今天來看一個遊戲的例子，遊戲引擎配合ECS設計結構可以不到五百行的程式就能有一個簡單的小遊戲，這樣的架構設計比較接近我所認知的軟體工程。

//! Eat the cakes. Eat them all. An example 3D game.

use std::f32::consts::PI;

use bevy::prelude::*;
use rand::Rng;

#[derive(Clone, Eq, PartialEq, Debug, Hash, Default, States)]
enum GameState {
    #[default]
    Playing,
    GameOver,
}

#[derive(Resource)]
struct BonusSpawnTimer(Timer);

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_resource::<Game>()
        .insert_resource(BonusSpawnTimer(Timer::from_seconds(
            5.0,
            TimerMode::Repeating,
        )))
        .add_state::<GameState>()
        .add_systems(Startup, setup_cameras)
        .add_systems(OnEnter(GameState::Playing), setup)
        .add_systems(
            Update,
            (
                move_player,
                focus_camera,
                rotate_bonus,
                scoreboard_system,
                spawn_bonus,
            )
                .run_if(in_state(GameState::Playing)),
        )
        .add_systems(OnExit(GameState::Playing), teardown)
        .add_systems(OnEnter(GameState::GameOver), display_score)
        .add_systems(
            Update,
            (
                gameover_keyboard.run_if(in_state(GameState::GameOver)),
                bevy::window::close_on_esc,
            ),
        )
        .add_systems(OnExit(GameState::GameOver), teardown)
        .run();
}

struct Cell {
    height: f32,
}

#[derive(Default)]
struct Player {
    entity: Option<Entity>,
    i: usize,
    j: usize,
    move_cooldown: Timer,
}

#[derive(Default)]
struct Bonus {
    entity: Option<Entity>,
    i: usize,
    j: usize,
    handle: Handle<Scene>,
}

#[derive(Resource, Default)]
struct Game {
    board: Vec<Vec<Cell>>,
    player: Player,
    bonus: Bonus,
    score: i32,
    cake_eaten: u32,
    camera_should_focus: Vec3,
    camera_is_focus: Vec3,
}

const BOARD_SIZE_I: usize = 14;
const BOARD_SIZE_J: usize = 21;

const RESET_FOCUS: [f32; 3] = [
    BOARD_SIZE_I as f32 / 2.0,
    0.0,
    BOARD_SIZE_J as f32 / 2.0 - 0.5,
];

fn setup_cameras(mut commands: Commands, mut game: ResMut<Game>) {
    game.camera_should_focus = Vec3::from(RESET_FOCUS);
    game.camera_is_focus = game.camera_should_focus;
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(
            -(BOARD_SIZE_I as f32 / 2.0),
            2.0 * BOARD_SIZE_J as f32 / 3.0,
            BOARD_SIZE_J as f32 / 2.0 - 0.5,
        )
        .looking_at(game.camera_is_focus, Vec3::Y),
        ..default()
    });
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>, mut game: ResMut<Game>) {
    // reset the game state
    game.cake_eaten = 0;
    game.score = 0;
    game.player.i = BOARD_SIZE_I / 2;
    game.player.j = BOARD_SIZE_J / 2;
    game.player.move_cooldown = Timer::from_seconds(0.3, TimerMode::Once);

    commands.spawn(PointLightBundle {
        transform: Transform::from_xyz(4.0, 10.0, 4.0),
        point_light: PointLight {
            intensity: 3000.0,
            shadows_enabled: true,
            range: 30.0,
            ..default()
        },
        ..default()
    });

    // spawn the game board
    let cell_scene = asset_server.load("models/AlienCake/tile.glb#Scene0");
    game.board = (0..BOARD_SIZE_J)
        .map(|j| {
            (0..BOARD_SIZE_I)
                .map(|i| {
                    let height = rand::thread_rng().gen_range(-0.1..0.1);
                    commands.spawn(SceneBundle {
                        transform: Transform::from_xyz(i as f32, height - 0.2, j as f32),
                        scene: cell_scene.clone(),
                        ..default()
                    });
                    Cell { height }
                })
                .collect()
        })
        .collect();

    // spawn the game character
    game.player.entity = Some(
        commands
            .spawn(SceneBundle {
                transform: Transform {
                    translation: Vec3::new(
                        game.player.i as f32,
                        game.board[game.player.j][game.player.i].height,
                        game.player.j as f32,
                    ),
                    rotation: Quat::from_rotation_y(-PI / 2.),
                    ..default()
                },
                scene: asset_server.load("models/AlienCake/alien.glb#Scene0"),
                ..default()
            })
            .id(),
    );

    // load the scene for the cake
    game.bonus.handle = asset_server.load("models/AlienCake/cakeBirthday.glb#Scene0");

