UIDocument Recover Unsaved Changes On App Crash/Force Quit

From what I understand, the UIDocument class can track unsaved changes to a file and even locks the file so it cannot be checked out by more than one person. But what happens if the user force quits the app without saving or the app crashes? How would I go about recovering the unsaved changes to a UIDocument so that when the app re-launches it reopens the UIDocument with the most recent unsaved changes? Do I need to make duplicate copies of each file before it is changed and alter the temporary duplicate until the user saves the changes? Or does Apple provide a simpler implementation? I have also considered encoding and storing the Data file contents and the undoManager of each UIDocument instance periodically as a cache. Would that work?