Systems as `bevy_immediate` ui components

widgets.rs

/////////////////////////////////////////////
// Trait + Implementation
/////////////////////////////////////////////

pub trait SystemWidgets<Caps: CapSet>: Sized {
    /// Calls the given system, providing this ui entity as its input.
    fn run_system(self, id: SystemId<In<Entity>>) -> Self;
}

impl<Caps: CapSet> SystemWidgets<Caps> for ImmEntity<'_, '_, '_, Caps> {
    fn run_system(self, id: SystemId<In<Entity>>) -> Self {
        self.at_this_moment_apply_commands(|commands| {
            let entity = commands.id();
            commands.commands().run_system_with(id, entity);
        })
    }
}

/////////////////////////////////////////////
// Example Usage
/////////////////////////////////////////////

#[derive(Component)
pub struct MyUi(SystemId<In<Entity>>);

fn my_ui_system(In(root): In<Entity>, ctx: ImmCtx<CapsUiFeathers>) {
    let mut ui = ctx.build_immediate_from("my_ui_system", root);

    // Now use `ui` as you would normally to build ui nodes
}

fn my_startup_system(mut commands: Commands) {
    // Register it and spawn it
    let system_id = commands.register_system(my_ui_system);

    commands.insert(MyUi(system_id));
}

#[derive(Component)]
pub struct UiRoot;

impl ImmediateAttach<CapsUiFeathers> for UiRoot {
    type Params = Query<'static, 'static, &'static MyUi>;

    fn construct(ui: &mut Imm<CapsUiFeathers>, query: &mut SystemParamItem<Self::Params>) {
        // query for your SystemId however you want
        let my_ui = query.single();

        ui.ch_id("my_ui")
            // This is the root entity for the ui system, so anything you spawn on it will
            // affect the ui system as its parent entity.
            .on_spawn_insert(|| Node {
                ..Default::default()
            })
            .run_system(my_ui.0);
    }
}