Called just before the view controller is added or removed from a container view controller.
- iOS 5.0+
- tvOS 9.0+
The parent view controller, or
nilif there is no parent.
Your view controller can override this method when it needs to know that it has been added to a container.
If you are implementing your own container view controller, it must call the
will method of the child view controller before calling the
remove method, passing in a parent value of