How to implement a "CallbackAction" in RealityKit like SceneKit's SCNAction.run()?

I am converting my SceneKit game to RealityKit. One difficulty is to convert SCNAction.run. For example:

node.runAction(
  .sequence[
    .scale(to:...),
    .run { /* do something */ }
    .fadeOut(...),
    .run { /* do something else */ },
  ]
)

This is a very common pattern that exist in most game libraries (e.g. SceneKit, SpriteKit, Cocos2D, etc).

However, looking at RealityKit's action API, it looks like they only provide several basic actions such as https://developer.apple.com/documentation/realitykit/fromtobyaction

I am wondering how to support CallbackAction like in SceneKit?

It looks like Apple allows you to create your own Action but the documentation isn't great: https://developer.apple.com/documentation/realitykit/entityaction

Apple's doc is indeed not helpful, but I was able to get callback action working:

    struct CallbackAction: EntityAction {
      var animatedValueType: (any AnimatableData.Type)? = nil
      let callback: (Entity?) -> Void
      init(callback: @escaping (Entity?) -> Void) {
        self.callback = callback
      }
      
    }
    
    struct CallbackActionHandler: ActionHandlerProtocol {
      typealias ActionType = CallbackAction
      let action: ActionType
      let entity: Entity?
      init(action: ActionType, entity: Entity?) {
        self.action = action
        self.entity = entity
      }
      
      func actionEnded(event: EventType) {
        action.callback(entity)
      }
    }
    

Before using it, make sure to register the handler. This will allow RealityKit to find the right handler when processing your action.

    CallbackActionHandler.register { event in
      return CallbackActionHandler(action: event.action, entity: event.targetEntity)
    }

    let callbackAction = CallbackAction { entity in
      print("callback")
    }