Track callsite for observers & hooks

[SpecificProtagonist]

Objective

Fixes #14708

Also fixes some commands not updating tracked location.

Solution

ObserverTrigger has a new caller field with the track_change_detection feature;
hooks take an additional caller parameter (which is Some(…) or None depending on the feature).

Testing

See the new tests in src/observer/mod.rs

---

Showcase

Observers now know from where they were triggered (if track_change_detection is enabled):

world.observe(move |trigger: Trigger<OnAdd, Foo>| {
    println!("Added Foo from {}", trigger.trigger.caller);
});

Migration

• hooks now take an additional Option<&'static Location> argument

[alice-i-cecile]

I like this strategy, but I really want to see benchmarks before we remove the feature flag on BundleInserter.

[SpecificProtagonist]

I tried running the various add_remove & insert_simple benches, but I need to figure out how to turn of frequency scaling for my laptop first (can't get consistent enough results).

Should this ends up having to sit behind a feature flag, would this be a new feature flag or the (possibly renamed) track_change_detection? And should hooks then take an Option<&Location>?

[NthTensor]

Personally, I'd rename track_change_detection and gate all tracking behind it.

For hooks, that's a bit harder. Ideally an optional Option<&Location> parameter, so that people don't have to litter their code with cfg attributes.

[SpecificProtagonist]

Personally, I'd rename track_change_detection and gate all tracking behind it.

For hooks, that's a bit harder. Ideally an optional Option<&Location> parameter, so that people don't have to litter their code with cfg attributes.

I think that would be the best, will do. Can you add the rename to your meta issue?

[github-actions]

Your PR increases Bevy Minimum Supported Rust Version. Please update the rust-version field in the root Cargo.toml file.

[github-actions]

Your PR increases Bevy Minimum Supported Rust Version. Please update the rust-version field in the root Cargo.toml file.