A key whose value is a unique identifier for the image request.
- iOS 8.0+
- macOS 10.13+
- tvOS 10.0+
This key provides information about an image loading result in the
result block for methods listed in Requesting Images. The corresponding value is an
NSNumber object containing an integer value.This identifier matches that returned when making a request. You can use it with the
cancel method to cancel requests with pending results that are no longer needed.