This example is running in WebGL2 and should work in most browsers. You can check the WebGPU examples here.
observers.rs:
//! Demonstrates how to observe life-cycle triggers as well as define custom ones.
use bevy::{
prelude::*,
utils::{HashMap, HashSet},
};
use rand::{Rng, SeedableRng};
use rand_chacha::ChaCha8Rng;
fn main() {
App::new()
.add_plugins(DefaultPlugins)
.init_resource::<SpatialIndex>()
.add_systems(Startup, setup)
.add_systems(Update, (draw_shapes, handle_click))
// Observers are systems that run when an event is "triggered". This observer runs whenever
// `ExplodeMines` is triggered.
.observe(
|trigger: Trigger<ExplodeMines>,
mines: Query<&Mine>,
index: Res<SpatialIndex>,
mut commands: Commands| {
// You can access the trigger data via the `Observer`
let event = trigger.event();
// Access resources
for e in index.get_nearby(event.pos) {
// Run queries
let mine = mines.get(e).unwrap();
if mine.pos.distance(event.pos) < mine.size + event.radius {
// And queue commands, including triggering additional events
// Here we trigger the `Explode` event for entity `e`
commands.trigger_targets(Explode, e);
}
}
},
)
// This observer runs whenever the `Mine` component is added to an entity, and places it in a simple spatial index.
.observe(on_add_mine)
// This observer runs whenever the `Mine` component is removed from an entity (including despawning it)
// and removes it from the spatial index.
.observe(on_remove_mine)
.run();
}
#[derive(Component)]
struct Mine {
pos: Vec2,
size: f32,
}
impl Mine {
fn random(rand: &mut ChaCha8Rng) -> Self {
Mine {
pos: Vec2::new(
(rand.gen::<f32>() - 0.5) * 1200.0,
(rand.gen::<f32>() - 0.5) * 600.0,
),
size: 4.0 + rand.gen::<f32>() * 16.0,
}
}
}
#[derive(Event)]
struct ExplodeMines {
pos: Vec2,
radius: f32,
}
#[derive(Event)]
struct Explode;
fn setup(mut commands: Commands) {
commands.spawn(Camera2dBundle::default());
commands.spawn(
TextBundle::from_section(
"Click on a \"Mine\" to trigger it.\n\
When it explodes it will trigger all overlapping mines.",
TextStyle {
color: Color::WHITE,
..default()
},
)
.with_style(Style {
position_type: PositionType::Absolute,
top: Val::Px(12.),
left: Val::Px(12.),
..default()
}),
);
let mut rng = ChaCha8Rng::seed_from_u64(19878367467713);
commands
.spawn(Mine::random(&mut rng))
// Observers can watch for events targeting a specific entity.
// This will create a new observer that runs whenever the Explode event
// is triggered for this spawned entity.
.observe(explode_mine);
// We want to spawn a bunch of mines. We could just call the code above for each of them.
// That would create a new observer instance for every Mine entity. Having duplicate observers
// generally isn't worth worrying about as the overhead is low. But if you want to be maximally efficient,
// you can reuse observers across entities.
//
// First, observers are actually just entities with the Observer component! The `observe()` functions
// you've seen so far in this example are just shorthand for manually spawning an observer.
let mut observer = Observer::new(explode_mine);
// As we spawn entities, we can make this observer watch each of them:
for _ in 0..1000 {
let entity = commands.spawn(Mine::random(&mut rng)).id();
observer.watch_entity(entity);
}
// By spawning the Observer component, it becomes active!
commands.spawn(observer);
}
fn on_add_mine(
trigger: Trigger<OnAdd, Mine>,
query: Query<&Mine>,
mut index: ResMut<SpatialIndex>,
) {
let mine = query.get(trigger.entity()).unwrap();
let tile = (
(mine.pos.x / CELL_SIZE).floor() as i32,
(mine.pos.y / CELL_SIZE).floor() as i32,
);
index.map.entry(tile).or_default().insert(trigger.entity());
}
// Remove despawned mines from our index
fn on_remove_mine(
trigger: Trigger<OnRemove, Mine>,
query: Query<&Mine>,
mut index: ResMut<SpatialIndex>,
) {
let mine = query.get(trigger.entity()).unwrap();
let tile = (
(mine.pos.x / CELL_SIZE).floor() as i32,
(mine.pos.y / CELL_SIZE).floor() as i32,
);
index.map.entry(tile).and_modify(|set| {
set.remove(&trigger.entity());
});
}
fn explode_mine(trigger: Trigger<Explode>, query: Query<&Mine>, mut commands: Commands) {
// If a triggered event is targeting a specific entity you can access it with `.entity()`
let id = trigger.entity();
let Some(mut entity) = commands.get_entity(id) else {
return;
};
info!("Boom! {:?} exploded.", id.index());
entity.despawn();
let mine = query.get(id).unwrap();
// Trigger another explosion cascade.
commands.trigger(ExplodeMines {
pos: mine.pos,
radius: mine.size,
});
}
// Draw a circle for each mine using `Gizmos`
fn draw_shapes(mut gizmos: Gizmos, mines: Query<&Mine>) {
for mine in &mines {
gizmos.circle_2d(
mine.pos,
mine.size,
Color::hsl((mine.size - 4.0) / 16.0 * 360.0, 1.0, 0.8),
);
}
}
// Trigger `ExplodeMines` at the position of a given click
fn handle_click(
mouse_button_input: Res<ButtonInput<MouseButton>>,
camera: Query<(&Camera, &GlobalTransform)>,
windows: Query<&Window>,
mut commands: Commands,
) {
let (camera, camera_transform) = camera.single();
if let Some(pos) = windows
.single()
.cursor_position()
.and_then(|cursor| camera.viewport_to_world(camera_transform, cursor))
.map(|ray| ray.origin.truncate())
{
if mouse_button_input.just_pressed(MouseButton::Left) {
commands.trigger(ExplodeMines { pos, radius: 1.0 });
}
}
}
#[derive(Resource, Default)]
struct SpatialIndex {
map: HashMap<(i32, i32), HashSet<Entity>>,
}
/// Cell size has to be bigger than any `TriggerMine::radius`
const CELL_SIZE: f32 = 64.0;
impl SpatialIndex {
// Lookup all entities within adjacent cells of our spatial index
fn get_nearby(&self, pos: Vec2) -> Vec<Entity> {
let tile = (
(pos.x / CELL_SIZE).floor() as i32,
(pos.y / CELL_SIZE).floor() as i32,
);
let mut nearby = Vec::new();
for x in -1..2 {
for y in -1..2 {
if let Some(mines) = self.map.get(&(tile.0 + x, tile.1 + y)) {
nearby.extend(mines.iter());
}
}
}
nearby
}
}