Asset Value Templates via asset_value() and HandleTemplate::Value

[cart]

Objective

Temporary simple / suboptimal solution to #23822

It would be very nice to be able to define assets inline in BSN.

Solution

• Add a new HandleTemplate::Value variant, which contains an Arc<Mutex<AssetOrHandle<T>>>
• This template, when first applied, will lock the mutex, insert the asset value as a new asset, cache the handle in the Arc<Mutex<HandleOrTemplate> and return the handle. Future calls will lock the mutex and return the cached handle.
• Add asset_value(SOME_ASSET) function to improve ergonomics.
• Port 3d_scene to illustrate

Doing a lock on every spawn is obviously suboptimal. The long term plan for "inline assets in BSN" is defined in #23822 and will not use this locking approach.

[andriyDev]

Keep in mind our AssetServer is also a Mutex, so this would be equivalent to a load I think? Which is like.... Not a big deal IMO? Or if it is we have bigger problems lol.

In fact, this should be even faster since the asset server is likely under more contention than each asset template.

[cart]

In fact, this should be even faster since the asset server is likely under more contention than each asset template.

The difference here is that in an ideal world, we hit the asset server once to retrieve the handle, then clone that directly rather than asking the asset server again. A non-BSN equivalent would be much faster, as it could just cache the handle somewhere. These are only on par when you spawn something exactly once.

[cart]

Just benchmarked spawning 100 scene instances each containing 10 asset handles (using scene inheritance to reuse the ResolvedScene):

The difference is present, but not to a degree that we shouldn't land this. I'll push the benchmark and do the doc work!

[andriyDev]

Nit: this drop does nothing.