Проектная работа - MVP

Создание нового проекта

snake-game/examples/bevy_learning.rs

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+/// Эксперименты в ходе прохождения туториала:
+/// https://github.com/fogarecious/bevy_tutorial
+use bevy::{prelude::*, winit::WinitSettings};
+use tracing::Level;
+
+fn main() {
+    App::new()
+        // .add_plugins(MinimalPlugins)
+        .add_plugins(
+            DefaultPlugins
+                .build()
+                .set(bevy::log::LogPlugin {
+                    level: Level::INFO,
+                    filter: "wgpu_hal=warn,bevy_learning=trace".to_string(),
+                    // fmt_layer: |_| snake_game::define_log_layer(),
+                    ..Default::default()
+                })
+                .set(TaskPoolPlugin {
+                    task_pool_options: TaskPoolOptions::with_num_threads(1),
+                }),
+        )
+        .insert_resource(WinitSettings::desktop_app())
+        .insert_resource(ClearColor(Color::srgb(0.3, 0.6, 0.8)))
+        .add_systems(Startup, setup)
+        .add_systems(PostStartup, (print_all_entities, inspect))
+        .run();
+}
+
+fn setup(mut commands: Commands, _window: Query<&Window>, _world: &World) {
+    let _window = _window.single().unwrap();
+
+    let _root = commands
+        .spawn((
+            Sprite::from_color(Color::srgb(1., 1., 1.), Vec2::new(100., 100.)),
+            Transform {
+                translation: vec3(0., 0., 0.),
+                // scale: Vec3::new(1., 1., 1.),
+                ..Default::default()
+            },
+        ))
+        .id();
+    let _red = commands
+        .spawn((
+            Sprite::from_color(Color::srgb(1., 0., 0.), Vec2::new(30., 30.)),
+            Transform {
+                translation: vec3(-20., 30., 0.),
+                ..Default::default()
+            },
+            ChildOf(_root),
+        ))
+        .id();
+    let _blue = commands
+        .spawn((
+            Sprite::from_color(Color::srgb(0., 0., 1.), Vec2::new(20., 20.)),
+            Transform {
+                translation: vec3(20., 30., 0.),
+                ..Default::default()
+            },
+            ChildOf(_root),
+        ))
+        .id();
+
+    commands.spawn(Camera2d);
+}
+
+fn print_all_entities(mut commands: Commands, query: Query<Entity>) {
+    for entity in query {
+        commands.entity(entity).log_components();
+    }
+}
+
+fn inspect(mut _commands: Commands, _query: Query<Entity>) {}

snake-game/examples/bevy_tutorial.rs

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+use bevy::prelude::*;
+
+fn main() {
+    App::new().add_plugins(DefaultPlugins).add_plugins(HelloPlugin).run();
+}
+
+#[derive(Component)]
+struct Person;
+
+#[derive(Component)]
+struct Name(String);
+
+#[derive(Resource)]
+struct GreetTimer(Timer);
+
+fn add_people(mut commands: Commands) {
+    commands.spawn((Person, Name("Elaina Proctor".to_string())));
+    commands.spawn((Person, Name("Renzo Hume".to_string())));
+    commands.spawn((Person, Name("Zaina Nieves".to_string())));
+}
+
+fn greet_people(time: Res<Time>, mut timer: ResMut<GreetTimer>, query: Query<&Name, With<Person>>) {
+    // update uor timer with the time elapsed since the last update
+    // if that causes the timer to finish, we say hello to everyone
+    if timer.0.tick(time.delta()).just_finished() {
+        for name in &query {
+            info!("hello {}!", name.0);
+        }
+    }
+}
+
+fn update_people(mut query: Query<&mut Name, With<Person>>) {
+    for mut name in &mut query {
+        if name.0 == "Elaina Proctor" {
+            name.0 = "Elaina Hume".to_string();
+            break; // We don't need to change any other names.
+        }
+    }
+}
+
+pub struct HelloPlugin;
+
+impl Plugin for HelloPlugin {
+    fn build(&self, app: &mut App) {
+        app.insert_resource(GreetTimer(Timer::from_seconds(2.0, TimerMode::Repeating)));
+        app.add_systems(Startup, add_people);
+        app.add_systems(Update, (update_people, greet_people).chain());
+    }
+}

