bevy/bevy_transform/examples/hierarchy.rs

extern crate legion;
extern crate legion_transform;

use legion::prelude::*;
use legion_transform::prelude::*;

#[allow(unused)]
fn tldr_sample() {
    // Create a normal Legion World
    let mut world = Universe::default().create_world();

    // Create a system bundle (vec of systems) for LegionTransform
    let transform_system_bundle = transform_system_bundle::build(&mut world);

    let parent_entity = *world
        .insert(
            (),
            vec![(
                // Always needed for an Entity that has any space transform
                LocalToWorld::identity(),
                // The only mutable space transform a parent has is a translation.
                Translation::new(100.0, 0.0, 0.0),
            )],
        )
        .first()
        .unwrap();

    world.insert(
        (),
        vec![
            (
                // Again, always need a `LocalToWorld` component for the Entity to have a custom
                // space transform.
                LocalToWorld::identity(),
                // Here we define a Translation, Rotation and uniform Scale.
                Translation::new(1.0, 2.0, 3.0),
                Rotation::from_euler_angles(3.14, 0.0, 0.0),
                Scale(2.0),
                // Add a Parent and LocalToParent component to attach a child to a parent.
                Parent(parent_entity),
                LocalToParent::identity(),
            );
            4
        ],
    );
}

fn main() {
    // Create a normal Legion World
    let mut world = Universe::default().create_world();

    // Create a system bundle (vec of systems) for LegionTransform
    let transform_system_bundle = transform_system_bundle::build(&mut world);

    // See `./types_of_transforms.rs` for an explanation of space-transform types.
    let parent_entity = *world
        .insert(
            (),
            vec![(LocalToWorld::identity(), Translation::new(100.0, 0.0, 0.0))],
        )
        .first()
        .unwrap();

    let four_children: Vec<_> = world
        .insert(
            (),
            vec![
                (
                    LocalToWorld::identity(),
                    Translation::new(1.0, 2.0, 3.0),
                    Rotation::from_euler_angles(3.14, 0.0, 0.0),
                    Scale(2.0),
                    // Add a Parent and LocalToParent component to attach a child to a parent.
                    Parent(parent_entity),
                    LocalToParent::identity(),
                );
                4
            ],
        )
        .iter()
        .cloned()
        .collect();

    // At this point the parent does NOT have a `Children` component attached to it. The `Children`
    // component is updated by the transform system bundle and thus can be out of date (or
    // non-existent for newly added members). By this logic, the `Parent` components should be
    // considered the always-correct 'source of truth' for any hierarchy.
    for system in transform_system_bundle.iter() {
        system.run(&mut world);
        system.command_buffer_mut().write(&mut world);
    }

    // At this point all parents with children have a correct `Children` component.
    let parents_children = world
        .get_component::<Children>(parent_entity)
        .unwrap()
        .0
        .clone();

    println!("Parent {}", parent_entity);
    for child in parents_children.iter() {
        println!(" -> Has child: {}", child);
    }

    // Each child will also have a `LocalToParent` component attached to it, which is a
    // space-transform from its local space to that of its parent.
    for child in four_children.iter() {
        println!("The child {}", child);
        println!(
            " -> Has a LocalToParent matrix: {}",
            *world.get_component::<LocalToParent>(*child).unwrap()
        );
        println!(
            " -> Has a LocalToWorld matrix: {}",
            *world.get_component::<LocalToWorld>(*child).unwrap()
        );
    }

    // Re-parent the second child to be a grandchild of the first.
    world.add_component(four_children[1], Parent(four_children[0]));

    // Re-running the system will cleanup and fix all `Children` components.
    for system in transform_system_bundle.iter() {
        system.run(&world);
        system.command_buffer_mut().write(&mut world);
    }

    println!("After the second child was re-parented as a grandchild of the first child...");

    for child in world
        .get_component::<Children>(parent_entity)
        .unwrap()
        .0
        .iter()
    {
        println!("Parent {} has child: {}", parent_entity, child);
    }

    for grandchild in world
        .get_component::<Children>(four_children[0])
        .unwrap()
        .0
        .iter()
    {
        println!("Child {} has grandchild: {}", four_children[0], grandchild);
    }

    println!("Grandchild: {}", four_children[1]);
    println!(
        " -> Has a LocalToWorld matrix: {}",
        *world
            .get_component::<LocalToWorld>(four_children[1])
            .unwrap()
    );
}