Submits a request to the focus engine for a focus update in this environment.
- iOS 9.1+
- tvOS 9.0+
If this environment does not currently contain the focused view, calling this method has no effect. Otherwise, and if the focus update is accepted by the focus engine, focus is reset to the preferred focused view on the run loop cycle. If a parent of this environment is also requesting focus, the parent's request takes priority.