snake-game/examples/cpu_draws.rs

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+//! Example of how to draw to a texture from the CPU.
+//!
+//! You can set the values of individual pixels to whatever you want.
+//! Bevy provides user-friendly APIs that work with [`Color`]
+//! values and automatically perform any necessary conversions and encoding
+//! into the texture's native pixel format.
+
+use bevy::asset::RenderAssetUsages;
+use bevy::color::{color_difference::EuclideanDistance, palettes::css};
+use bevy::prelude::*;
+use bevy::render::render_resource::{Extent3d, TextureDimension, TextureFormat};
+use rand::{RngExt, SeedableRng};
+use rand_chacha::ChaCha8Rng;
+
+const IMAGE_WIDTH: u32 = 256;
+const IMAGE_HEIGHT: u32 = 256;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        // In this example, we will use a fixed timestep to draw a pattern on the screen
+        // one pixel at a time, so the pattern will gradually emerge over time, and
+        // the speed at which it appears is not tied to the framerate.
+        // Let's make the fixed update very fast, so it doesn't take too long. :)
+        .insert_resource(Time::<Fixed>::from_hz(1024.0))
+        .add_systems(Startup, setup)
+        .add_systems(FixedUpdate, draw)
+        .run();
+}
+
+/// Store the image handle that we will draw to, here.
+#[derive(Resource)]
+struct MyProcGenImage(Handle<Image>);
+
+#[derive(Resource)]
+struct SeededRng(ChaCha8Rng);
+
+fn setup(mut commands: Commands, mut images: ResMut<Assets<Image>>) {
+    commands.spawn(Camera2d);
+
+    // Create an image that we are going to draw into
+    let mut image = Image::new_fill(
+        // 2D image of size 256x256
+        Extent3d {
+            width: IMAGE_WIDTH,
+            height: IMAGE_HEIGHT,
+            depth_or_array_layers: 1,
+        },
+        TextureDimension::D2,
+        // Initialize it with a beige color
+        &(css::BEIGE.to_u8_array()),
+        // Use the same encoding as the color we set
+        TextureFormat::Rgba8UnormSrgb,
+        RenderAssetUsages::MAIN_WORLD | RenderAssetUsages::RENDER_WORLD,
+    );
+
+    // To make it extra fancy, we can set the Alpha of each pixel,
+    // so that it fades out in a circular fashion.
+    for y in 0..IMAGE_HEIGHT {
+        for x in 0..IMAGE_WIDTH {
+            let center = Vec2::new(IMAGE_WIDTH as f32 / 2.0, IMAGE_HEIGHT as f32 / 2.0);
+            let max_radius = IMAGE_HEIGHT.min(IMAGE_WIDTH) as f32 / 2.0;
+            let r = Vec2::new(x as f32, y as f32).distance(center);
+            let a = 1.0 - (r / max_radius).clamp(0.0, 1.0);
+
+            // Here we will set the A value by accessing the raw data bytes.
+            // (it is the 4th byte of each pixel, as per our `TextureFormat`)
+
+            // Find our pixel by its coordinates
+            let pixel_bytes = image.pixel_bytes_mut(UVec3::new(x, y, 0)).unwrap();
+            // Convert our f32 to u8
+            pixel_bytes[3] = (a * u8::MAX as f32) as u8;
+        }
+    }
+
+    // Add it to Bevy's assets, so it can be used for rendering
+    // this will give us a handle we can use
+    // (to display it in a sprite, or as part of UI, etc.)
+    let handle = images.add(image);
+
+    // Create a sprite entity using our image
+    commands.spawn(Sprite::from_image(handle.clone()));
+    commands.insert_resource(MyProcGenImage(handle));
+
+    // We're seeding the PRNG here to make this example deterministic for testing purposes.
+    // This isn't strictly required in practical use unless you need your app to be deterministic.
+    let seeded_rng = ChaCha8Rng::seed_from_u64(19878367467712);
+    commands.insert_resource(SeededRng(seeded_rng));
+}
+
+/// Every fixed update tick, draw one more pixel to make a spiral pattern
+fn draw(
+    my_handle: Res<MyProcGenImage>,
+    mut images: ResMut<Assets<Image>>,
+    // Used to keep track of where we are
+    mut i: Local<u32>,
+    mut draw_color: Local<Color>,
+    mut seeded_rng: ResMut<SeededRng>,
+) {
+    if *i == 0 {
+        // Generate a random color on first run.
+        *draw_color = Color::linear_rgb(seeded_rng.0.random(), seeded_rng.0.random(), seeded_rng.0.random());
+    }
+
+    // Get the image from Bevy's asset storage.
+    let image = images.get_mut(&my_handle.0).expect("Image not found");
+
+    // Compute the position of the pixel to draw.
+
+    let center = Vec2::new(IMAGE_WIDTH as f32 / 2.0, IMAGE_HEIGHT as f32 / 2.0);
+    let max_radius = IMAGE_HEIGHT.min(IMAGE_WIDTH) as f32 / 2.0;
+    let rot_speed = 0.0123;
+    let period = 0.12345;
+
+    let r = ops::sin(*i as f32 * period) * max_radius;
+    let xy = Vec2::from_angle(*i as f32 * rot_speed) * r + center;
+    let (x, y) = (xy.x as u32, xy.y as u32);
+
+    // Get the old color of that pixel.
+    let old_color = image.get_color_at(x, y).unwrap();
+
+    // If the old color is our current color, change our drawing color.
+    let tolerance = 1.0 / 255.0;
+    if old_color.distance(&draw_color) <= tolerance {
+        *draw_color = Color::linear_rgb(seeded_rng.0.random(), seeded_rng.0.random(), seeded_rng.0.random());
+    }
+
+    // Set the new color, but keep old alpha value from image.
+    image.set_color_at(x, y, draw_color.with_alpha(old_color.alpha())).unwrap();
+
+    *i += 1;
+}

