The calendar item’s unique identifier.
- iOS 6.0+
- macOS 10.8+
- watchOS 2.0+
This property is set when the calendar item is created and can be used as a local identifier. Use
calendar to look up the item by this value.
A full sync with the calendar will lose this identifier. You should have a plan for dealing with a calendar whose identifier is no longer fetch-able by caching its other properties.