bevy/release-content/migration-guides/required_components_rework.md

title	pull_requests
Required components refactor	20110

The required components feature has been reworked to be more consistent around the priority of the required components and fix some soundness issues. In particular:

• the priority of required components will now always follow a priority given by the depth-first/preorder traversal of the dependency tree. This was mostly the case before with a couple of exceptions that we are now fixing:
  • when deriving the Component trait, sometimes required components at depth 1 had priority over components at depth 2 even if they came after in the depth-first ordering;
  • registering runtime required components followed a breadth-first ordering and used the wrong inheritance depth for derived required components.
• uses of the inheritance depth were removed from the RequiredComponent struct and from the methods for registering runtime required components, as it's not unused for the depth-first ordering;
• Component::register_required_components, RequiredComponents::register and RequiredComponents::register_by_id are now unsafe;
• RequiredComponentConstructor's only field is now private for safety reasons.

The Component::register_required_components method has also changed signature. It now takes the ComponentId of the component currently being registered and a single other parameter RequiredComponentsRegistrator which combines the old components and required_components parameters, since exposing both of them was unsound. As previously discussed the inheritance_depth is now useless and has been removed, while the recursion_check_stack has been moved into ComponentsRegistrator and will be handled automatically.