Add warning when a hierarchy component is missing

A common pitfall since 0.8 is the requirement on `ComputedVisibility`
being present on all ancestors of an entity that itself has
`ComputedVisibility`, without which, the entity becomes invisible.

I myself hit the issue and got very confused, and saw a few people hit
it as well, so it makes sense to provide a hint of what to do when such
a situation is encountered.

We now check that all entities with both a `Parent` and a
`ComputedVisibility` component have parents that themselves have a
`ComputedVisibility` component.

Note that the warning is only printed once.

We also add a similar warning to `GlobalTransform`.

This only emits a warning. Because sometimes it could be an intended
behavior.

Alternatives:
- Do nothing and keep repeating to newcomers how to avoid recurring
  pitfalls
- Make the transform and visibility propagations tolerant to missing
  components

crates/bevy_hierarchy/Cargo.toml

@@ -15,6 +15,7 @@ trace = []
 # bevy
 bevy_app = { path = "../bevy_app", version = "0.9.0-dev" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.9.0-dev", features = ["bevy_reflect"] }
+bevy_log = { path = "../bevy_log", version = "0.9.0-dev" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.9.0-dev", features = ["bevy"] }
 bevy_utils = { path = "../bevy_utils", version = "0.9.0-dev" }
 

crates/bevy_hierarchy/src/hierarchy.rs

@@ -1,10 +1,13 @@
 use crate::components::{Children, Parent};
 use bevy_ecs::{
+    component::Component,
     entity::Entity,
-    system::{Command, EntityCommands},
+    query::{Added, Changed, Or, With},
+    system::{Command, EntityCommands, Local, Query},
     world::{EntityMut, World},
 };
-use bevy_utils::tracing::debug;
+use bevy_log::warn;
+use bevy_utils::{get_short_name, tracing::debug};
 
 /// Despawns the given entity and all its children recursively
 #[derive(Debug)]
@@ -20,6 +23,29 @@ pub struct DespawnChildrenRecursive {
     pub entity: Entity,
 }
 
+/// System to print a warning if at any time we detect an inconsistent hierarchy.
+pub fn hierarchy_healthcheck_system<T: Component>(
+    parent_query: Query<&Parent, (With<T>, Or<(Changed<Parent>, Added<T>)>)>,
+    component_query: Query<(), With<T>>,
+    mut already_diagnosed: Local<bool>,
+) {
+    if *already_diagnosed {
+        return;
+    }
+    for parent_of_component in &parent_query {
+        let parent = parent_of_component.get();
+        if !component_query.contains(parent) {
+            *already_diagnosed = true;
+            warn!(
+                "warning[B0004]: An entity 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>()),
+            );
+            return;
+        }
+    }
+}
+
 /// Function for despawning an entity and all its children
 pub fn despawn_with_children_recursive(world: &mut World, entity: Entity) {
     // first, make the entity's own parent forget about it

crates/bevy_render/src/view/visibility/mod.rs

@@ -5,7 +5,7 @@ pub use render_layers::*;
 use bevy_app::{CoreStage, Plugin};
 use bevy_asset::{Assets, Handle};
 use bevy_ecs::prelude::*;
-use bevy_hierarchy::{Children, Parent};
+use bevy_hierarchy::{hierarchy_healthcheck_system, Children, Parent};
 use bevy_reflect::std_traits::ReflectDefault;
 use bevy_reflect::Reflect;
 use bevy_transform::components::GlobalTransform;
@@ -170,6 +170,9 @@ pub enum VisibilitySystems {
     /// Label for the [`check_visibility()`] system updating each frame the [`ComputedVisibility`]
     /// of each entity and the [`VisibleEntities`] of each view.
     CheckVisibility,
+    /// Check if [`ComputedVisibility`]
+    /// entities' parents also have a [`ComputedVisibility`] component.
+    HealthCheck,
 }
 
 pub struct VisibilityPlugin;
@@ -214,6 +217,10 @@ impl Plugin for VisibilityPlugin {
                 .after(UpdateProjectionFrusta)
                 .after(VisibilityPropagate)
                 .after(TransformSystem::TransformPropagate),
+        )
+        .add_system_to_stage(
+            CoreStage::Last,
+            hierarchy_healthcheck_system::<ComputedVisibility>.label(HealthCheck),
         );
     }
 }

crates/bevy_transform/src/lib.rs

@@ -14,6 +14,7 @@ pub mod prelude {
 
 use bevy_app::prelude::*;
 use bevy_ecs::prelude::*;
+use bevy_hierarchy::hierarchy_healthcheck_system;
 use prelude::{GlobalTransform, Transform};
 
