track cursor

src/boids.rs

@@ -3,6 +3,7 @@ use std::f32::consts::PI;
 
 use crate::{
     config::ModelConfig,
+    pointer::PointerTarget,
     spatial_grid::{SpatialGrid, update_spatial_grid},
 };
 
@@ -122,6 +123,7 @@ fn update_boids(
     window: Single<&Window, With<bevy::window::PrimaryWindow>>,
     time: Res<Time<Fixed>>,
     grid: Res<SpatialGrid>,
+    pointer: Res<PointerTarget>,
     mut query: Query<(Entity, &mut Boid, &mut Transform)>,
 ) {
     let dt = time.delta_secs();
@@ -194,8 +196,18 @@ fn update_boids(
         // Push each boid forward along its current heading
         let self_force = boid.velocity.normalize_or_zero() * model_config.self_acceleration;
 
-        let acceleration =
+        // Pull towards the mouse cursor or touch point
-            boundary_force + separation_force + alignment_force + cohesion_force + self_force;
+        let attraction_force = pointer.0.map_or(Vec2::ZERO, |target| {
+            (target - transform.translation.xy()).normalize_or_zero()
+                * model_config.attraction_force
+        });
+
+        let acceleration = boundary_force
+            + separation_force
+            + alignment_force
+            + cohesion_force
+            + self_force
+            + attraction_force;
         boid.velocity += acceleration * dt;
         boid.velocity = boid.velocity.clamp_length_max(model_config.max_velocity);
 

src/config.rs

@@ -22,6 +22,7 @@ pub struct ModelConfig {
     pub cohesion_factor: f32,
     pub separation_factor: f32,
     pub self_acceleration: f32,
+    pub attraction_force: f32,
 }
 
 impl Default for ModelConfig {
@@ -36,7 +37,8 @@ impl Default for ModelConfig {
             alignment_factor: 0.3,
             cohesion_factor: 0.2,
             separation_factor: 500.0,
-            self_acceleration: 30.0,
+            self_acceleration: 100.0,
+            attraction_force: 100.0,
         }
     }
 }

src/main.rs

@@ -1,5 +1,6 @@
 mod boids;
 mod config;
+mod pointer;
 mod spatial_grid;
 mod ui;
 
@@ -18,7 +19,12 @@ fn main() {
             }),
             ..default()
         }))
-        .add_plugins((config::ConfigPlugin, boids::BoidsPlugin, ui::ConfigUiPlugin))
+        .add_plugins((
+            config::ConfigPlugin,
+            pointer::PointerPlugin,
+            boids::BoidsPlugin,
+            ui::ConfigUiPlugin,
+        ))
         .add_systems(Startup, |mut commands: Commands| {
             commands.spawn(Camera2d);
         })

src/pointer.rs

@@ -0,0 +1,28 @@
+use bevy::{prelude::*, window::PrimaryWindow};
+
+pub struct PointerPlugin;
+
+impl Plugin for PointerPlugin {
+    fn build(&self, app: &mut App) {
+        app.init_resource::<PointerTarget>()
+            .add_systems(Update, update_pointer_target);
+    }
+}
+
+/// World position of the active touch or mouse cursor, if any.
+#[derive(Resource, Default)]
+pub struct PointerTarget(pub Option<Vec2>);
+
+fn update_pointer_target(
+    window: Single<&Window, With<PrimaryWindow>>,
+    camera: Single<(&Camera, &GlobalTransform)>,
+    touches: Res<Touches>,
+    mut target: ResMut<PointerTarget>,
+) {
+    let (camera, camera_transform) = *camera;
+    // Touch and cursor positions are both in logical window pixels
+    target.0 = touches
+        .first_pressed_position()
+        .or_else(|| window.cursor_position())
+        .and_then(|position| camera.viewport_to_world_2d(camera_transform, position).ok());
+}

src/ui.rs

@@ -28,6 +28,7 @@ const CONFIG_FIELDS: &[(&str, f32)] = &[
     ("cohesion_factor", 0.05),
     ("separation_factor", 10.0),
     ("self_acceleration", 5.0),
+    ("attraction_force", 10.0),
 ];
 
 #[derive(Component)]