Mark and Modify Blender Objects, Materials, and Meshes

When Components are inserted onto an Entity, such as the ones we've defined in Blender, Hook and Observer events will fire and allow us to write code that runs in reaction.

Hooks

Bevy's Component Hooks run basically immediately when a Component is added to an Entity. It is possible to define at least the following hooks for a Component:

on_add
on_insert
on_replace
on_remove
on_despawn

A practial use case for hooks is shown in Replace a Blender Material.

Defining a hook can be done via attribute macro:

#[derive(Component, Reflect)]
#[reflect(Component)]
#[component(on_add = on_add_use_force_field_material)]
struct UseForceFieldMaterial;

Its notable that the hook function has a different signature than the systems you might be more used to writing, and things like commands need to be accessed via the DeferredWorld.

fn on_add_use_force_field_material(
    mut world: DeferredWorld,
    HookContext { entity, .. }: HookContext,
) {
    let force_field = world
        .resource::<MaterialStore>()
        .force_field
        .clone();

    world
        .commands()
        .entity(entity)
        .remove::<MeshMaterial3d<StandardMaterial>>()
        .insert(MeshMaterial3d(force_field));
}

Observers

Observers can react to more events than hooks and are thus more flexible. Observers run just after Hooks.

Similar to Hooks, Observers can trigger on many of the same events:

A global observer can be added at the App level.

App::new()
    .add_observer(on_add_force_field)
    .run()

and Observers look a lot more like regular systems.

fn on_add_force_field(
    trigger: Trigger<OnAdd, UseForceFieldMaterial>,
    mut commands: Commands,
    material_store: Res<MaterialStore>,
) {
    let force_field = material_store
        .force_field
        .clone();

    commands
        .entity(trigger.target())
        .remove::<MeshMaterial3d<StandardMaterial>>()
        .insert(MeshMaterial3d(force_field));
}