snake-game/examples/observers.rs

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+use bevy::{
+    platform::collections::{HashMap, HashSet},
+    prelude::*,
+};
+use rand::{RngExt, 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.
+        .add_observer(
+            |explode_mines: On<ExplodeMines>, mines: Query<&Mine>, index: Res<SpatialIndex>, mut commands: Commands| {
+                // Access resources
+                for entity in index.get_nearby(explode_mines.pos) {
+                    // Run queries
+                    let mine = mines.get(entity).unwrap();
+                    if mine.pos.distance(explode_mines.pos) < mine.size + explode_mines.radius {
+                        // And queue commands, including triggering additional events
+                        // Here we trigger the `Explode` event for entity `e`
+                        commands.trigger(Explode { entity });
+                    }
+                }
+            },
+        )
+        // This observer runs whenever the `Mine` component is added to an entity, and places it in a simple spatial index.
+        .add_observer(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.
+        .add_observer(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.random::<f32>() - 0.5) * 1200.0, (rand.random::<f32>() - 0.5) * 600.0),
+            size: 4.0 + rand.random::<f32>() * 16.0,
+        }
+    }
+}
+
+/// This is a normal [`Event`]. Any observer that watches for it will run when it is triggered.
+#[derive(Event)]
+struct ExplodeMines {
+    pos: Vec2,
+    radius: f32,
+}
+
+/// An [`EntityEvent`] is a specialized type of [`Event`] that can target a specific entity. In addition to
+/// running normal "top level" observers when it is triggered (which target _any_ entity that Explodes), it will
+/// also run any observers that target the _specific_ entity for that event.
+#[derive(EntityEvent)]
+struct Explode {
+    entity: Entity,
+}
+
+fn setup(mut commands: Commands) {
+    commands.spawn(Camera2d);
+    commands.spawn((
+        Text::new(
+            "Click on a \"Mine\" to trigger it.\n\
+            When it explodes it will trigger all overlapping mines.",
+        ),
+        Node {
+            position_type: PositionType::Absolute,
+            top: px(12),
+            left: 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(add: On<Add, Mine>, query: Query<&Mine>, mut index: ResMut<SpatialIndex>) {
+    let mine = query.get(add.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(add.entity);
+}
+
+// Remove despawned mines from our index
+fn on_remove_mine(remove: On<Remove, Mine>, query: Query<&Mine>, mut index: ResMut<SpatialIndex>) {
+    let mine = query.get(remove.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(&remove.entity);
+    });
+}
+
+fn explode_mine(explode: On<Explode>, query: Query<&Mine>, mut commands: Commands) {
+    // Explode is an EntityEvent. `explode.entity` is the entity that Explode was triggered for.
+    let Ok(mut entity) = commands.get_entity(explode.entity) else {
+        return;
+    };
+    info!("Boom! {} exploded.", explode.entity);
+    entity.despawn();
+    let mine = query.get(explode.entity).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: Single<(&Camera, &GlobalTransform)>,
+    windows: Query<&Window>,
+    mut commands: Commands,
+) {
+    let Ok(windows) = windows.single() else {
+        return;
+    };
+
+    let (camera, camera_transform) = *camera;
+    if let Some(pos) = windows
+        .cursor_position()
+        .and_then(|cursor| camera.viewport_to_world(camera_transform, cursor).ok())
+        .map(|ray| ray.origin.truncate())
+        && 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
+    }
+}

