Generic asset data loader

[will-hart]

I asked on discord a while back about implementing custom asset loaders for game data files. There was a bit of discussion here https://discordapp.com/channels/691052431525675048/742884593551802431/758904560223518720

What it boils down to is I'd like a way to use the existing AssetLoader pipeline to load in game data files (e.g. character stats, random level data, whatever) stored in RON files. I could be wrong here, but I think this is distinct from using the scenes feature because its more likely to be static / resource type data than something you instantiate as game entities.

My idea was essentially to implement a generic DataFileLoader asset loader which looks something like this:

pub struct DataFileLoader {
    matching_extensions: Vec<&'static str>,
}

impl DataFileLoader {
    pub fn from_extensions(matching_extensions: Vec<&'static str>) -> Self {
        DataFileLoader {
            matching_extensions,
        }
    }
}

impl<TAsset> AssetLoader<TAsset> for DataFileLoader
where
    for<'de> TAsset: Deserialize<'de>,
{
    fn from_bytes(&self, _asset_path: &Path, bytes: Vec<u8>) -> Result<TAsset, anyhow::Error> {
        Ok(ron::de::from_bytes::<TAsset>(bytes.as_slice())?)
    }

    fn extensions(&self) -> &[&str] {
        self.matching_extensions.as_slice()
    }
}

The current AppBuilder::add_asset_loader function takes 0 arguments and FromResources to create the loader instance to add. I'm proposing adding an AppBuilder::add_asset_loader_from_instance function that allows providing a concrete instance of an asset loader, and would be used something like this:

app.add_asset_loader_from_instance<MyCharacterData, DataFileLoader>(DataFileLoader::from_extensions(vec![".char"]));

Internally the existing add_asset_loader could call this function i.e. self.add_asset_loader_from_instance(TLoader::from_resources()) to maintain backwards compatibility. I'm happy to submit a PR (I have a local branch which implements this) but I'm wondering if this is something others would find useful or if there is a different / better way to manage "static" game data?

EDIT: Draft PR here with example to aid discussion #580

[will-hart]

#580 was merged into master