InsertIf for Bevy

insert_if.rs

use bevy::{ecs::system::EntityCommand, prelude::*};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, (startup, log).chain())
        .run();
}

fn startup(mut commands: Commands) {
    let is_true = true;
    commands.spawn_empty().add(
        move |mut entity: EntityWorldMut| {
            if is_true {
                entity.insert(SpriteBundle::default());
            }
        },
    );

    commands.spawn_empty().add(InsertIf {
        bundle: SpriteBundle::default(),
        cond: || true,
    });

    commands.spawn_empty().add(InsertIf::new(
        SpriteBundle::default(),
        || true,
    ));
}

fn log(mut commands: Commands, query: Query<Entity>) {
    for entity in &query {
        commands.entity(entity).log_components();
    }
}

/// custom version
struct InsertIf<
    F: FnMut() -> bool + Send + 'static,
    B: Bundle,
> {
    cond: F,
    bundle: B,
}

impl<B: Bundle, F: FnMut() -> bool + Send + 'static>
    EntityCommand for InsertIf<F, B>
{
    fn apply(mut self, id: Entity, world: &mut World) {
        if (self.cond)() {
            world.entity_mut(id).insert(self.bundle);
        }
    }
}

impl<B: Bundle, F: FnMut() -> bool + Send + 'static>
    InsertIf<F, B>
{
    fn new(b: B, f: F) -> Self {
        Self { cond: f, bundle: b }
    }
}