website/app/src/main.rs

use avian2d::prelude::*;
use bevy::{
    input::common_conditions::input_just_pressed,
    prelude::*,
    window::{PrimaryWindow, WindowResized},
};

fn main() {
    App::new()
        .insert_resource(ClearColor(Color::srgb_u8(26, 26, 46)))
        .add_plugins((
            DefaultPlugins.set(WindowPlugin {
                primary_window: Some(Window {
                    fit_canvas_to_parent: true,
                    prevent_default_event_handling: false,
                    canvas: Some("#bevy-canvas".into()),
                    ..default()
                }),
                ..default()
            }),
            PhysicsPlugins::default(),
        ))
        .insert_resource(Time::<Physics>::default().with_relative_speed(5.0))
        .add_systems(Startup, setup)
        .add_systems(
            Update,
            (
                update_screen_edges,
                spawn_ball.run_if(input_just_pressed(MouseButton::Left)),
            ),
        )
        .run();
}

#[derive(Component, Clone, Copy)]
enum ScreenEdge {
    Top,
    Bottom,
    Left,
    Right,
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
    window: Query<&Window, With<PrimaryWindow>>,
) {
    commands.spawn(Camera2d);

    if let Ok(window) = window.single() {
        spawn_screen_edges(&mut commands, &mut meshes, &mut materials, window.size());
    }
}

#[derive(Component, Clone, Copy)]
struct Ball;

fn spawn_ball(
    mouse_button: Res<ButtonInput<MouseButton>>,
    window: Single<&Window, With<PrimaryWindow>>,
    camera_query: Single<(&Camera, &GlobalTransform)>,
    time: Res<Time>,
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    const CIRCLE_RADIUS: f32 = 5.0;

    let (camera, camera_transform) = *camera_query;
    if mouse_button.just_pressed(MouseButton::Left)
        && let Some(cursor_position) = window.cursor_position()
        && let Ok(world_position) = camera.viewport_to_world_2d(camera_transform, cursor_position)
    {
        commands.spawn((
            RigidBody::Dynamic,
            Collider::circle(CIRCLE_RADIUS),
            Restitution::PERFECTLY_ELASTIC.with_combine_rule(CoefficientCombine::Max),
            Mesh2d(meshes.add(Circle::new(CIRCLE_RADIUS))),
            MeshMaterial2d(materials.add(Color::hsl(time.elapsed_secs().sin() * 360.0, 1.0, 0.5))),
            Transform::from_xyz(world_position.x, world_position.y, 0.0),
            Ball,
        ));
    }
}

fn spawn_screen_edges(
    commands: &mut Commands,
    meshes: &mut Assets<Mesh>,
    materials: &mut Assets<ColorMaterial>,
    size: Vec2,
) {
    const EDGE_COLOR: Color = Color::srgb_u8(255, 255, 255);

    let material = materials.add(EDGE_COLOR);

    for edge in [
        ScreenEdge::Top,
        ScreenEdge::Bottom,
        ScreenEdge::Left,
        ScreenEdge::Right,
    ] {
        let (collider, transform, rect_size) = edge_shape(edge, size);
        commands.spawn((
            RigidBody::Static,
            collider,
            transform,
            edge,
            Mesh2d(meshes.add(Rectangle::from_size(rect_size))),
            MeshMaterial2d(material.clone()),
        ));
    }
}

fn edge_shape(edge: ScreenEdge, size: Vec2) -> (Collider, Transform, Vec2) {
    const EDGE_THICKNESS: f32 = 5.0;

    let half = size / 2.0;
    let thickness = EDGE_THICKNESS;

    match edge {
        ScreenEdge::Top => {
            let rect = Vec2::new(size.x, thickness);
            (
                Collider::rectangle(rect.x, rect.y),
                Transform::from_xyz(0.0, half.y - thickness / 2.0, 0.0),
                rect,
            )
        }
        ScreenEdge::Bottom => {
            let rect = Vec2::new(size.x, thickness);
            (
                Collider::rectangle(rect.x, rect.y),
                Transform::from_xyz(0.0, -half.y + thickness / 2.0, 0.0),
                rect,
            )
        }
        ScreenEdge::Left => {
            let rect = Vec2::new(thickness, size.y);
            (
                Collider::rectangle(rect.x, rect.y),
                Transform::from_xyz(-half.x + thickness / 2.0, 0.0, 0.0),
                rect,
            )
        }
        ScreenEdge::Right => {
            let rect = Vec2::new(thickness, size.y);
            (
                Collider::rectangle(rect.x, rect.y),
                Transform::from_xyz(half.x - thickness / 2.0, 0.0, 0.0),
                rect,
            )
        }
    }
}

fn update_screen_edges(
    mut resized: PopulatedMessageReader<WindowResized>,
    mut edges: Query<(&ScreenEdge, &mut Collider, &mut Transform, &Mesh2d)>,
    mut meshes: ResMut<Assets<Mesh>>,
) {
    for event in resized.read() {
        let size = Vec2::new(event.width, event.height);
        for (edge, mut collider, mut transform, mesh2d) in &mut edges {
            let (new_collider, new_transform, rect_size) = edge_shape(*edge, size);
            *collider = new_collider;
            *transform = new_transform;
            if let Some(mut mesh) = meshes.get_mut(&mesh2d.0) {
                *mesh = Rectangle::from_size(rect_size).into();
            }
        }
    }
}