300 lines
13 KiB
Rust
300 lines
13 KiB
Rust
use bevy::{
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app::{AppExit, ScheduleRunnerPlugin},
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prelude::*,
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};
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use rand::random;
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use std::time::Duration;
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/// This is a guided introduction to Bevy's "Entity Component System" (ECS)
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/// All Bevy app logic is built using the ECS pattern, so definitely pay attention!
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///
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/// Why ECS?
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/// * Data oriented: Functionality is driven by data
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/// * Clean Architecture: Loose coupling of functionality / prevents deeply nested inheritance
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/// * High Performance: Massively parallel and cache friendly
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///
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/// ECS Definitions:
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///
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/// Component: just a normal Rust data type. generally scoped to a single piece of functionality
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/// Examples: position, velocity, health, color, name
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///
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/// Entity: a collection of components with a unique id
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/// Examples: Entity1 { Name("Alice"), Position(0, 0) }, Entity2 { Name("Bill"), Position(10, 5) }
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/// Resource: a shared global piece of data
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/// Examples: asset_storage, events, system state
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///
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/// System: runs logic on entities, components, and resources
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/// Examples: move_system, damage_system
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///
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/// Now that you know a little bit about ECS, lets look at some Bevy code!
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/// We will now make a simple "game" to illustrate what Bevy's ECS looks like in practice.
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//
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// COMPONENTS: Pieces of functionality we add to entities. These are just normal Rust data types
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//
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// Our game will have a number of "players". Each player has a name that identifies them
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struct Player {
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name: String,
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}
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// Each player also has a score. This component holds on to that score
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struct Score {
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value: usize,
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}
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//
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// RESOURCES: "Global" state accessible by systems. These are also just normal Rust data types!
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//
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// This resource holds information about the game:
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#[derive(Default)]
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struct GameState {
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current_round: usize,
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total_players: usize,
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winning_player: Option<String>,
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}
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// This resource provides rules for our "game".
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struct GameRules {
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winning_score: usize,
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max_rounds: usize,
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max_players: usize,
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}
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//
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// SYSTEMS: Logic that runs on entities, components, and resources. These generally run once each time the app updates.
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//
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// This is the simplest type of system. It just prints "This game is fun!" on each run:
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fn print_message_system() {
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println!("This game is fun!");
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}
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// Systems can also read and modify resources. This system starts a new "round" on each update:
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// NOTE: "mut" denotes that the resource is "mutable"
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// Res<GameRules> is read-only. ResMut<GameState> can modify the resource
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fn new_round_system(game_rules: Res<GameRules>, mut game_state: ResMut<GameState>) {
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game_state.current_round += 1;
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println!(
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"Begin round {} of {}",
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game_state.current_round, game_rules.max_rounds
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);
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}
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// This system updates the score for each entity with the "Player" and "Score" component.
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fn score_system(mut query: Query<(&Player, &mut Score)>) {
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for (player, mut score) in &mut query.iter() {
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let scored_a_point = random::<bool>();
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if scored_a_point {
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score.value += 1;
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println!(
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"{} scored a point! Their score is: {}",
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player.name, score.value
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);
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} else {
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println!(
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"{} did not score a point! Their score is: {}",
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player.name, score.value
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);
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}
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}
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// this game isn't very fun is it :)
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}
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// This system runs on all entities with the "Player" and "Score" components, but it also
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// accesses the "GameRules" resource to determine if a player has won.
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// NOTE: resources must always come before worlds/queries in system functions
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fn score_check_system(
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game_rules: Res<GameRules>,
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mut game_state: ResMut<GameState>,
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mut query: Query<(&Player, &Score)>,
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) {
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for (player, score) in &mut query.iter() {
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if score.value == game_rules.winning_score {
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game_state.winning_player = Some(player.name.clone());
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}
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}
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}
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// This system ends the game if we meet the right conditions. This fires an AppExit event, which tells our
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// App to quit. Check out the "event.rs" example if you want to learn more about using events.
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fn game_over_system(
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game_rules: Res<GameRules>,
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game_state: Res<GameState>,
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mut app_exit_events: ResMut<Events<AppExit>>,
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) {
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if let Some(ref player) = game_state.winning_player {
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println!("{} won the game!", player);
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app_exit_events.send(AppExit);
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} else if game_state.current_round == game_rules.max_rounds {
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println!("Ran out of rounds. Nobody wins!");
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app_exit_events.send(AppExit);
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}
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println!();
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}
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// This is a "startup" system that runs exactly once when the app starts up. Startup systems are generally used to create
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// the initial "state" of our game. The only thing that distinguishes a "startup" system from a "normal" system is how it is registered:
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// Startup: app.add_startup_system(startup_system)
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// Normal: app.add_system(normal_system)
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// This startup system needs direct access to the ECS World and Resources, which makes it a "thread local system".
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// That being said, startup systems can use any of the system forms we've covered. We will also cover thread local systems more in a bit.
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fn startup_system(world: &mut World, resources: &mut Resources) {
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// Create our game rules resource
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resources.insert(GameRules {
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max_rounds: 10,
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winning_score: 4,
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max_players: 4,
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});
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// Add some players to our world. Players start with a score of 0 ... we want our game to be fair!
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world.spawn_batch(vec![
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(
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Player {
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name: "Alice".to_string(),
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},
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Score { value: 0 },
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),
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(
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Player {
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name: "Bob".to_string(),
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},
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Score { value: 0 },
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),
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]);
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// set the total players to "2"
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let mut game_state = resources.get_mut::<GameState>().unwrap();
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game_state.total_players = 2;
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}
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// This system uses a command buffer to (potentially) add a new player to our game on each iteration.
