Question

I have come across an odd behaviour in NSKeyedUnarchiver.

I archive a data, for this example I just use a string:

NSMutableData       *myData = [ NSMutableData data ];
NSKeyedArchiver     *archiver = [[ NSKeyedArchiver alloc ] initForWritingWithMutableData:myData ];
NSString            *testString = [ NSString stringWithString:@"Hello World!" ];
[ archiver encodeObject:testString forKey:@"test" ];
[ archiver finishEncoding ];

and then I want to unarchive it. if I use

NSKeyedUnarchiver  *unarchiver = [ NSKeyedUnarchiver unarchiveObjectWithData:myData ];
NSString     *result = [ unarchiver decodeObjectForKey:@"test" ];

I get nil result, but if I use:

NSKeyedUnarchiver  *unarchiver = [[ NSKeyedUnarchiver alloc ] initForReadingWithData:myData ];
NSString     *result = [ unarchiver decodeObjectForKey:@"test" ];

I get my string back. Now I thought that [ NSKeyedUnarchiver unarchiveObjectWithData:myData ] is just a convenience method for [[ NSKeyedUnarchiver alloc ] initForReadingWithData:myData ], but it does not seem to behave in that way. Am I wrong.

Thanks
Reza

Answer 1

unarchiveObjectWithData

Apple: “Decodes and returns the object graph previously encoded by NSKeyedArchiver and stored in a given NSData object.”

Which means it unarchives a complete class, not preparing to unarchive individual objects.

initForReadingWithData

Apple: “Initializes the receiver for decoding an archive previously encoded by NSKeyedArchiver.

Returns: a NSKeyedUnarchiver object initialized for for decoding data.”

Which means just initializing an un-archiver ready to accept unarchive method calls.