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iphoneiosmacosnskeyedarchiver

NSKeyedUnarchiver not behaving as expected

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

Solution

  • 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.