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// Normal systems cannot safely access the World instance directly because they run in parallel.
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// Our World contains all of our components, so mutating arbitrary parts of it in parallel is not thread safe.
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// Command buffers give us the ability to queue up changes to our World without directly accessing it
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// NOTE: Command buffers must always come before resources and queries in system functions
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fn new_player_system(
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mut commands: Commands,
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game_rules: Res<GameRules>,
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mut game_state: ResMut<GameState>,
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) {
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// Randomly add a new player
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let add_new_player = random::<bool>();
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if add_new_player && game_state.total_players < game_rules.max_players {
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game_state.total_players += 1;
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commands.spawn((
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Player {
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name: format!("Player {}", game_state.total_players),
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},
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Score { value: 0 },
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));
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println!("Player {} joined the game!", game_state.total_players);
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}
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}
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// If you really need full, immediate read/write access to the world or resources, you can use a "thread local system".
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// These run on the main app thread (hence the name "thread local")
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// WARNING: These will block all parallel execution of other systems until they finish, so they should generally be avoided if you
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// care about performance
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// NOTE: You may notice that this function signature looks exactly like the "startup_system" above.
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// Thats because they are both thread local!
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#[allow(dead_code)]
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fn thread_local_system(world: &mut World, resources: &mut Resources) {
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// this does the same thing as "new_player_system"
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let mut game_state = resources.get_mut::<GameState>().unwrap();
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let game_rules = resources.get::<GameRules>().unwrap();
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// Randomly add a new player
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let add_new_player = random::<bool>();
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if add_new_player && game_state.total_players < game_rules.max_players {
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world.spawn((
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Player {
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name: format!("Player {}", game_state.total_players),
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},
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Score { value: 0 },
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));
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game_state.total_players += 1;
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}
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}
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// Sometimes systems need their own unique "local" state. Bevy's ECS provides Local<T> resources for this case.
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// Local<T> resources are unique to their system and are automatically initialized on your behalf (if they don't already exist).
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// If you have a system's id, you can also access local resources directly in the Resources collection using `Resources::get_local()`.
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// In general you should only need this feature in the following cases:
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// 1. You have multiple instances of the same system and they each need their own unique state
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// 2. You already have a global version of a resource that you don't want to overwrite for your current system
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// 3. You are too lazy to register the system's resource as a global resource
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#[derive(Default)]
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struct State {
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counter: usize,
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}
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// NOTE: this doesn't do anything relevant to our game, it is just here for illustrative purposes
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#[allow(dead_code)]
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fn local_state_system(mut state: Local<State>, mut query: Query<(&Player, &Score)>) {
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for (player, score) in &mut query.iter() {
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println!("processed: {} {}", player.name, score.value);
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}
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println!("this system ran {} times", state.counter);
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state.counter += 1;
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}
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// Our Bevy app's entry point
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fn main() {
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// Bevy apps are created using the builder pattern. We use the builder to add systems, resources, and plugins to our app
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App::build()
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// Plugins are just a grouped set of app builder calls (just like we're doing here).
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// We could easily turn our game into a plugin, but you can check out the plugin example for that :)
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// The plugin below runs our app's "system schedule" once every 5 seconds.
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.add_plugin(ScheduleRunnerPlugin::run_loop(Duration::from_secs(5)))
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// Resources can be added to our app like this
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.add_resource(State { counter: 0 })
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// Resources that implement the Default or FromResources trait can be added like this:
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.init_resource::<GameState>()
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// Startup systems run exactly once BEFORE all other systems. These are generally used for
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// app initialization code (ex: adding entities and resources)
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.add_startup_system(startup_system.thread_local_system())
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// my_system.system() calls converts normal rust functions into ECS systems:
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.add_system(print_message_system.system())
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//
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// SYSTEM EXECUTION ORDER
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//
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// By default, all systems run in parallel. This is efficient, but sometimes order matters.
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// For example, we want our "game over" system to execute after all other systems to ensure we don't
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// accidentally run the game for an extra round.
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//
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// First, if a system writes a component or resource (ComMut / ResMut), it will force a synchronization.
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// Any systems that access the data type and were registered BEFORE the system will need to finish first.
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// Any systems that were registered _after_ the system will need to wait for it to finish. This is a great
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// default that makes everything "just work" as fast as possible without us needing to think about it ... provided
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// we don't care about execution order. If we do care, one option would be to use the rules above to force a synchronization
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// at the right time. But that is complicated and error prone!
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//
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// This is where "stages" come in. A "stage" is a group of systems that execute (in parallel). Stages are executed in order,
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// and the next stage won't start until all systems in the current stage have finished.
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// add_system(system) adds systems to the UPDATE stage by default
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// However we can manually specify the stage if we want to. The following is equivalent to add_system(score_system.system())
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.add_system_to_stage(stage::UPDATE, score_system.system())
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// We can also create new stages. Here is what our games stage order will look like:
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// "before_round": new_player_system, new_round_system
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// "update": print_message_system, score_system
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// "after_round": score_check_system, game_over_system
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.add_stage_before(stage::UPDATE, "before_round")
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.add_stage_after(stage::UPDATE, "after_round")
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.add_system_to_stage("before_round", new_round_system.system())
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.add_system_to_stage("before_round", new_player_system.system())
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.add_system_to_stage("after_round", score_check_system.system())
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.add_system_to_stage("after_round", game_over_system.system())
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// score_check_system will run before game_over_system because score_check_system modifies GameState and game_over_system
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// reads GameState. This works, but it's a bit confusing. In practice, it would be clearer to create a new stage that runs
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// before "after_round"
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// This call to run() starts the app we just built!
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.run();
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}
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