b34833f00c
# Objective After #17398, Bevy now has relations! We don't teach users how to make / work with these in the examples yet though, but we definitely should. ## Solution - Add a simple abstract example that goes over defining, spawning, traversing and removing a custom relations. - ~~Add `Relationship` and `RelationshipTarget` to the prelude: the trait methods are really helpful here.~~ - this causes subtle ambiguities with method names and weird compiler errors. Not doing it here! - Clean up related documentation that I referenced when writing this example. ## Testing `cargo run --example relationships` ## Notes to reviewers 1. Yes, I know that the cycle detection code could be more efficient. I decided to reduce the caching to avoid distracting from the broader point of "here's how you traverse relationships". 2. Instead of using an `App`, I've decide to use `World::run_system_once` + system functions defined inside of `main` to do something closer to literate programming. --------- Co-authored-by: Joona Aalto <jondolf.dev@gmail.com> Co-authored-by: MinerSebas <66798382+MinerSebas@users.noreply.github.com> Co-authored-by: Kristoffer Søholm <k.soeholm@gmail.com>
94 lines
3.3 KiB
Rust
94 lines
3.3 KiB
Rust
//! Demonstrates techniques for creating a hierarchy of parent and child entities.
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//!
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//! When [`DefaultPlugins`] are added to your app, systems are automatically added to propagate
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//! [`Transform`] and [`Visibility`] from parents to children down the hierarchy,
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//! resulting in a final [`GlobalTransform`] and [`InheritedVisibility`] component for each entity.
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use std::f32::consts::*;
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use bevy::{color::palettes::css::*, prelude::*};
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_systems(Startup, setup)
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.add_systems(Update, rotate)
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.run();
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}
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fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
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commands.spawn(Camera2d);
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let texture = asset_server.load("branding/icon.png");
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// Spawn a root entity with no parent
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let parent = commands
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.spawn((
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Sprite::from_image(texture.clone()),
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Transform::from_scale(Vec3::splat(0.75)),
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))
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// With that entity as a parent, run a lambda that spawns its children
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.with_children(|parent| {
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// parent is a ChildSpawnerCommands, which has a similar API to Commands
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parent.spawn((
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Transform::from_xyz(250.0, 0.0, 0.0).with_scale(Vec3::splat(0.75)),
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Sprite {
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image: texture.clone(),
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color: BLUE.into(),
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..default()
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},
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));
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})
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// Store parent entity for next sections
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.id();
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// Another way is to use the add_child function to add children after the parent
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// entity has already been spawned.
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let child = commands
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.spawn((
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Sprite {
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image: texture,
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color: LIME.into(),
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..default()
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},
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Transform::from_xyz(0.0, 250.0, 0.0).with_scale(Vec3::splat(0.75)),
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))
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.id();
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// Add child to the parent.
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commands.entity(parent).add_child(child);
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}
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// A simple system to rotate the root entity, and rotate all its children separately
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fn rotate(
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mut commands: Commands,
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time: Res<Time>,
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mut parents_query: Query<(Entity, &Children), With<Sprite>>,
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mut transform_query: Query<&mut Transform, With<Sprite>>,
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) {
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for (parent, children) in &mut parents_query {
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if let Ok(mut transform) = transform_query.get_mut(parent) {
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transform.rotate_z(-PI / 2. * time.delta_secs());
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}
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// To iterate through the entities children, just treat the Children component as a Vec
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// Alternatively, you could query entities that have a ChildOf component
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for child in children {
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if let Ok(mut transform) = transform_query.get_mut(*child) {
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transform.rotate_z(PI * time.delta_secs());
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}
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}
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// To demonstrate removing children, we'll remove a child after a couple of seconds.
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if time.elapsed_secs() >= 2.0 && children.len() == 2 {
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let child = children.last().unwrap();
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commands.entity(*child).despawn();
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}
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if time.elapsed_secs() >= 4.0 {
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// This will remove the entity from its parent's list of children, as well as despawn
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// any children the entity has.
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commands.entity(parent).despawn();
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}
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}
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}
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