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system_param.rs
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system_param.rs
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//! This example creates a custom [`SystemParam`] struct that counts the number of players.
use bevy::{ecs::system::SystemParam, prelude::*};
fn main() {
App::new()
.insert_resource(PlayerCount(0))
.add_systems(Startup, spawn)
.add_systems(Update, count_players)
.run();
}
#[derive(Component)]
struct Player;
#[derive(Resource)]
struct PlayerCount(usize);
/// The [`SystemParam`] struct can contain any types that can also be included in a
/// system function signature.
///
/// In this example, it includes a query and a mutable resource.
#[derive(SystemParam)]
struct PlayerCounter<'w, 's> {
players: Query<'w, 's, &'static Player>,
count: ResMut<'w, PlayerCount>,
}
impl<'w, 's> PlayerCounter<'w, 's> {
fn count(&mut self) {
self.count.0 = self.players.iter().len();
}
}
/// Spawn some players to count
fn spawn(mut commands: Commands) {
commands.spawn(Player);
commands.spawn(Player);
commands.spawn(Player);
}
/// The [`SystemParam`] can be used directly in a system argument.
fn count_players(mut counter: PlayerCounter) {
counter.count();
println!("{} players in the game", counter.count.0);
}