Dynamically changing the material of an entity

[vincent-herlemont]

I am trying to dynamically change the material of an entity via an event (websocket in my case). I am able to retrieve the IDs of my different materials and meshes (I am basing this on the glTF specification).

I can retrieve the material assets via the world.resource_mut command. Then I manage to obtain the material corresponding to the specific ID using material_assets.get_strong_handle, and I can use a Bevy Query to insert the new material.

Problem: The first modification is performed perfectly ✅ . However, the second one seems to remove the previous materials ❌ .

Do you have any debugging tips, or where could this be coming from?

fn refresh_mesh_primitive(world: &mut World, mesh_primitive: &MeshPrimitive) {
    let mut material_assets = world.resource_mut::<Assets<StandardMaterial>>();
    let mesh_primitive_material_asset = if let Some(material_id) = mesh_primitive.material {
        let uuid: uuid::Uuid = material_id.into();
        let asset_id = AssetId::from(uuid);
        material_assets.get_strong_handle(asset_id).unwrap()
    };

    let entities_to_update: Vec<Entity> = {
        let mut query = world.query::<(Entity, &MeshPrimitiveId)>();
        query
            .iter(world)
            .filter(|(_, mesh_primitive_id)| mesh_primitive_id.0 == mesh_primitive.id)
            .map(|(entity, _)| entity)
            .collect::<Vec<_>>()
    };

    let mut commands = world.commands();
    for entity in entities_to_update {
        commands
            .entity(entity)
            .insert(mesh_primitive_material_asset.clone());
    }
}

Here is a video of two modifications (the first one works, the second does not)

1. The central cube turns blue ✅
2. The central cube turns green (issue: the other cube in the back with the blue texture disappears ❌)

Screen.Recording.2024-12-12.at.17.31.29.mp4

[vincent-herlemont]

Note: when the same material is inserted twice into an entity, it disappears from view (the second time).

        commands
            .entity(entity)
            .insert(mesh_primitive_material_asset.clone());

[vincent-herlemont]

I understood why there's this behavior, because Assets.get_strong_handle upgrades the ID (and in my case changes the UUID) to another one, consequently, the indexing in Bevy is no longer correct.

However, I don't see how to use Assets.get to insert it into the entity, because the get method returns Option<&A>, which is a reference.

let new_material_asset: &StandardMaterial = material_assets.get(asset_id).unwrap();
commands.entity(entity).insert(new_material_asset);

Solution

Actually, I understood that I can use the ID directly instead of retrieving the assets, and everything works perfectly ✅

let handle_material_asset_id = Handle::Weak(asset_id);
commands.entity(entity).insert(handle_material_asset_id);