A delegate to be called during the animation loop.
- iOS 8.0+
- macOS 10.10+
- tvOS 9.0+
- watchOS 2.0+
When a delegate is present, when any of the animation loop methods steps are executed, your delegate is called. Typically, you use a delegate when you do not want to implement a scene subclass or if you want to dynamically change the scene behavior at runtime.