hierarchy/
hierarchy.rs

1//! Demonstrates techniques for creating a hierarchy of parent and child entities.
2//!
3//! When [`DefaultPlugins`] are added to your app, systems are automatically added to propagate
4//! [`Transform`] and [`Visibility`] from parents to children down the hierarchy,
5//! resulting in a final [`GlobalTransform`] and [`InheritedVisibility`] component for each entity.
6
7use std::f32::consts::*;
8
9use bevy::{color::palettes::css::*, prelude::*};
10
11fn main() {
12    App::new()
13        .add_plugins(DefaultPlugins)
14        .add_systems(Startup, setup)
15        .add_systems(Update, rotate)
16        .run();
17}
18
19fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
20    commands.spawn(Camera2d);
21    let texture = asset_server.load("branding/icon.png");
22
23    // Spawn a root entity with no parent
24    let parent = commands
25        .spawn((
26            Sprite::from_image(texture.clone()),
27            Transform::from_scale(Vec3::splat(0.75)),
28        ))
29        // With that entity as a parent, run a lambda that spawns its children
30        .with_children(|parent| {
31            // parent is a ChildSpawnerCommands, which has a similar API to Commands
32            parent.spawn((
33                Transform::from_xyz(250.0, 0.0, 0.0).with_scale(Vec3::splat(0.75)),
34                Sprite {
35                    image: texture.clone(),
36                    color: BLUE.into(),
37                    ..default()
38                },
39            ));
40        })
41        // Store parent entity for next sections
42        .id();
43
44    // Another way is to use the add_child function to add children after the parent
45    // entity has already been spawned.
46    let child = commands
47        .spawn((
48            Sprite {
49                image: texture,
50                color: LIME.into(),
51                ..default()
52            },
53            Transform::from_xyz(0.0, 250.0, 0.0).with_scale(Vec3::splat(0.75)),
54        ))
55        .id();
56
57    // Add child to the parent.
58    commands.entity(parent).add_child(child);
59}
60
61// A simple system to rotate the root entity, and rotate all its children separately
62fn rotate(
63    mut commands: Commands,
64    time: Res<Time>,
65    mut parents_query: Query<(Entity, &Children), With<Sprite>>,
66    mut transform_query: Query<&mut Transform, With<Sprite>>,
67) {
68    for (parent, children) in &mut parents_query {
69        if let Ok(mut transform) = transform_query.get_mut(parent) {
70            transform.rotate_z(-PI / 2. * time.delta_secs());
71        }
72
73        // To iterate through the entities children, just treat the Children component as a Vec
74        // Alternatively, you could query entities that have a ChildOf component
75        for child in children {
76            if let Ok(mut transform) = transform_query.get_mut(*child) {
77                transform.rotate_z(PI * time.delta_secs());
78            }
79        }
80
81        // To demonstrate removing children, we'll remove a child after a couple of seconds.
82        if time.elapsed_secs() >= 2.0 && children.len() == 2 {
83            let child = children.last().unwrap();
84            commands.entity(*child).despawn();
85        }
86
87        if time.elapsed_secs() >= 4.0 {
88            // This will remove the entity from its parent's list of children, as well as despawn
89            // any children the entity has.
90            commands.entity(parent).despawn();
91        }
92    }
93}
hierarchy/hierarchy.rs

hierarchy/
hierarchy.rs
