How do I save additional content into my UIManagedDocument file packages?

Question

I\'m having a lot of trouble deciphering Apple\'s documentation around UIManagedDocument, specifically the following methods:

• - (id)additionalContentForU

Answer 1

The documentation for -additionalContentForURL:error: indicates that returning a nil supposed to signal an error.

  A return value of nil indicates an error condition. To avoid generating 
  an exception, you must return a value from this method. If it is not always
  the case that there will be additional content, you should return a sentinel value (for example, an NSNull instance) that you check for in
  writeAdditionalContent:toURL:originalContentsURL:error:.

I override -writeContents:andAttributes:safelyToURL:forSaveOperation:error: for another purpose (doing some stuff on first save of a new document), and calling super invokes the NSException gods because contents value is nil, not an NSDictionary as seemingly expected by UIManagedDocument. Hmm.

The more you know...

P.S. I guess it depends on the time of day with -writeContents:andAttributes:... It once threw an exception complaining about expecting an NSDictionary, but later threw an exception complaining that I didn't pass it an NSData. My eyebrow could not be raised in a more Spock-like fashion than it is right now.