Make bevy_app and reflect opt-out for bevy_hierarchy. (#10721)

# Objective

Bevy_hierarchy is very useful for ECS only usages of bevy_ecs, but it
currently pulls in bevy_reflect, bevy_app and bevy_core with no way to
opt out.

## Solution

This PR provides features `bevy_app` and `reflect` that are enabled by
default. If disabled, they should remove these dependencies from
bevy_hierarchy.

---

## Changelog

Added features `bevy_app` and `reflect` to bevy_hierarchy.

crates/bevy_hierarchy/Cargo.toml

@@ -9,19 +9,20 @@ license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
 [features]
+default = ["bevy_app"]
 trace = []
+bevy_app = ["reflect", "dep:bevy_app", "bevy_core", "bevy_log"]
+reflect = ["bevy_ecs/bevy_reflect", "bevy_reflect"]
 
 [dependencies]
 # bevy
-bevy_app = { path = "../bevy_app", version = "0.12.0" }
-bevy_core = { path = "../bevy_core", version = "0.12.0" }
-bevy_ecs = { path = "../bevy_ecs", version = "0.12.0", features = [
-  "bevy_reflect",
-] }
-bevy_log = { path = "../bevy_log", version = "0.12.0" }
+bevy_app = { path = "../bevy_app", version = "0.12.0", optional = true }
+bevy_core = { path = "../bevy_core", version = "0.12.0", optional = true }
+bevy_ecs = { path = "../bevy_ecs", version = "0.12.0", default-features = false }
+bevy_log = { path = "../bevy_log", version = "0.12.0", optional = true }
 bevy_reflect = { path = "../bevy_reflect", version = "0.12.0", features = [
   "bevy",
-] }
+], optional = true }
 bevy_utils = { path = "../bevy_utils", version = "0.12.0" }
 
 # other

crates/bevy_hierarchy/src/components/children.rs

@@ -1,11 +1,11 @@
+#[cfg(feature = "reflect")]
+use bevy_ecs::reflect::{ReflectComponent, ReflectMapEntities};
 use bevy_ecs::{
     component::Component,
     entity::{Entity, EntityMapper, MapEntities},
     prelude::FromWorld,
-    reflect::{ReflectComponent, ReflectMapEntities},
     world::World,
 };
-use bevy_reflect::Reflect;
 use core::slice;
 use smallvec::SmallVec;
 use std::ops::Deref;
@@ -23,8 +23,9 @@ use std::ops::Deref;
 /// [`Query`]: bevy_ecs::system::Query
 /// [`Parent`]: crate::components::parent::Parent
 /// [`BuildChildren::with_children`]: crate::child_builder::BuildChildren::with_children
-#[derive(Component, Debug, Reflect)]
-#[reflect(Component, MapEntities)]
+#[derive(Component, Debug)]
+#[cfg_attr(feature = "reflect", derive(bevy_reflect::Reflect))]
+#[cfg_attr(feature = "reflect", reflect(Component, MapEntities))]
 pub struct Children(pub(crate) SmallVec<[Entity; 8]>);
 
 impl MapEntities for Children {

crates/bevy_hierarchy/src/components/parent.rs

@@ -1,10 +1,10 @@
+#[cfg(feature = "reflect")]
+use bevy_ecs::reflect::{ReflectComponent, ReflectMapEntities};
 use bevy_ecs::{
     component::Component,
     entity::{Entity, EntityMapper, MapEntities},
-    reflect::{ReflectComponent, ReflectMapEntities},
     world::{FromWorld, World},
 };
-use bevy_reflect::Reflect;
 use std::ops::Deref;
 
