bevy/release-content/release-notes/new_querydata.md

title	authors	pull_requests
New QueryData Types	@ecoskey	19602

Bevy queries have some new powers for advanced users. Namely, custom WorldQuery implementations can store and apply "deferred" mutations, just like Commands! This release includes a few new types making use of this capability, and we're sure third-party crates will find all kinds of new ways to do cool stuff with this.

DeferredMut

When working with immutable components in Bevy, the acts of reading and writing component values are very clearly separated. This can be valuable, especially if a component has expensive hooks or observers attached and inserting it has a significant cost, but in some cases it can feel like boilerplate.

DeferredMut is meant to improve the ergonomics of the latter case, by providing "fake" mutable access to any component, even immutable ones! Internally, it keeps track of any modifications and inserts them into the ECS at the next sync point.

// without `DeferredMut`
pub fn tick_poison(
    mut commands: Commands,
    query: Query<(Entity, &Health), With<Poisoned>>
) {
    for (entity, Health(health_points)) in query {
        commands.insert(entity, Health(health_points - 1))
    }
}

// with `DeferredMut`
pub fn tick_poison(
    mut health_query: Query<DeferredMut<Health>, With<Poisoned>>
) {
    for mut health in &health_query {
         health.0 -= 1;
    }
}