Schedules a document-reading or document-writing operation on a concurrent background queue.
- iOS 5.0+
A block that is invoked as the task to execute on the background queue. The block returns no value and takes no parameters.
The default implementations of
open(completion call this method to serialize file access. If you override these methods and do not call
super, you should call this method to serialize file access on a background queue. If you directly call the
read(from:) method, you should wrap that call in the block passed into