snake-game/examples/paddle_demo.rs

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+use bevy::math::bounding::{Aabb2d, BoundingCircle, BoundingVolume, IntersectsVolume};
+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .insert_resource(ClearColor(Color::srgb(0.8, 0.8, 1.0)))
+        .add_systems(Startup, setup)
+        .add_systems(FixedUpdate, (move_paddle, apply_velocity, check_for_collisions))
+        .run();
+}
+
+const PADDLE_GAP_Y: f32 = 50.0;
+const PADDLE_COLOR: Color = Color::srgb(0.5, 0.5, 1.0);
+const PADDLE_SIZE: Vec2 = Vec2::new(120.0, 20.0);
+const PADDLE_SPEED: f32 = 500.0;
+const PADDLE_PADDING: f32 = 10.0;
+
+const LEFT_WALL: f32 = -600.0;
+const RIGHT_WALL: f32 = 600.0;
+const BOTTOM_WALL: f32 = -300.0;
+const TOP_WALL: f32 = 300.0;
+const WALL_WIDTH: f32 = 20.0;
+
+const BALL_START_POS: Vec2 = Vec2::new(0.0, -100.0);
+const BALL_COLOR: Color = Color::srgb(0.2, 0.5, 0.2);
+const BALL_DIAMETER: f32 = 20.0;
+const INIT_BALL_SPEED: Vec2 = Vec2::new(100.0, 100.0);
+
+#[derive(Component)]
+struct Paddle;
+
+#[derive(Component)]
+struct Ball;
+
+#[derive(Component)]
+struct Collider;
+
+fn setup(mut commands: Commands, mut meshes: ResMut<Assets<Mesh>>, mut materials: ResMut<Assets<ColorMaterial>>) {
+    commands.spawn(Camera2d);
+
+    let paddle_y = BOTTOM_WALL + PADDLE_GAP_Y;
+    commands.spawn((
+        Sprite::from_color(PADDLE_COLOR, Vec2::ONE),
+        Transform {
+            translation: Vec3::new(0.0, paddle_y, 0.0),
+            scale: PADDLE_SIZE.extend(1.0),
+            ..Default::default()
+        },
+        Paddle,
+        Collider,
+    ));
+
+    commands.spawn((
+        Mesh2d(meshes.add(Circle::default())),
+        MeshMaterial2d(materials.add(BALL_COLOR)),
+        Transform {
+            translation: BALL_START_POS.extend(0.0),
+            scale: Vec3::splat(BALL_DIAMETER),
+            ..Default::default()
+        },
+        Velocity(INIT_BALL_SPEED),
+        Ball,
+    ));
+
+    commands.spawn(Wall::new(WallLocation::Left));
+    commands.spawn(Wall::new(WallLocation::Right));
+    commands.spawn(Wall::new(WallLocation::Top));
+    commands.spawn(Wall::new(WallLocation::Bottom));
+}
+
+fn move_paddle(input: Res<ButtonInput<KeyCode>>, time: Res<Time>, mut paddle_transform: Single<&mut Transform, With<Paddle>>) {
+    let direction = if input.pressed(KeyCode::ArrowLeft) {
+        -1.0
+    } else if input.pressed(KeyCode::ArrowRight) {
+        1.0
+    } else {
+        0.0
+    };
+
+    let new_pos = paddle_transform.translation.x + direction * PADDLE_SPEED * time.delta_secs();
+
+    const LEFT_BOUND: f32 = LEFT_WALL + WALL_WIDTH / 2.0 + PADDLE_SIZE.x / 2.0 + PADDLE_PADDING;
+    const RIGHT_BOUND: f32 = RIGHT_WALL - WALL_WIDTH / 2.0 - PADDLE_SIZE.x / 2.0 - PADDLE_PADDING;
+
+    paddle_transform.translation.x = new_pos.clamp(LEFT_BOUND, RIGHT_BOUND);
+}
+
+#[derive(Component, Deref, DerefMut)]
+struct Velocity(Vec2);
+
+fn apply_velocity(time: Res<Time>, mut query: Query<(&mut Transform, &Velocity)>) {
+    for (mut transform, velocity) in &mut query {
+        transform.translation += velocity.extend(0.0) * time.delta_secs();
+    }
+}
+
+fn check_for_collisions(ball_query: Single<(&mut Velocity, &Transform), With<Ball>>, collider_query: Query<&Transform, With<Collider>>) {
+    let (mut ball_velocity, ball_transform) = ball_query.into_inner();
+
+    for collider_transform in &collider_query {
+        let collision = ball_collision(
+            BoundingCircle::new(ball_transform.translation.truncate(), BALL_DIAMETER / 2.0),
+            Aabb2d::new(collider_transform.translation.truncate(), collider_transform.scale.truncate() / 2.0),
+        );
+
+        if let Some(collision) = collision {
+            let mut reflect_x = false;
+            let mut reflect_y = false;
+
+            match collision {
+                Collision::Left => reflect_x = ball_velocity.x > 0.0,
+                Collision::Right => reflect_x = ball_velocity.x < 0.0,
+                Collision::Top => reflect_y = ball_velocity.y < 0.0,
+                Collision::Bottom => reflect_y = ball_velocity.y > 0.0,
+            };
+
+            if reflect_x {
+                ball_velocity.x *= -1.0;
+            }
+
+            if reflect_y {
+                ball_velocity.y *= -1.0;
+            }
+        }
+    }
+}
+
+enum Collision {
+    Left,
+    Right,
+    Top,
+    Bottom,
+}
+
+fn ball_collision(circle: BoundingCircle, rect: Aabb2d) -> Option<Collision> {
+    if !circle.intersects(&rect) {
+        return None;
+    }
+
+    let closest = rect.closest_point(circle.center());
+    let offset = circle.center() - closest;
+
+    let side = if offset.x.abs() > offset.y.abs() {
+        if offset.x < 0.0 { Collision::Left } else { Collision::Right }
+    } else if offset.y > 0.0 {
+        Collision::Top
+    } else {
+        Collision::Bottom
+    };
+
+    Some(side)
+}
+
+#[derive(Component)]
+#[require(Sprite, Transform)]
+struct Wall;
+
+impl Wall {
+    pub fn new(location: WallLocation) -> (Wall, Sprite, Transform, Collider) {
+        (
+            Wall,
+            Sprite::from_color(Color::srgb(1.0, 0.4, 0.4), Vec2::ONE),
+            Transform {
+                translation: location.position().extend(0.0),
+                scale: location.size().extend(1.0),
+                ..Default::default()
+            },
+            Collider,
+        )
+    }
+}
+
+enum WallLocation {
+    Left,
+    Right,
+    Top,
+    Bottom,
+}
+
+impl WallLocation {
+    fn position(&self) -> Vec2 {
+        match self {
+            WallLocation::Left => Vec2::new(LEFT_WALL, 0.0),
+            WallLocation::Right => Vec2::new(RIGHT_WALL, 0.0),
+            WallLocation::Top => Vec2::new(0.0, TOP_WALL),
+            WallLocation::Bottom => Vec2::new(0.0, BOTTOM_WALL),
+        }
+    }
+
+    fn size(&self) -> Vec2 {
+        match self {
+            WallLocation::Left | WallLocation::Right => Vec2::new(WALL_WIDTH, TOP_WALL - BOTTOM_WALL),
+            WallLocation::Top | WallLocation::Bottom => Vec2::new(RIGHT_WALL - LEFT_WALL, WALL_WIDTH),
+        }
+    }
+}

