Add helper macro's for logging only once

[josfeenstra]

Objective

Fixes #10291

This adds a way to easily log messages once within system which are called every frame.

Solution

Opted for a macro-based approach. The fact that the 'once' call is tracked per call site makes the log_once!() macro very versatile and easy-to-use. I suspect it will be very handy for all of us, but especially beginners, to get some initial feedback from systems without spamming up the place!

I've made the macro's return its internal has_fired state, for situations in which that might be useful to know (trigger something else alongside the log, for example).

Please let me know if I placed the macro's in the right location, and if you would like me to do something more clever with the macro's themselves, since its looking quite copy-pastey at the moment. I've tried ways to replace 5 with 1 macro's, but no success yet.

One downside of this approach is: Say you wish to warn the user if a resource is invalid. In this situation, the
resource.is_valid() check would still be performed every frame:

fn my_system(my_res: Res<MyResource>) {
   if !my_res.is_valid() {
      warn_once!("resource is invalid!");
   }
}

If you want to prevent that, you would still need to introduce a local boolean. I don't think this is a very big deal, as expensive checks shouldn't be called every frame in any case.

Changelog

Added: trace_once!(), debug_once!(), info_once!(), warn_once!(), and error_once!() log macros which fire only once per call site.

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[MrGVSV]

Nice! I like the simplicity of this rather than more explicit options (like a Local<bool> or some other method of tracking).

[josfeenstra]

Nice! I like the simplicity of this rather than more explicit options (like a Local<bool> or some other method of tracking).

Me too @MrGVSV, and thanks for the review! What do you think of this latest change, in which the once!() mechanic is abstracted away in yet another macro?

[josfeenstra]

Some remaining questions:

• Should once!() be moved to bevy_util?
• AtomicBool is from std. Does it work on all platforms, in particular webassembly?

[alice-i-cecile]

Really simple and clean. Nice stuff!

[alice-i-cecile]

Should once!() be moved to bevy_util?

I'm content with where it is for now. I don't particularly want to promote this as a general-purpose tool, or add things to bevy_utils if we have another plausible choice.

AtomicBool is from std. Does it work on all platforms, in particular webassembly?

https://github.com/rust-lang/rust/issues/77839suggests that Rust can't access WASM atomic primitives, but I'm not sure if that's the same as std's atomic types simply failing to compile. It feels like falling back to a simple static in the context of a purely single-threaded program should be straightforward.

A quick test before merging would be great.

[josfeenstra]

A quick test before merging would be great.

@alice-i-cecile

Seems to work just fine!