Run a plugin conditionally

Sometimes your plugins might depend on other plugins running. Trying to include a plugin twice will cause an error.

Instead, as long as your plugins impl Plugin and are not just a basic function you can check if they have been added:

#[derive(Resource, Default)]
pub struct MousePosition(Vec2);

pub(crate) fn plugin(app: &mut App) {
  app.init_resource::<MousePosition>()
     .init_resource::<ButtonInput<KeyCode>>()
     .add_systems(Update, handle_keyboard_input);

  if !app.is_plugin_added::<physics::PhysicsPlugin>() {
    app.add_plugins(physics::PhysicsPlugin);
  }
}

Plugins are statically configured when your App is built so there is no real way to add or remove them from your app during runtime.

You can however use run conditions or some other logic to have them run when your game is in a certain state.

use bevy::prelude::*;

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

fn system_a() {
  println!("System A");
}

fn system_b() {
  println!("System B");
}

fn main() {
  App::new()
    .add_plugins(DefaultPlugins)
    .add_systems(Update, system_a.run_if(in_state(GameState::MainMenu)))
    .add_systems(Update, system_b.run_if(in_state(GameState::Game)))
    .run();
}