snake-game/examples/schedules_test.rs

@@ -0,0 +1,69 @@
+use bevy::prelude::*;
+
+fn main() {
+    App::new() //
+        .add_plugins(DefaultPlugins)
+        .add_systems(PreStartup, pre_startup)
+        .add_systems(Startup, (startup, setup))
+        .add_systems(PostStartup, post_startup)
+        .add_systems(First, first)
+        .add_systems(PreUpdate, pre_update)
+        .add_systems(Update, (update, check_space_pressed))
+        .add_systems(PostUpdate, post_update)
+        .run();
+}
+
+fn pre_startup() {
+    info!("PreStartup");
+}
+
+fn startup() {
+    info!("Startup");
+}
+
+fn post_startup() {
+    info!("PostStartup");
+}
+
+fn first() {
+    info!("First");
+}
+
+fn pre_update() {
+    info!("PreUpdate");
+}
+
+fn update() {
+    info!("Update");
+}
+
+fn post_update() {
+    info!("PostUpdate");
+}
+
+fn check_space_pressed(input: Res<ButtonInput<KeyCode>>, mut commands: Commands) {
+    if input.just_pressed(KeyCode::Space) {
+        commands.trigger(TestEvent0);
+    }
+}
+
+fn setup(mut commands: Commands) {
+    commands.spawn((Observer::new(test_event_0_handler),));
+    commands.spawn((Observer::new(test_event_1_handler),));
+}
+
+#[derive(Event, Debug)]
+struct TestEvent0;
+
+#[derive(Event, Debug)]
+struct TestEvent1;
+
+fn test_event_0_handler(event: On<TestEvent0>, mut commands: Commands) {
+    info!("handle {:?}", event.event());
+    commands.trigger(TestEvent1);
+}
+
+fn test_event_1_handler(event: On<TestEvent1>, mut commands: Commands) {
+    info!("handle {:?}", event.event());
+    commands.trigger(TestEvent0);
+}

