Notifies the delegate that the specified supplementary view is about to be displayed by the collection view.
- macOS 10.11+
The collection view that is adding the supplementary view.
The supplementary view being added.
The type of the supplementary view. Layouts are responsible for defining the types of supplementary views they support.
The index path associated with the supplementary view.
The collection view calls this method before adding new supplementary views to its content. You can use this method to track the addition of those views and perform related tasks.