SystemId scene templating

[ItsDoot]

Objective

• Simplified alternative to Arcify one-shot systems to enable SystemId templating #24072

I have a bunch of Bundle-style code that I want to replace with the new bsn! Scene-style macro:

pub fn rest_ui(/* system params */) {
    // my ui system...
}

// From
pub fn rest(mut commands: Commands) -> impl Bundle {
    (
        Rest,
        Name::new("Rest"),
        Activity {
            render: commands.register_system(rest_ui),
        },
    )
}

// To (ideally)
pub fn rest() -> impl Scene {
    bsn! {
        Rest
        Name("Rest")
        Activity {
            render: rest_ui
        }
    }
}

Solution

This solution is more inspired by how HandleTemplate works.

1. Added SystemIdTemplate; it stores either a SystemId or a Arc<Mutex<Either<SystemId, Box<dyn System>>>>
2. Added a system_value function for wrapping system functions (see Future Work for potentially removing the need)

Testing

• Added a unit test for SystemIdTemplate.
• Added to the callbacks example demonstrating how to spawn SystemIds via BSN scenes.

Future work

• I believe we can remove the need for wrapping with system_value by introducing SuperFrom/SuperInto traits a la Dioxus and using it in the bsn! macros in-place of the implicit .into()s.

---

Showcase

You can now spawn components containing SystemIds via bsn! macros:

#[derive(Component, FromTemplate)]
struct Callback {
    system_id: SystemId<(), ()>,
}

fn my_scene() -> impl Scene {
    bsn! {
        Callback {
            system_id: system_value(|| {
                println!("This is a callback spawned via a scene.");
            })
        }
    }
}

[laundmo]

Glad to see my idea from #24072 (comment) had some merit! I won't approve yet, since i've only looked through it on my phone, but it already seems far simpler.

[SkiFire13]

This was proposed basically 1:1 in #24026 (this was later changed though). The issue is that it's unclear who owns these systems, that is who is responsible for unregistering them once they are no longer needed. Given that recreating the template will spawn the system again this basically becomes a memory leak.

[laundmo]

This was proposed basically 1:1 in #24026 (this was later changed though). The issue is that it's unclear who owns these systems, that is who is responsible for unregistering them once they are no longer needed. Given that recreating the template will spawn the system again this basically becomes a memory leak.

Hm, are you sure this is the case even tho in build_template it only registers the system the first time its called, switching over to storing the SystemId after the first call?

[SkiFire13]

Hm, are you sure this is the case even tho in build_template it only registers the system the first time its called, switching over to storing the SystemId after the first call?

If you recreate the template (e.g. you call my_scene() again) then you will create a new instance of the system. And since the system is not scoped to the scene once the scene is despawned the system entity will be leaked.