snake-game/examples/time.rs

@@ -0,0 +1,115 @@
+//! An example that illustrates how Time is handled in ECS.
+
+use bevy::{app::AppExit, prelude::*};
+
+use std::{
+    io::{self, BufRead},
+    time::Duration,
+};
+
+fn banner() {
+    println!("This example is meant to intuitively demonstrate how Time works in Bevy.");
+    println!();
+    println!("Time will be printed in three different schedules in the app:");
+    println!("- PreUpdate: real time is printed");
+    println!("- FixedUpdate: fixed time step time is printed, may be run zero or multiple times");
+    println!("- Update: virtual game time is printed");
+    println!();
+    println!("Max delta time is set to 5 seconds. Fixed timestep is set to 1 second.");
+    println!();
+}
+
+fn help() {
+    println!("The app reads commands line-by-line from standard input.");
+    println!();
+    println!("Commands:");
+    println!("  empty line: Run app.update() once on the Bevy App");
+    println!("  q: Quit the app.");
+    println!("  f: Set speed to fast, 2x");
+    println!("  n: Set speed to normal, 1x");
+    println!("  s: Set speed to slow, 0.5x");
+    println!("  p: Pause");
+    println!("  u: Unpause");
+}
+
+fn runner(mut app: App) -> AppExit {
+    banner();
+    help();
+    let stdin = io::stdin();
+    for line in stdin.lock().lines() {
+        if let Err(err) = line {
+            println!("read err: {err:#}");
+            break;
+        }
+        match line.unwrap().as_str() {
+            "" => {
+                app.update();
+            }
+            "f" => {
+                println!("FAST: setting relative speed to 2x");
+                app.world_mut().resource_mut::<Time<Virtual>>().set_relative_speed(2.0);
+            }
+            "n" => {
+                println!("NORMAL: setting relative speed to 1x");
+                app.world_mut().resource_mut::<Time<Virtual>>().set_relative_speed(1.0);
+            }
+            "s" => {
+                println!("SLOW: setting relative speed to 0.5x");
+                app.world_mut().resource_mut::<Time<Virtual>>().set_relative_speed(0.5);
+            }
+            "p" => {
+                println!("PAUSE: pausing virtual clock");
+                app.world_mut().resource_mut::<Time<Virtual>>().pause();
+            }
+            "u" => {
+                println!("UNPAUSE: resuming virtual clock");
+                app.world_mut().resource_mut::<Time<Virtual>>().unpause();
+            }
+            "q" => {
+                println!("QUITTING!");
+                break;
+            }
+            _ => {
+                help();
+            }
+        }
+    }
+
+    AppExit::Success
+}
+
+fn print_real_time(time: Res<Time<Real>>) {
+    println!(
+        "PreUpdate: this is real time clock, delta is {:?} and elapsed is {:?}",
+        time.delta(),
+        time.elapsed()
+    );
+}
+
+fn print_fixed_time(time: Res<Time>) {
+    println!(
+        "FixedUpdate: this is generic time clock inside fixed, delta is {:?} and elapsed is {:?}",
+        time.delta(),
+        time.elapsed()
+    );
+}
+
+fn print_time(time: Res<Time>) {
+    println!(
+        "Update: this is generic time clock, delta is {:?} and elapsed is {:?}",
+        time.delta(),
+        time.elapsed()
+    );
+}
+
+fn main() {
+    App::new()
+        .add_plugins(MinimalPlugins)
+        .insert_resource(Time::<Virtual>::from_max_delta(Duration::from_secs(5)))
+        .insert_resource(Time::<Fixed>::from_duration(Duration::from_secs(1)))
+        .add_systems(PreUpdate, print_real_time)
+        .add_systems(FixedUpdate, print_fixed_time)
+        .add_systems(Update, print_time)
+        .set_runner(runner)
+        .run();
+}