Demonstrate how to dispatch assets of the same extension to different Rust types

[alice-i-cecile]

          > So @bushrat011899, extending on my proposed use case a bit.

1. I have multiple types which should be stored as a .json file.

2. I want to parse the json file to determine what Rust type to load this in as, and then add it to a central collection.

Is that feasible with this PR?

@alice-i-cecile Yes that should be entirely possible:

1. First you'd create a plain JSON AssetLoader to give you the parsed contents to inspect.

// Handle<JsonAsset> implied by .json file extension if
// `JsonAssetLoader` is configured to be default handler of .json
let my_data_json_handle = asset_server.load("data/my_data.json");

2. Once loaded, you can then inspect the JSON data for whatever information you'd need to determine the type it should be:

let my_data_json = json_assets.get(&my_data_json_handle)?;

let serde_json::Value::String(magic_token) = my_data_json.get("magic_token")? else {
    return None;
};

// Can re-use the path value for DRY reasons
let path = my_data_json_handle.path().unwrap();

// All match arms need to have the same return type, using UntypedHandle to avoid an enum
let my_data_handle = match magic_token {
    "key_bindings" => asset_server.load::<KeyBindingsAsset>(&path).untyped(),
    "settings" => asset_server.load::<SettingsAsset>(&path).untyped(),
    "scoreboard" => asset_server.load::<ScoreboardAsset>(&path).untyped(),
    _ => my_data_json_handle.untyped(),
};

Some(my_data_handle)

I imagine you'd want an enum for the central collection, and that would probably be the type of my_data_handle in the above match statement instead of using UntypedHandle's. Additionally, this behaviour is extended to the LoadContext::load method, so you could put all of this functionality into a custom AssetLoader if the load_with_reader method @nicopap suggested in #10565 was added (would need another PR to add).

Originally posted by @bushrat011899 in #10153 (comment)

[torsteingrindvik]

I tried asking a similar question here: #9714 (comment)

My TL;DR is I wish I could just

let handle_foo: Handle<Foo> = asset_server.load("data/foo.json");
let handle_bar: Handle<Bar> = asset_server.load("data/bar.json");

[bushrat011899]

I tried asking a similar question here: #9714 (comment)

My TL;DR is I wish I could just

let handle_foo: Handle<Foo> = asset_server.load("data/foo.json");
let handle_bar: Handle<Bar> = asset_server.load("data/bar.json");

@torsteingrindvik with #10153, you will be able to do this!

[torsteingrindvik]

I am doing extensionless mapped to lots of types now so I'm not actively using e.g. json-to-many myself but recent PRs and tests added by @bushrat011899 means this can be closed I think

EDIT: Noticing the title says "Demonstrate how to" so I suppose an example is needed, haven't checked if any of the asset examples cover this feature right now.