bevy/errors/B0004.md

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 defined in bevy include:

• ComputedVisibility
• GlobalTransform

Third party plugins may also define their own hierarchy components, so read the warning message carefully and pay attention to the exact type of the missing component.

To fix this warning, add the missing hierarchy component to all ancestors of entities with the hierarchy component you wish to use.

The following code will cause a warning to be emitted:

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(TransformBundle::default())
        .with_children(|parent| {
            // cube
            parent.spawn(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(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_systems(Startup, setup_cube)
        .run();
}

This code will not show a cube on screen. This is because the entity spawned with commands.spawn(…) 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 follows:

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(SpatialBundle::default())
        .with_children(|parent| {
            // cube
            parent.spawn(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(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_systems(Startup, 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 encounter this warning when loading a scene as a child of a pre-existing Entity that does not have the proper components.