Allow Creating Custom AssetIO Implementations That Support Hot Reload

[zicklag]

What problem does this solve or what need does it fill?

Currently, when creating your own AssetIo implementation, it is actually impossible to implement a custom equivalent to the filesystem_watcher_sysem, because that function depends on asset_server.server.asset_io, which is a private field of AssetServer.

In my use-case, I am creating a custom AssetIo implementation that allows loading assets from zip bundles, but I want to fall back completely to the default asset loader. Unfortunately, when I did this, I lost the ability to do hot reload, because the built-in filesystem_watcher_system will only work when the current AssetIo implementation is FileAssetIo.

What solution would you like?

I don't have a specific idea for the implementation, but however necessary, I need a way to be able to copy the exact functionality of the current filesystem_watcher_system in my own AssetIo implementation.

What alternative(s) have you considered?

Waiting on the Distil integration? 🤷‍♂️ But I would like to get this into my app now, so I'd hope there's at least a short-term solution would could come up with now.

Additional context

If the strategy doesn't require a ton of work, I'd be willing to try to do this, but I'm not exactly sure what the best strategy is. The obvious easy techinque would just be to make the types that are necessary to access public types instead of private, but I'm not sure that is the best long-term design. Also, with the integration with Distil comming up, I'm not sure if we need it to be a great long term solution?

Anyway, I need some directional guidance on how we want to go about this.

[johanhelsing]

Was struggling with this today. @zicklag did you figure out a way around it?

[zicklag]

No, I never made any progress with this.

[johanhelsing]

Looked at this again today. AssetServer::asset_io was added in 0.7 (98938a8), which makes it possible to get the AssetIO and downcast it to FileAssetIo yourself. However, It's still not possible to replicate the default implementation (filesystem_watcher_system) because FileAssetIo::filesystem_watcher is private and has no getter.

[Neopallium]

It is now possible to re-implement "watch for changes" support in custom AssetIo implementations. It should be possible to reuse the FilesystemWatcher in custom AssetIo impls, but it does require copying some code from FileAssetIo.

This PR [0] merged a public method AssetServer::reload_asset which can be used to trigger the reloads as is done in filesystem_watcher_system. The reload_asset method can be used for more specialized reloaders too (network event, editor "deploy" trigger, etc..).

I have this working in my custom AssetIo that works like bevy_web_asset crate.

0. [Merged by Bors] - Add reload_asset method to AssetServer. #5106

[zicklag]

Sounds good, thank you!