Default Mesh3d with Bevy 0.15 require components

[TanJunKiat]

The new require components is really helpful to simplify the components required to define for a bundle and/or entity.

I want to create an entity where it has a default Mesh3d of a 1 meter cube and using the scale attribute in Transform to resize it, because it is easier to query and recover the size in other systems.

Right now, I am struggling to define a Default trait for my mesh

#[derive(Component)]
#[require(Transform, Mesh3d(default_mesh))]
struct Parent;

fn default_mesh() -> Mesh3d {
    let mut mesh = Mesh3d::default();
    /// what to do here
    return mesh;
}

What i am familiar with is using the assets server to spawn meshes. Not sure how should I go about doing this.

[Henauxg]

While you can't access the Assets resources in a default constructor, you can achieve what you want with Component Hooks. Here's an example:

#[derive(Component)]
#[require(Transform, MeshMaterial3d<StandardMaterial>)]
#[component(on_add = add_default_cube)]
struct CustomComponent;

pub fn add_default_cube(mut world: DeferredWorld<'_>, HookContext { entity, .. }: HookContext) {
    // Retrieve a mesh handle. This handle could also be retrieved from a Resource instead of created here
    let Some(mut meshes) = world.get_resource_mut::<Assets<Mesh>>() else {
        return;
    };
    let mesh_handle = meshes.add(Cuboid::from_length(1.));
    // Add the component
    world.commands().entity(entity).insert(Mesh3d(mesh_handle));
}

Now simply spawning CustomComponent will spawn default cubes.

    cmd.spawn(CustomComponent);

I added MeshMaterial3d<StandardMaterial> as a required component, so the default material will be applied, but you could add a custom material in the Hook the same way.