Point queries

[fluffysquirrels]

I'm trying to do a point query in one of my systems, by which I mean looking up
a component belonging to an entity that I know the entity id of.

Here's my code so far:

pub type Hp = i32;
pub struct HealthComponent {
    pub current: Hp,
    pub max: Hp,
}
pub struct Player;
pub struct DamageEvent {
    pub target: Entity,
    pub hp: Hp,
}

fn setup(
    mut commands: Commands,
) {
    commands
        .spawn()
        .insert(Player)
        .insert(HealthComponent {
            current: 10,
            max: 10,
        }

}

fn apply_damage(
    mut commands: Commands,
    dmg_reader: mut EventReader<DamageEvent>,
) {
    for damage in dmg_reader.iter() {
        let entity = commands.entity(damage.target);
        let mut health: HealthComponent = todo!();
        health.current -= damage.hp;
    }
}

The bit I'm uncertain of is what to replace the todo!() with.

If I had access to a World I could just do
world.entity(entity).get::<HealthComponent>(), but I don't know how
to get access to a World. I suppose I could stash the World while
using my AppBuilder but that doesn't seem very ergonomic.

Commands lets me modify the entity but not query it. Is there a
reason for this? Perhaps ad-hoc access to components prevents
lock-free parallelisation?

I could inject a Query into apply_damage to loop over every entity
with a HealthComponent component, but that sounds like it would
require O(^2) iterations where n is the number of entities that
have a HealthComponent component.

Perhaps there is a more idiomatic way to represent HealthComponent or DamageEvent?

Any help is appreciated.

[TheRawMeatball]

You can use query.get or query.get_component :)

[fluffysquirrels]

Ooops! Thanks!

[tatref]

Here is a complete example:

fn apply_damage(
    mut commands: Commands,
    dmg_reader: mut EventReader<DamageEvent>,
    mut q: Query<&mut HealthComponent>
) {
    for damage in dmg_reader.iter() {
        let entity = commands.entity(damage.target);
        let mut health = q.get_mut(entity).expect("Entity not found or no HealthComponent");
        health.current -= damage.hp;
    }
}

You can also use if let Some(health) = q.get_mut(... for better error handeling.