How to Define Custom Formats

This guide explains how to define a new asset format. This will allow Amethyst to load assets stored in a particular encoding.

There is a trait in Amethyst for implementing a format: Format<A: Asset::Data>. Format provides a loading implementation that provides detection when an asset should be reloaded for hot reloading; you don't need to implement it since it has a default implementation. A blanket implementation will implement Format::import and we only need to implement Format::import_simple.

Format takes a type parameter for the asset data type it supports. This guide covers a type parameterized implementation of Format<D> where D is an arbitrary Asset::Data, so we can reuse it for any asset which can be loaded from deserializable asset data.

If you are defining a new format that may be useful to others, please send us a PR!

  1. Define a struct that represents the format.

    In most cases a unit struct is sufficient. When possible, this should implement Clone and Copy for ergonomic usage.

    /// Format for loading from `.mylang` files.
    #[derive(Clone, Copy, Debug, Default)]
    pub struct MyLangFormat;
    
  2. Implement the Format trait.

    This is where the logic to deserialize the asset data type is provided. Fields of the format struct can be used to specify additional parameters for deserialization; use a unit struct if this is not needed.

    In this example the RON deserializer is used, though it is already a supported format.

    # extern crate amethyst;
    # extern crate ron;
    # extern crate serde;
    #
    use amethyst::{
        error::Error,
        assets::{Asset, Format},
    };
    use serde::Deserialize;
    use ron::de::Deserializer; // Replace this in your implementation.
    
    /// Format for loading from `.mylang` files.
    #[derive(Clone, Copy, Debug, Default)]
    pub struct MyLangFormat;
    
    impl<D> Format<D> for MyLangFormat
    where
        D: for<'a> Deserialize<'a> + Send + Sync + 'static,
    {
        fn name(&self) -> &'static str {
            "MyLangFormat"
        }
    
        fn import_simple(&self, bytes: Vec<u8>) -> Result<D, Error> {
            let mut deserializer = Deserializer::from_bytes(&bytes)?;
            let val = D::deserialize(&mut deserializer)?;
            deserializer.end()?;
    
            Ok(val)
        }
    }
    

    The custom format can now be used:

    # extern crate amethyst;
    # extern crate ron;
    # extern crate serde;
    # extern crate serde_derive;
    #
    # use amethyst::{
    #     error::Error,
    #     assets::{
    #         Asset, AssetStorage, Handle, Loader, Processor, ProgressCounter,
    #         ProcessingState, Format,
    #     },
    #     ecs::{VecStorage, World, WorldExt},
    #     prelude::*,
    #     utils::application_root_dir,
    # };
    # use ron::de::Deserializer;
    # use serde::Deserialize as DeserializeTrait;
    # use serde_derive::{Deserialize, Serialize};
    #
    # /// Custom asset representing an energy blast.
    # #[derive(Clone, Debug, Default, PartialEq, Eq, Deserialize, Serialize)]
    # pub struct EnergyBlast {
    #     /// How much HP to subtract.
    #     pub hp_damage: u32,
    #     /// How much MP to subtract.
    #     pub mp_damage: u32,
    # }
    #
    # /// A handle to a `EnergyBlast` asset.
    # pub type EnergyBlastHandle = Handle<EnergyBlast>;
    #
    # impl Asset for EnergyBlast {
    #     const NAME: &'static str = "my_crate::EnergyBlast";
    #     type Data = Self;
    #     type HandleStorage = VecStorage<EnergyBlastHandle>;
    # }
    #
    # impl From<EnergyBlast> for Result<ProcessingState<EnergyBlast>, Error> {
    #     fn from(energy_blast: EnergyBlast) -> Result<ProcessingState<EnergyBlast>, Error> {
    #       Ok(ProcessingState::Loaded(energy_blast))
    #     }
    # }
    #
    # pub struct LoadingState {
    #     /// Tracks loaded assets.
    #     progress_counter: ProgressCounter,
    #     /// Handle to the energy blast.
    #     energy_blast_handle: Option<EnergyBlastHandle>,
    # }
    #
    # /// Format for loading from `.mylang` files.
    #  #[derive(Clone, Copy, Debug, Default)]
    #  pub struct MyLangFormat;
    #
    #  impl<D> Format<D> for MyLangFormat
    #  where
    #      D: for<'a> DeserializeTrait<'a> + Send + Sync + 'static,
    #  {
    #      fn name(&self) -> &'static str {
    #          "MyLangFormat"
    #      }
    #
    #      fn import_simple(&self, bytes: Vec<u8>) -> Result<D, Error> {
    #          let mut deserializer = Deserializer::from_bytes(&bytes)?;
    #          let val = D::deserialize(&mut deserializer)?;
    #          deserializer.end()?;
    #
    #          Ok(val)
    #      }
    #  }
    #
    impl SimpleState for LoadingState {
        fn on_start(&mut self, data: StateData<'_, GameData<'_, '_>>) {
            let loader = &data.world.read_resource::<Loader>();
            let energy_blast_handle = loader.load(
                "energy_blast.mylang",
                MyLangFormat,
                &mut self.progress_counter,
                &data.world.read_resource::<AssetStorage<EnergyBlast>>(),
            );
    
            self.energy_blast_handle = Some(energy_blast_handle);
        }
    #
    #     fn update(
    #         &mut self,
    #         _data: &mut StateData<'_, GameData<'_, '_>>,
    #     ) -> SimpleTrans {
    #         Trans::Quit
    #     }
    }
    #
    # fn main() -> amethyst::Result<()> {
    #     amethyst::start_logger(Default::default());
    #     let app_root = application_root_dir()?;
    #     let assets_dir = app_root.join("assets");
    #
    #     let game_data = GameDataBuilder::default()
    #         .with(Processor::<EnergyBlast>::new(), "", &[]);
    #     let mut game = Application::new(
    #         assets_dir,
    #         LoadingState {
    #             progress_counter: ProgressCounter::new(),
    #             energy_blast_handle: None,
    #         },
    #         game_data,
    #     )?;
    #
    #     game.run();
    #     Ok(())
    # }