Add BeforeAdd and AfterRemove lifecycle events

[jonas-meyer]

Objective

• Add BeforeAdd and AfterRemove lifecycle events to complete the symmetric lifecycle: BeforeAdd -> Add -> Insert -> Discard -> Remove -> AfterRemove
• Without these events, there is no point in the lifecycle where the entity is observable in the "component absent" state — Add fires after the component exists, and Remove fires before it's gone
• These are prerequisites for query-based observers like DataAdded<Without> (Lifecycle event observers for queries #20817), which need to evaluate queries when the component is not yet/no longer present
• Part of the structured lifecycle proposal (Make lifecycle events ordering and meaning more intuitive #20729)

Solution

Added before_add and after_remove as both component hooks and observer events, following the same patterns as existing lifecycle events:

• BeforeAdd fires before Add and Insert. During insert, it fires before the archetype move (component data is not yet accessible). During spawn, it fires after data is written but before Add/Insert hooks.
• AfterRemove fires after Remove. Component data has already been dropped and is not accessible. Fires during both remove() and despawn().

Note

BeforeAdd/AfterRemove are interim names. A future PR will introduce a Before<E>/After<E> wrapper scheme.

Testing

• Extended 3 existing hook ordering tests to verify before_add fires at position 0 and after_remove fires at position 5 in the full lifecycle sequence
• Added 7 new tests covering: replace skips before_add, component data inaccessibility during before_add/after_remove, entity inaccessibility during spawn's before_add, and observer-before-hook ordering for both events

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Showcase

Click to view showcase

  // Component hooks
  world.register_component_hooks::<Health>()
      .before_add(|world, ctx| {
          // Fires before the component exists on the entity
          // Good for "about to gain component" logic
      })
      .after_remove(|world, ctx| {
          // Fires after the component is fully gone
          // Good for "component is now absent" cleanup
      });

  // Observers
  world.add_observer(|_: On<BeforeAdd, Health>, /* ... */| {
      // Entity does not yet have Health
  });

  world.add_observer(|_: On<AfterRemove, Health>, /* ... */| {
      // Entity no longer has Health
  });

  // Derive macro
  #[derive(Component)]
  #[component(before_add = on_before, after_remove = on_after)]
  struct Health(f32);

[Copilot]

Pull request overview

This PR extends bevy_ecs’s component lifecycle by introducing symmetric “component absent” observation points via the new BeforeAdd and AfterRemove lifecycle events, implemented both as component hooks and as observer-triggered events.

Changes:

• Added BeforeAdd and AfterRemove lifecycle event types, event keys, and prelude exports.
• Wired new lifecycle triggers into spawn/insert/remove/despawn paths, including new archetype flags and observer caching.
• Expanded and added tests to validate hook/event ordering and inaccessibility semantics.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
crates/bevy_ecs/src/world/mod.rs	Registers new lifecycle event keys during world initialization.
crates/bevy_ecs/src/world/entity_access/world_mut.rs	Triggers AfterRemove during despawn after component data drop.
crates/bevy_ecs/src/world/deferred_world.rs	Adds DeferredWorld helpers to run before_add / after_remove hooks.
crates/bevy_ecs/src/observer/centralized_storage.rs	Adds cached observer buckets and archetype-flag mapping for the new events.
crates/bevy_ecs/src/lifecycle.rs	Defines new event types/keys and adds hook registration APIs + documentation updates.
crates/bevy_ecs/src/lib.rs	Exports BeforeAdd / AfterRemove in the public prelude.
crates/bevy_ecs/src/component/mod.rs	Adds Component::before_add() / Component::after_remove() hook providers and docs.
crates/bevy_ecs/src/component/info.rs	Sets new archetype hook flags based on component hook presence.
crates/bevy_ecs/src/component/constants.rs	Reserves new constant component indices for lifecycle observer components.
crates/bevy_ecs/src/bundle/tests.rs	Extends hook-order tests and adds new tests for new lifecycle semantics.
crates/bevy_ecs/src/bundle/spawner.rs	Triggers BeforeAdd during spawn prior to writing component data.
crates/bevy_ecs/src/bundle/remove.rs	Triggers AfterRemove after removal archetype move / data drop.
crates/bevy_ecs/src/bundle/insert.rs	Triggers BeforeAdd for newly-added components before the archetype move.
crates/bevy_ecs/src/archetype.rs	Adds archetype flags and helper methods for new hook/observer presence checks.
crates/bevy_ecs/macros/src/component.rs	Extends derive macro parsing/codegen to support before_add and after_remove attributes.

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[alice-i-cecile]

I'm in favor of something here: I really don't think our existing setup is adequate or clear. This PR looks fine at first blush, but I would like to have a proper design effort before moving forward though.

[jonas-meyer]

I've written up something here: #20729 (comment)

Is this what you had in mind? Happy to move it to an issue as well.