A key whose value indicates whether the image request was canceled.
- 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 a Boolean value.If you call the
cancel method to cancel a request, Photos calls your result handler block with the value
true for this key.