inexistent_children.rs

//! produce inexistent children using DynamicScene duplication

use bevy::{prelude::*, ecs::entity::EntityHashMap};

#[derive(Component, Reflect, Default)]
#[reflect(Component)]
struct Cat;

#[derive(Component, Reflect, Default)]
#[reflect(Component)]
struct Kitten;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, (setup, print).chain())
        .register_type::<Cat>()
        .register_type::<Kitten>()
        .run();
}

fn setup(world: &mut World) {
    // spawn a kitten and its parent
    let kitten = world.spawn(Kitten).id();
    world.spawn(Cat).add_child(kitten);

    // create a dynamic scene containing the parent (and its children component)
    let mut query = world.query_filtered::<Entity, With<Cat>>();
    let scene = DynamicSceneBuilder::from_world(&world)
        .allow::<Children>()
        .allow::<Cat>()
        .extract_entities(query.iter(&world))
        .build();
    let mut entity_map = EntityHashMap::default();
    // write the scene, now we have an extra cat without a kitten
    // but it has a children component which contains an inexistent id
    let _ = scene.write_to_world(world, &mut entity_map);
}

fn print(
    cat_query: Query<(Entity, &Children), With<Cat>>,
    existent_query: Query<Entity>,
) {
    for (id, children) in cat_query.iter() {
        info!("cat_id: {:?}, children: {:?}", id, children);
        for kitten_id in children {
            if existent_query.contains(*kitten_id) {
                info!("kitten {:?} exists! mew!", kitten_id);
            } else {
                info!("kitten {:?} doesn't exists!", kitten_id);
            }
        }
    }
}

output.md

2024-03-12T18:02:13.165911Z  INFO hierarchy: cat_id: 2v1, children: Children([1v1])
2024-03-12T18:02:13.165943Z  INFO hierarchy: kitten 1v1 exists! mew!
2024-03-12T18:02:13.165950Z  INFO hierarchy: cat_id: 3v1, children: Children([4v1])
2024-03-12T18:02:13.165956Z  INFO hierarchy: kitten 4v1 doesn't exists!