Named observer

[hukasu]

Objective

Closes #18726

Solution

• Add add_named_observer to App and World
• Add observe_named to EntityCommands and EntityWorldMut

Showcase

commands.spawn_empty().observe_named(my_observer, "MyObserver");
world.entity(entity).observe_named(my_observer, "MyObserver");
world.add_named_observer(my_observer, "MyObserver");
app.add_named_observer(my_observer, "MyObserver");

TODO

• Maybe rename add_named_observer
• Maybe rename observe_named
• Maybe add names to some of built-in observers

[cart]

I would prefer to not introduce a special-cased API for this. Observers will be increasingly component driven (I might even push for deprecation of entity.observe() ). Instead, we should use commands.spawn((Observer::new(..), Name::new("My Observer")).

[hukasu]

in this component driven style, the Observer would lose the entity list from the descriptor and each entity observing that observer would have the component?

[alice-i-cecile]

I agree with Cart in #18726 (comment). Can you explain why you feel this should be merged as is, rather than pursuing that line?

[hukasu]

This is as a draft exactly because i don't think it should be merged as-is, either it requires changes or the path that we will take is a different one, which is more likely given the talks

[cart]

in this component driven style, the Observer would lose the entity list from the descriptor and each entity observing that observer would have the component?

Observers would still be standalone entities, but they would define their targets via relationships (either in addition to manually defined targets on the descriptor, or instead of that). Final design is TBD but that is the general idea.

[hukasu]

on the lines of Observing ObservedBy? that would require n-m relationships, right?

[alice-i-cecile]

on the lines of Observing ObservedBy? that would require n-m relationships, right?

Indeed, that's why #17607 is stalled out :)