use bevy_ecs::prelude::*;
use rand::Rng;
use std::ops::Deref;
fn main() {
let mut world = World::new();
world.insert_resource(EntityCounter { value: 0 });
let mut schedule = Schedule::default();
let mut update = SystemStage::parallel();
update.add_system(spawn_entities.label(SimulationSystem::Spawn));
update.add_system(print_counter_when_changed.after(SimulationSystem::Spawn));
update.add_system(age_all_entities.label(SimulationSystem::Age));
update.add_system(remove_old_entities.after(SimulationSystem::Age));
update.add_system(print_changed_entities.after(SimulationSystem::Age));
#[derive(StageLabel)]
struct Update;
schedule.add_stage(Update, update);
for iteration in 1..=10 {
println!("Simulating frame {iteration}/10");
schedule.run(&mut world);
}
}
#[derive(Debug, Resource)]
struct EntityCounter {
pub value: i32,
}
#[derive(Component, Default, Debug)]
struct Age {
frames: i32,
}
#[derive(SystemLabel, Debug, Clone, PartialEq, Eq, Hash)]
enum SimulationSystem {
Spawn,
Age,
}
fn spawn_entities(mut commands: Commands, mut entity_counter: ResMut<EntityCounter>) {
if rand::thread_rng().gen_bool(0.6) {
let entity_id = commands.spawn(Age::default()).id();
println!(" spawning {entity_id:?}");
entity_counter.value += 1;
}
}
fn print_changed_entities(
entity_with_added_component: Query<Entity, Added<Age>>,
entity_with_mutated_component: Query<(Entity, &Age), Changed<Age>>,
) {
for entity in &entity_with_added_component {
println!(" {entity:?} has it's first birthday!");
}
for (entity, value) in &entity_with_mutated_component {
println!(" {entity:?} is now {value:?} frames old");
}
}
fn age_all_entities(mut entities: Query<&mut Age>) {
for mut age in &mut entities {
age.frames += 1;
}
}
fn remove_old_entities(mut commands: Commands, entities: Query<(Entity, &Age)>) {
for (entity, age) in &entities {
if age.frames > 2 {
println!(" despawning {entity:?} due to age > 2");
commands.entity(entity).despawn();
}
}
}
fn print_counter_when_changed(entity_counter: Res<EntityCounter>) {
if entity_counter.is_changed() {
println!(
" total number of entities spawned: {}",
entity_counter.deref().value
);
}
}