The date and time the document file was last modified.
- iOS 5.0+
The modification date is updated by the
revert(to methods. Its value is
nil if none of these methods has completed successfully at least once. If you override any of these methods, you should be sure to set this property in your implementation.
UIKit sets this property before it calls the completion handlers of the
revert(to. If, outside of these methods or their completion handlers, you want to wait for any pending file operations to complete before you access this property, you can call
perform and access the property value in the block parameter.