Adding just a Node component to an empty entity causes a panic

[SludgePhD]

Bevy version

0.11

What you did

Spawned an entity with a SpatialBundle and added a Node component to it.

What went wrong

Since this can easily be done from an editor or a script, this shouldn't cause a crash, but it caused a panic inside bevy:

thread 'Compute Task Pool (8)' panicked at 'called `Option::unwrap()` on a `None` value', /home/sludge/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ui-0.11.0/src/layout/mod.rs:169:52

[bushrat011899]

Can confirm this harness will reproduce this behaviour on the main branch:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn((
        SpatialBundle::default(),
        Node::default(),
    ));
}

Looks like the ui_layout_system finds an Entity with the query root_node_query: Query<Entity, (With<Node>, Without<Parent>)> that breaks the assumptions made for this line.

This indicates that the Entity found in root_node_query was never added to the UiSurface. Scrolling up to here, it's clear that the Entity is only added to the surface if it has a Style component.

Modifying my test to add a default Style component removes the error and the app runs as expected:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn((
        SpatialBundle::default(),
        Node::default(),
        Style::default(),
    ));
}

My question for someone more familiar with Bevy's UI system would then be: is it intended that a Node should exist on an Entity even if it does not have a Style component?

[nicopap]

You are supposed to use NodeBunlde for UI. But I don't think a panic is an OK thing to do. In a loading scenario, it is expected that part of the UI might be ready while others are still loading. Panicking unconditionally makes bevy_ui pretty much unusable in such a common scenario.

[ickshonpe]

My question for someone more familiar with Bevy's UI system would then be: is it intended that a Node should exist on an Entity even if it does not have a Style component?

Ideally yes. Users might want to write their own layout system but use Bevy UI's rendering.

[SludgePhD]

A panic also happens when spawning an entity without Node or Style as a child of a UI node. It seems to me that all UI systems probably need to have all of their .unwrap()s reviewed.

[nicopap]

@SludgePhD the specific issue of panicking on non-UI child of UI should be fixed by #9621

[nicoburns]

My question for someone more familiar with Bevy's UI system would then be: is it intended that a Node should exist on an Entity even if it does not have a Style component?

This seems like it ought to be allowed to me. The UI system can set the entity's style to Style::default() in the Taffy tree if it doesn't have a Style component.