Objective

Fixes #13403

Solution

Created a new temp:// asset source which can be configured through the AssetPlugin via a new optional field temporary_file_path. If a path is provided, it will be considered as a persistent temporary directory, otherwise a directory is created in a platform appropriate location which will be automatically deleted on application exit.

Example Usage

A typical use for a temporary directory is the storage of data which you may need at some point, but don't right now (e.g., pre-fetching a map from a game server). In the below example, we create an asset my_text_asset, save it to the temp:// source, and later retrieve it.

// This is the asset we will attempt to save.
let my_text_asset = TextAsset("Hello World!".to_owned());

// To ensure the `Task` can outlive this function, we must provide owned versions
// of the `AssetServer` and our desired path.
let path = AssetPath::from("temp://message.txt").into_owned();
let server = assets.clone();

// Pass to the IoTaskPool to complete in the background
IoTaskPool::get().spawn(async move {
    save_asset(my_text_asset, path, server, TextSaver)
        .await
        .unwrap();
}).detach();

// ...

// Can later retrieve the saved asset
let my_text_asset = assets.load("temp://message.txt");

Since the temp:// source is on-disk, the unloaded asset will free memory for the rest of the application to use. In the above example, the memory used is trivial, but in a more complex game, a pre-fetched level could be significantly larger.

Testing

Tested the included example on my local devices (Windows and Linux).
Have not tested on mobile (I suspect iOS may be problematic here but cannot confirm).
Tested in Firefox when compiling to WASM.

Changelog

Added temp:// Asset Source
Added pub temporary_file_path: Option<String> to AssetPlugin (defaults to None)
Added a new example, temp_asset, which demonstrates how to save and load arbitrary assets in the new temporary asset source.
Added OriginPrivateFileSystem for the WASM platform as an AssetReader / AssetWriter for the Origin Private File System API

Migration Guide

If you were using the temp:// asset source ID, please use an alternate ID (e.g., my_temp://)
If constructing AssetPlugin manually, use ..default() to populate the new field with an appropriate default, or configure it yourself.

Converting to draft as I have a rough implementation of this now working on WASM as well as desktop (mobile is not something I am equipped to test so that is still outside of my scope). WASM support is built on top of the Origin Private File System, which provides a file-level API in the browser. I have implemented an AssetReader and an AssetWriter which are suitable for use with the proposed temp:// asset source.

I'm marking this as draft because while I have these implementations working in the browser (tested on Firefox using temp_asset example), I believe there is room for improvement in the exact implementations, and the potential for an AssetWatcher as well for completeness.

I would encourage anyone with wasm-bindgen experience (especially if you've worked with the OPFS API!) to leave any remarks as I will gladly accept and greatly appreciate any feedback!

bevyengine / bevy

Implementation of `temp://` Asset Source #13406