 /// A [`Bundle`] of the [`Transform`] and [`GlobalTransform`]
@@ -81,6 +82,9 @@ impl From<Transform> for TransformBundle {
 pub enum TransformSystem {
     /// Propagates changes in transform to children's [`GlobalTransform`](crate::components::GlobalTransform)
     TransformPropagate,
+    /// Check if [`GlobalTransform`](crate::components::GlobalTransform)
+    /// entities' parents also have a `GlobalTransform` component.
+    HealthCheck,
 }
 
 /// The base plugin for handling [`Transform`] components
@@ -99,6 +103,10 @@ impl Plugin for TransformPlugin {
             .add_system_to_stage(
                 CoreStage::PostUpdate,
                 systems::transform_propagate_system.label(TransformSystem::TransformPropagate),
+            )
+            .add_system_to_stage(
+                CoreStage::Last,
+                hierarchy_healthcheck_system::<GlobalTransform>.label(TransformSystem::HealthCheck),
             );
     }
 }

errors/B0004.md

@@ -0,0 +1,113 @@
+# B0004
+
+A runtime warning.
+
+An [`Entity`] with a hierarchy-inherited component has a [`Parent`]
+without the hierarchy-inherited component in question.
+
+The hierarchy-inherited components are:
+
+- [`ComputedVisibility`]
+- [`GlobalTransform`]
+
+For example, the following code will cause a warning to be emitted:
+
+```rust,no_run
+use bevy::prelude::*;
+
+// WARNING: this code is buggy
+fn setup_cube(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    commands
+        .spawn_bundle(TransformBundle::default())
+        .with_children(|parent| {
+            // cube
+            parent.spawn_bundle(PbrBundle {
+                mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+                material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
+                transform: Transform::from_xyz(0.0, 0.5, 0.0),
+                ..default()
+            });
+        });
+
+    // camera
+    commands.spawn_bundle(Camera3dBundle {
+        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..default()
+    });
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup_cube)
+        .run();
+}
+```
+
+This code **will not** show a cube on screen.
+This is because the entity spawned with `commands.spawn_bundle(…)`
+doesn't have a [`ComputedVisibility`] component.
+Since the cube is spawned as a child of an entity without the
+[`ComputedVisibility`] component, it will not be visible at all.
+
+To fix this, you must use [`SpatialBundle`] over [`TransformBundle`],
+as follow:
+
+
+```rust,no_run
+use bevy::prelude::*;
+
+fn setup_cube(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    commands
+        // We use SpatialBundle instead of TransformBundle, it contains the
+        // ComputedVisibility component needed to display the cube,
+        // In addition to the Transform and GlobalTransform components.
+        .spawn_bundle(SpatialBundle::default())
+        .with_children(|parent| {
+            // cube
+            parent.spawn_bundle(PbrBundle {
+                mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+                material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
+                transform: Transform::from_xyz(0.0, 0.5, 0.0),
+                ..default()
+            });
+        });
+
+    // camera
+    commands.spawn_bundle(Camera3dBundle {
+        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..default()
+    });
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup_cube)
+        .run();
+}
+```
+
+A similar problem occurs when the [`GlobalTransform`] component is missing.
+However, when a parent [`GlobalTransform`] is missing,
+it will simply prevent all transform propagation,
+including when updating the [`Transform`] component of the child.
+
+You will most likely encouter this warning when loading a scene
+as a child of a pre-existing [`Entity`] that does not have the proper components.
+
+[`ComputedVisibility`]: https://docs.rs/bevy/*/bevy/render/view/struct.ComputedVisibility.html
+[`GlobalTransform`]: https://docs.rs/bevy/*/bevy/transform/components/struct.GlobalTransform.html
+[`Transform`]: https://docs.rs/bevy/*/bevy/transform/components/struct.Transform.html
+[`Parent`]: https://docs.rs/bevy/*/bevy/hierarchy/struct.Parent.html
+[`Entity`]: https://docs.rs/bevy/*/bevy/ecs/entity/struct.Entity.html
+[`SpatialBundle`]: https://docs.rs/bevy/*/bevy/render/prelude/struct.SpatialBundle.html
+[`TransformBundle`]: https://docs.rs/bevy/*/bevy/transform/struct.TransformBundle.html

errors/src/lib.rs

@@ -6,3 +6,6 @@ pub struct B0002;
 
 #[doc = include_str!("../B0003.md")]
 pub struct B0003;
+
+#[doc = include_str!("../B0004.md")]
+pub struct B0004;