observer_triggers.md

---

title: Observer Triggers
pull_requests: [19440, 19596]

---

The Trigger type used inside observers has been renamed to On for a cleaner API.

// Old
commands.add_observer(|trigger: Trigger<OnAdd, Player>| {
    info!("Spawned player {}", trigger.entity());
});

// New
commands.add_observer(|event: On<Add, Player>| {
    info!("Spawned player {}", event.entity());
});

To reduce repetition and improve readability, the OnAdd, OnInsert, OnReplace, OnRemove, and OnDespawn observer events have also been renamed to Add, Insert, Replace, Remove, and Despawn respectively. In rare cases where the Add event conflicts with the std::ops::Add trait, you may need to disambiguate, for example by using ops::Add for the trait.

Observers may be triggered on particular entities or globally. Previously, a global trigger would claim to trigger on a particular Entity, Entity::PLACEHOLDER. For correctness and transparency, triggers have been changed to Option<Entity>.

On::entity (previously Trigger::target) now returns Option<Entity>, and ObserverTrigger::target is now of type Option<Entity>. If you were checking for Entity::PLACEHOLDER, migrate to handling the None case. If you were not checking for Entity::PLACEHOLDER, migrate to unwrapping, as Entity::PLACEHOLDER would have caused a panic before, at a later point.