    // scoreboard
    commands.spawn(
        TextBundle::from_section(
            "Score:",
            TextStyle {
                font_size: 40.0,
                color: Color::rgb(0.5, 0.5, 1.0),
                ..default()
            },
        )
        .with_style(Style {
            position_type: PositionType::Absolute,
            top: Val::Px(5.0),
            left: Val::Px(5.0),
            ..default()
        }),
    );
}

// remove all entities that are not a camera or window
fn teardown(mut commands: Commands, entities: Query<Entity, (Without<Camera>, Without<Window>)>) {
    for entity in &entities {
        commands.entity(entity).despawn();
    }
}

// control the game character
fn move_player(
    mut commands: Commands,
    keyboard_input: Res<Input<KeyCode>>,
    mut game: ResMut<Game>,
    mut transforms: Query<&mut Transform>,
    time: Res<Time>,
) {
    if game.player.move_cooldown.tick(time.delta()).finished() {
        let mut moved = false;
        let mut rotation = 0.0;

        if keyboard_input.pressed(KeyCode::Up) {
            if game.player.i < BOARD_SIZE_I - 1 {
                game.player.i += 1;
            }
            rotation = -PI / 2.;
            moved = true;
        }
        if keyboard_input.pressed(KeyCode::Down) {
            if game.player.i > 0 {
                game.player.i -= 1;
            }
            rotation = PI / 2.;
            moved = true;
        }
        if keyboard_input.pressed(KeyCode::Right) {
            if game.player.j < BOARD_SIZE_J - 1 {
                game.player.j += 1;
            }
            rotation = PI;
            moved = true;
        }
        if keyboard_input.pressed(KeyCode::Left) {
            if game.player.j > 0 {
                game.player.j -= 1;
            }
            rotation = 0.0;
            moved = true;
        }

        // move on the board
        if moved {
            game.player.move_cooldown.reset();
            *transforms.get_mut(game.player.entity.unwrap()).unwrap() = Transform {
                translation: Vec3::new(
                    game.player.i as f32,
                    game.board[game.player.j][game.player.i].height,
                    game.player.j as f32,
                ),
                rotation: Quat::from_rotation_y(rotation),
                ..default()
            };
        }
    }

    // eat the cake!
    if let Some(entity) = game.bonus.entity {
        if game.player.i == game.bonus.i && game.player.j == game.bonus.j {
            game.score += 2;
            game.cake_eaten += 1;
            commands.entity(entity).despawn_recursive();
            game.bonus.entity = None;
        }
    }
}

// change the focus of the camera
fn focus_camera(
    time: Res<Time>,
    mut game: ResMut<Game>,
    mut transforms: ParamSet<(Query<&mut Transform, With<Camera3d>>, Query<&Transform>)>,
) {
    const SPEED: f32 = 2.0;
    // if there is both a player and a bonus, target the mid-point of them
    if let (Some(player_entity), Some(bonus_entity)) = (game.player.entity, game.bonus.entity) {
        let transform_query = transforms.p1();
        if let (Ok(player_transform), Ok(bonus_transform)) = (
            transform_query.get(player_entity),
            transform_query.get(bonus_entity),
        ) {
            game.camera_should_focus = player_transform
                .translation
                .lerp(bonus_transform.translation, 0.5);
        }
        // otherwise, if there is only a player, target the player
    } else if let Some(player_entity) = game.player.entity {
        if let Ok(player_transform) = transforms.p1().get(player_entity) {
            game.camera_should_focus = player_transform.translation;
        }
        // otherwise, target the middle
    } else {
        game.camera_should_focus = Vec3::from(RESET_FOCUS);
    }
    // calculate the camera motion based on the difference between where the camera is looking
    // and where it should be looking; the greater the distance, the faster the motion;
    // smooth out the camera movement using the frame time
    let mut camera_motion = game.camera_should_focus - game.camera_is_focus;
    if camera_motion.length() > 0.2 {
        camera_motion *= SPEED * time.delta_seconds();
        // set the new camera's actual focus
        game.camera_is_focus += camera_motion;
    }
    // look at that new camera's actual focus
    for mut transform in transforms.p0().iter_mut() {
        *transform = transform.looking_at(game.camera_is_focus, Vec3::Y);
    }
}

// despawn the bonus if there is one, then spawn a new one at a random location
fn spawn_bonus(
    time: Res<Time>,
    mut timer: ResMut<BonusSpawnTimer>,
    mut next_state: ResMut<NextState<GameState>>,
    mut commands: Commands,
    mut game: ResMut<Game>,
) {
    // make sure we wait enough time before spawning the next cake
    if !timer.0.tick(time.delta()).finished() {
        return;
    }

    if let Some(entity) = game.bonus.entity {
        game.score -= 3;
        commands.entity(entity).despawn_recursive();
        game.bonus.entity = None;
        if game.score <= -5 {
            next_state.set(GameState::GameOver);
            return;
        }
    }