 /// Holds a reference to the parent entity of this entity.
@@ -20,8 +20,9 @@ use std::ops::Deref;
 /// [`Query`]: bevy_ecs::system::Query
 /// [`Children`]: super::children::Children
 /// [`BuildChildren::with_children`]: crate::child_builder::BuildChildren::with_children
-#[derive(Component, Debug, Eq, PartialEq, Reflect)]
-#[reflect(Component, MapEntities, PartialEq)]
+#[derive(Component, Debug, Eq, PartialEq)]
+#[cfg_attr(feature = "reflect", derive(bevy_reflect::Reflect))]
+#[cfg_attr(feature = "reflect", reflect(Component, MapEntities, PartialEq))]
 pub struct Parent(pub(crate) Entity);
 
 impl Parent {

crates/bevy_hierarchy/src/lib.rs

@@ -25,18 +25,21 @@ pub use query_extension::*;
 #[doc(hidden)]
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{
-        child_builder::*, components::*, hierarchy::*, query_extension::*, HierarchyPlugin,
-        ValidParentCheckPlugin,
-    };
+    pub use crate::{child_builder::*, components::*, hierarchy::*, query_extension::*};
+
+    #[doc(hidden)]
+    #[cfg(feature = "bevy_app")]
+    pub use crate::{HierarchyPlugin, ValidParentCheckPlugin};
 }
 
+#[cfg(feature = "bevy_app")]
 use bevy_app::prelude::*;
 
 /// The base plugin for handling [`Parent`] and [`Children`] components
 #[derive(Default)]
 pub struct HierarchyPlugin;
 
+#[cfg(feature = "bevy_app")]
 impl Plugin for HierarchyPlugin {
     fn build(&self, app: &mut App) {
         app.register_type::<Children>()

crates/bevy_hierarchy/src/valid_parent_check_plugin.rs

@@ -1,12 +1,10 @@
 use std::marker::PhantomData;
 
-use bevy_app::{App, Last, Plugin};
-use bevy_core::Name;
-use bevy_ecs::prelude::*;
-use bevy_log::warn;
-use bevy_utils::{get_short_name, HashSet};
-
+#[cfg(feature = "bevy_app")]
 use crate::Parent;
+use bevy_ecs::prelude::*;
+#[cfg(feature = "bevy_app")]
+use bevy_utils::{get_short_name, HashSet};
 
 /// When enabled, runs [`check_hierarchy_component_has_valid_parent<T>`].
 ///
@@ -45,6 +43,7 @@ impl<T> Default for ReportHierarchyIssue<T> {
     }
 }
 
+#[cfg(feature = "bevy_app")]
 /// System to print a warning for each [`Entity`] with a `T` component
 /// which parent hasn't a `T` component.
 ///
@@ -55,7 +54,7 @@ impl<T> Default for ReportHierarchyIssue<T> {
 /// (See B0004 explanation linked in warning message)
 pub fn check_hierarchy_component_has_valid_parent<T: Component>(
     parent_query: Query<
-        (Entity, &Parent, Option<&Name>),
+        (Entity, &Parent, Option<&bevy_core::Name>),
         (With<T>, Or<(Changed<Parent>, Added<T>)>),
     >,
     component_query: Query<(), With<T>>,
@@ -65,7 +64,7 @@ pub fn check_hierarchy_component_has_valid_parent<T: Component>(
         let parent = parent.get();
         if !component_query.contains(parent) && !already_diagnosed.contains(&entity) {
             already_diagnosed.insert(entity);
-            warn!(
+            bevy_log::warn!(
                 "warning[B0004]: {name} with the {ty_name} component has a parent without {ty_name}.\n\
                 This will cause inconsistent behaviors! See https://bevyengine.org/learn/errors/#b0004",
                 ty_name = get_short_name(std::any::type_name::<T>()),
@@ -94,10 +93,11 @@ impl<T: Component> Default for ValidParentCheckPlugin<T> {
     }
 }
 
-impl<T: Component> Plugin for ValidParentCheckPlugin<T> {
-    fn build(&self, app: &mut App) {
+#[cfg(feature = "bevy_app")]
+impl<T: Component> bevy_app::Plugin for ValidParentCheckPlugin<T> {
+    fn build(&self, app: &mut bevy_app::App) {
         app.init_resource::<ReportHierarchyIssue<T>>().add_systems(
-            Last,
+            bevy_app::Last,
             check_hierarchy_component_has_valid_parent::<T>
                 .run_if(resource_equals(ReportHierarchyIssue::<T>::new(true))),
         );