//! This example demonstrates how to use the `Camera::viewport_to_world` method.

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, draw_cursor)
        .run();
}

fn draw_cursor(
    camera_query: Single<(&Camera, &GlobalTransform)>,
    ground: Single<&GlobalTransform, With<Ground>>,
    windows: Single<&Window>,
    mut gizmos: Gizmos,
) {
    let (camera, camera_transform) = *camera_query;

    let Some(cursor_position) = windows.cursor_position() else {
        return;
    };

    // Calculate a ray pointing from the camera into the world based on the cursor's position.
    let Ok(ray) = camera.viewport_to_world(camera_transform, cursor_position) else {
        return;
    };

    // Calculate if and where the ray is hitting the ground plane.
    let Some(distance) =
        ray.intersect_plane(ground.translation(), InfinitePlane3d::new(ground.up()))
    else {
        return;
    };
    let point = ray.get_point(distance);

    // Draw a circle just above the ground plane at that position.
    gizmos.circle(
        Isometry3d::new(
            point + ground.up() * 0.01,
            Quat::from_rotation_arc(Vec3::Z, ground.up().as_vec3()),
        ),
        0.2,
        Color::WHITE,
    );
}

#[derive(Component)]
struct Ground;

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // plane
    commands.spawn((
        Mesh3d(meshes.add(Plane3d::default().mesh().size(20., 20.))),
        MeshMaterial3d(materials.add(Color::srgb(0.3, 0.5, 0.3))),
        Ground,
    ));

    // light
    commands.spawn((
        DirectionalLight::default(),
        Transform::from_translation(Vec3::ONE).looking_at(Vec3::ZERO, Vec3::Y),
    ));

    // camera
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(15.0, 5.0, 15.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
}