    // ensure bonus doesn't spawn on the player
    loop {
        game.bonus.i = rand::thread_rng().gen_range(0..BOARD_SIZE_I);
        game.bonus.j = rand::thread_rng().gen_range(0..BOARD_SIZE_J);
        if game.bonus.i != game.player.i || game.bonus.j != game.player.j {
            break;
        }
    }
    game.bonus.entity = Some(
        commands
            .spawn(SceneBundle {
                transform: Transform::from_xyz(
                    game.bonus.i as f32,
                    game.board[game.bonus.j][game.bonus.i].height + 0.2,
                    game.bonus.j as f32,
                ),
                scene: game.bonus.handle.clone(),
                ..default()
            })
            .with_children(|children| {
                children.spawn(PointLightBundle {
                    point_light: PointLight {
                        color: Color::rgb(1.0, 1.0, 0.0),
                        intensity: 1000.0,
                        range: 10.0,
                        ..default()
                    },
                    transform: Transform::from_xyz(0.0, 2.0, 0.0),
                    ..default()
                });
            })
            .id(),
    );
}

// let the cake turn on itself
fn rotate_bonus(game: Res<Game>, time: Res<Time>, mut transforms: Query<&mut Transform>) {
    if let Some(entity) = game.bonus.entity {
        if let Ok(mut cake_transform) = transforms.get_mut(entity) {
            cake_transform.rotate_y(time.delta_seconds());
            cake_transform.scale =
                Vec3::splat(1.0 + (game.score as f32 / 10.0 * time.elapsed_seconds().sin()).abs());
        }
    }
}

// update the score displayed during the game
fn scoreboard_system(game: Res<Game>, mut query: Query<&mut Text>) {
    let mut text = query.single_mut();
    text.sections[0].value = format!("Sugar Rush: {}", game.score);
}

// restart the game when pressing spacebar
fn gameover_keyboard(
    mut next_state: ResMut<NextState<GameState>>,
    keyboard_input: Res<Input<KeyCode>>,
) {
    if keyboard_input.just_pressed(KeyCode::Space) {
        next_state.set(GameState::Playing);
    }
}

// display the number of cake eaten before losing
fn display_score(mut commands: Commands, game: Res<Game>) {
    commands
        .spawn(NodeBundle {
            style: Style {
                width: Val::Percent(100.),
                align_items: AlignItems::Center,
                justify_content: JustifyContent::Center,
                ..default()
            },
            ..default()
        })
        .with_children(|parent| {
            parent.spawn(TextBundle::from_section(
                format!("Cake eaten: {}", game.cake_eaten),
                TextStyle {
                    font_size: 80.0,
                    color: Color::rgb(0.5, 0.5, 1.0),
                    ..default()
                },
            ));
        });
}

目標與概述:

這是一個3D遊戲，玩家需要控制一個外星人去吃蛋糕。每當玩家吃到一塊蛋糕，他們的分數增加，但如果沒有在一定時間內吃到蛋糕，分數會減少。

結構和資料:

• GameState: 遊戲的主要狀態。分為正在玩 (Playing) 和遊戲結束 (GameOver)。
• Cell: 表示遊戲版面上的一個格子。
• Player: 玩家的資料，包括位置、實體引用等。
• Bonus: 蛋糕的資料，包括位置、實體引用等。
• Game: 主要的遊戲狀態和資料，包括遊戲板、玩家、蛋糕、分數等。

初始化遊戲:

遊戲開始時，通過 setup_cameras 函數設置遊戲的初始視角。
setup 函數用於初始化遊戲場景，設置遊戲板、玩家位置等。

遊戲邏輯:

• move_player: 控制玩家的移動。玩家使用方向鍵控制移動，並可以吃到放在遊戲板上的蛋糕。
• focus_camera: 改變攝像頭的焦點以便更好地查看玩家和蛋糕。
• spawn_bonus: 在遊戲板上隨機生成新的蛋糕。
• rotate_bonus: 讓蛋糕自轉，並根據分數動態調整蛋糕的大小。
• scoreboard_system: 更新和顯示遊戲分數。
• gameover_keyboard: 在遊戲結束時，允許玩家按空格鍵重新開始遊戲。
• display_score: 在遊戲結束時顯示玩家所吃蛋糕的數量。

結束遊戲:

如果玩家的分數低於 -5，遊戲將結束，並顯示玩家所吃的蛋糕數。

其他:

遊戲還包括一些其他函數，如 teardown 用於清理遊戲場景。