cached,
6. The various registries are cached and only updated when the resources are reloaded.