bevyjam/src/particle_effect.rs

use bevy::{prelude::*, sprite::Mesh2dHandle};
use rand::Rng;
use rand_distr::{UnitCircle, Distribution};

pub const POOL_COUNT: usize = 100;

pub const MIN_VELOCITY: f32 = 10.0;
pub const MAX_VELOCITY: f32 = 100.0;
pub const VELOCITY_SCALE: f32 = 0.1;
pub const PARTICLE_EFFECT_RADIUS: f32 = 70.0;

pub struct ParticleEffectPlugin;

impl Plugin for ParticleEffectPlugin {
    fn build(&self, app: &mut App) {
        app
			.init_resource::<ParticleMesh>()
            .insert_resource(ParticleEffectResource::new(Vec3::new(10000.0, 10000.0, 0.0)))
            .add_startup_system(particle_effect_startup)
			.add_system(particle_effect_system);
    }
}

pub struct ParticleMesh {
	square: Mesh2dHandle,
}

impl FromWorld for ParticleMesh {
	fn from_world(world: &mut World) -> Self {
		let mut meshes = world.get_resource_mut::<Assets<Mesh>>().unwrap();

		Self {
			square: meshes
				.add(Mesh::from(shape::Quad {
					size: Vec2 { x: 4.0, y: 4.0 },
					flip: false,
				}))
				.into(),
		}
	}
}

#[derive(bevy_inspector_egui::Inspectable)]
pub struct ParticleEffectResource {
    pub translation: Vec3,
	pub prev_translation: Vec3,
    pub radius_squared: f32,
	pub color: Color,
}

impl ParticleEffectResource {
	fn new(init_translation: Vec3) -> Self {
		Self {
			translation: init_translation,
			prev_translation: init_translation,
			radius_squared: PARTICLE_EFFECT_RADIUS * PARTICLE_EFFECT_RADIUS,
			color: Color::WHITE,
		}
	}
}

#[derive(Default, Component)]
pub struct ParticleComponent {
	pub velocity: Vec3,
}

impl ParticleComponent {
	pub fn new() -> Self {
		let mut particle_component: Self = Self::default();
        particle_component.randomize_velocity(MIN_VELOCITY, MAX_VELOCITY);
        return particle_component;
	}

	pub fn randomize_velocity(&mut self, min_velocity: f32, max_velocity: f32) {
        let random_direction: [f32; 2] = UnitCircle.sample(&mut rand::thread_rng());
        let random_magnitude: f32 = rand::thread_rng().gen_range(min_velocity .. max_velocity);
        self.velocity = Vec3::new(random_direction[0], random_direction[1], 0.0) * random_magnitude;
	}
}

fn particle_effect_startup(
    mut commands: Commands,

	particle_mesh: Res<ParticleMesh>,
	mut materials: ResMut<Assets<ColorMaterial>>,
) {
    for _p in 0 .. POOL_COUNT {
        let color_mesh = ColorMesh2dBundle {
			mesh: particle_mesh.square.clone(),
			material: materials.add(ColorMaterial::from(Color::WHITE)),
			..default()
		};

        commands
            .spawn_bundle(color_mesh)
            .insert(ParticleComponent::new());
    }
}

fn particle_effect_system(
	mut materials: ResMut<Assets<ColorMaterial>>,
    mut query: Query<(&mut Transform, &mut ParticleComponent, &Handle<ColorMaterial>)>,
    mut particle_effect: ResMut<ParticleEffectResource>,
    time: Res<Time>,
) {
    let delta_seconds: f32 = f32::max(0.001, time.delta_seconds());
	let delta_position: Vec3 = particle_effect.translation - particle_effect.prev_translation;
	// let overall_velocity: Vec3 = delta_position / delta_seconds;
	particle_effect.prev_translation = particle_effect.translation;

	for (mut transform, mut particle_component, color_material) in query.iter_mut() {
        // particle_component.velocity -= overall_velocity * VELOCITY_SCALE;
        transform.translation += particle_component.velocity * delta_seconds + delta_position;

		let squared_distance: f32 = transform.translation.distance_squared(particle_effect.translation);
        if squared_distance > particle_effect.radius_squared {
			transform.translation = particle_effect.translation;
			particle_component.randomize_velocity(MIN_VELOCITY, MAX_VELOCITY);
		}

		if let Some(material) = materials.get_mut(color_material) {
			material.color = particle_effect.color;
			material.color.set_a((particle_effect.radius_squared - squared_distance) / particle_effect.radius_squared);
		}
	}
}