Returns the item associated with the specified index path.
- macOS 10.11+
The index path whose item you want.
The item for the specified index path or
nil if no item is available.
For efficiency, the collection view does not create items until they are needed, and usually it creates them only when they need to be displayed onscreen. If the collection view does not currently have an item for the specified index path, because that item would be positioned offscreen, this method returns