mirror of
https://github.com/bevyengine/bevy
synced 2024-11-22 20:53:53 +00:00
113 lines
4.2 KiB
Rust
113 lines
4.2 KiB
Rust
use bevy_ecs::prelude::*;
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use rand::Rng;
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use std::ops::Deref;
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// In this example we will simulate a population of entities. In every tick we will:
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// 1. spawn a new entity with a certain possibility
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// 2. age all entities
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// 3. despawn entities with age > 2
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//
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// To demonstrate change detection, there are some console outputs based on changes in
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// the EntityCounter resource and updated Age components
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fn main() {
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// Create a new empty World to hold our Entities, Components and Resources
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let mut world = World::new();
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// Add the counter resource to remember how many entities where spawned
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world.insert_resource(EntityCounter { value: 0 });
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// Create a new Schedule, which stores systems and controls their relative ordering
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let mut schedule = Schedule::default();
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// Add systems to the Schedule to execute our app logic
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// We can label our systems to force a specific run-order between some of them
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schedule.add_systems((
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spawn_entities.in_set(SimulationSet::Spawn),
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print_counter_when_changed.after(SimulationSet::Spawn),
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age_all_entities.in_set(SimulationSet::Age),
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remove_old_entities.after(SimulationSet::Age),
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print_changed_entities.after(SimulationSet::Age),
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));
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// Simulate 10 frames in our world
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for iteration in 1..=10 {
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println!("Simulating frame {iteration}/10");
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schedule.run(&mut world);
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}
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}
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// This struct will be used as a Resource keeping track of the total amount of spawned entities
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#[derive(Debug, Resource)]
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struct EntityCounter {
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pub value: i32,
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}
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// This struct represents a Component and holds the age in frames of the entity it gets assigned to
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#[derive(Component, Default, Debug)]
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struct Age {
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frames: i32,
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}
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// System sets can be used to group systems and configured to control relative ordering
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#[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
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enum SimulationSet {
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Spawn,
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Age,
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}
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// This system randomly spawns a new entity in 60% of all frames
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// The entity will start with an age of 0 frames
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// If an entity gets spawned, we increase the counter in the EntityCounter resource
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fn spawn_entities(mut commands: Commands, mut entity_counter: ResMut<EntityCounter>) {
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if rand::thread_rng().gen_bool(0.6) {
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let entity_id = commands.spawn(Age::default()).id();
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println!(" spawning {entity_id:?}");
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entity_counter.value += 1;
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}
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}
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// This system prints out changes in our entity collection
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// For every entity that just got the Age component added we will print that it's the
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// entities first birthday. These entities where spawned in the previous frame.
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// For every entity with a changed Age component we will print the new value.
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// In this example the Age component is changed in every frame, so we don't actually
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// need the `Changed` here, but it is still used for the purpose of demonstration.
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fn print_changed_entities(
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entity_with_added_component: Query<Entity, Added<Age>>,
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entity_with_mutated_component: Query<(Entity, &Age), Changed<Age>>,
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) {
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for entity in &entity_with_added_component {
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println!(" {entity:?} has it's first birthday!");
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}
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for (entity, value) in &entity_with_mutated_component {
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println!(" {entity:?} is now {value:?} frames old");
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}
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}
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// This system iterates over all entities and increases their age in every frame
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fn age_all_entities(mut entities: Query<&mut Age>) {
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for mut age in &mut entities {
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age.frames += 1;
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}
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}
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// This system iterates over all entities in every frame and despawns entities older than 2 frames
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fn remove_old_entities(mut commands: Commands, entities: Query<(Entity, &Age)>) {
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for (entity, age) in &entities {
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if age.frames > 2 {
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println!(" despawning {entity:?} due to age > 2");
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commands.entity(entity).despawn();
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}
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}
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}
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// This system will print the new counter value every time it was changed since
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// the last execution of the system.
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fn print_counter_when_changed(entity_counter: Res<EntityCounter>) {
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if entity_counter.is_changed() {
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println!(
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" total number of entities spawned: {}",
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entity_counter.deref().value
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);
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}
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}
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