Plugins

Plugins are used for code organization, often in their own files.

extern crate bevy;
extern crate bevy_ecs;
extern crate bevy_state;
use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(WindowPlugin {
            primary_window: Some(Window {
                title: "Bevy Workshop".into(),
                ..default()
            }),
            ..default()
        }))
        .init_state::<ApplicationState>()
        .add_plugins(splash::SplashPlugin)           // adding our new plugin
        .run();
}

#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash, States, Default)]
enum ApplicationState {
    #[default]
    Splash,
    Menu,
}

mod splash {
    use bevy::prelude::*;

    use crate::ApplicationState;

    pub struct SplashPlugin;

    impl Plugin for SplashPlugin {
        fn build(&self, app: &mut App) {
            app.add_systems(OnEnter(ApplicationState::Splash), display_title)
                .add_systems(Update, switch_to_menu.run_if(in_state(ApplicationState::Splash)));
        }
    }

    fn display_title(mut commands: Commands) {
        commands.spawn(Camera2d);

        commands.spawn((
            Node {
                width: Val::Percent(100.0),
                height: Val::Percent(100.0),
                align_items: AlignItems::Center,
                justify_content: JustifyContent::Center,
                flex_direction: FlexDirection::Column,
                ..default()
            },
            children![
                (
                    Text::new("Bevy Workshop"),
                    TextFont {
                        font_size: 130.0,
                        ..default()
                    },
                ),
                (
                    Text::new("EuroRust 2025"),
                    TextFont {
                        font_size: 100.0,
                        ..default()
                    },
                )
            ],
            DespawnOnExit(ApplicationState::Splash),
        ));

        commands.insert_resource(SplashScreenTimer(Timer::from_seconds(2.0, TimerMode::Once)));
    }

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

    fn switch_to_menu(
        mut next: ResMut<NextState<ApplicationState>>,
        mut timer: ResMut<SplashScreenTimer>,
        time: Res<Time>,
    ) {
        if timer.0.tick(time.delta()).just_finished() {
            next.set(ApplicationState::Menu);
        }
    }
}

For most cases, a plugin can be a free function:

extern crate bevy;
extern crate bevy_state;
use bevy::prelude::*;
#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash, States, Default)]
enum ApplicationState {
    #[default]
    Splash,
}
fn main() {
    App::new()
        // ...
        .add_plugins(splash::splash_plugin)           // adding our new plugin
        .run();
}

mod splash {
    use bevy::prelude::*;
    use crate::ApplicationState;
    fn display_title() {}
    fn load_assets() {}
    fn switch_to_menu() {}
    pub fn splash_plugin(app: &mut App) {
        app.add_systems(OnEnter(ApplicationState::Splash), display_title)
            .add_systems(Update, switch_to_menu.run_if(in_state(ApplicationState::Splash)));
    }
}