NSKeyedArchiver+butWithNSError

NSKeyedArchiver+butWithNSError is a very small project that provides two categories: one on NSKeyedArchiver and one on NSKeyedUnarchiver:

@interface NSKeyedArchiver (butWithNSError)
+ (NSData*)jr_archivedDataWithRootObject:(id)rootObject
					requiresSecureCoding:(BOOL)requiresSecureCoding
								   error:(NSError**)error
@end

@interface NSKeyedUnarchiver (butWithNSError)
+ (id)jr_unarchiveData:(NSData*)data
  requiresSecureCoding:(BOOL)requiresSecureCoding
			 whitelist:(NSArray*)customClassWhitelist
				 error:(NSError**)error;
@end

These categories make it easier to use NSKeyedArchiver and NSKeyedUnarchiver correctly, securely and cope with the fact NSKeyedUnarchiver throws exceptions.

These should be compatible with both ARC and MRC.

Use

The categories are mostly self-evident.

Call +[NSKeyedArchiver(butWithNSError) jr_archivedDataWithRootObject:requiresSecureCoding:error:] to serialize rootObject's object graph into an NSData. Set requiresSecureCoding to YES if you want an error returned if any object doesn't support NSSecureCoding.

Call +[NSKeyedUnarchiver(butWithNSError) jr_unarchiveData:requiresSecureCoding:whitelist:error:] to deserialize the data back into objects. requiresSecureCoding works the same way as above.

whitelist can be nil or empty, but if supplied will be added to the list of trusted classes allowed to be deserialized. Naturally these classes must support NSSecureCoding otherwise you'll get a runtime error.

The Plist-standard are automatically whitelisted for you: NSArray, NSDictionary, NSString, NSNumber, NSDate, NSData.

Ease

In the Good Old Days, you'd call

+[NSKeyedArchiver archivedDataWithRootObject:]

and

+[NSKeyedUnarchiver unarchiveObjectWithData:]

and would be done for the day and catch an X-Files rerun. I hope the Smoking Man's in this one.

But then NSSecureCoding burst onto the scene. Now you need to manually create your archivers and unarchivers just so you can call their concealed -setRequiresSecureCoding: methods.

Not only are you now in the creation and configuration business, you need to use it correctly. And it turns out there's a trap for the unwary.

Having to reimplement +archivedDataWithRootObject:, you'd be forgiven thinking that you should utilize -[NSKeyedArchiver encodeRootObject:]. That's what I did.

Bzzt. Unfortunately it turns out generates a binary plist in a format (I call this "Format 1") that's incompatible with +[NSKeyedUnarchiver unarchiveObjectWithData:] (which requires what I call "Format 2").

Here's a table of which methods play with which format. I think.

Method	Binary Plist Format	NSSecureCoding compatible
-[NSKeyedArchiver encodeRootObject:]	Format 1	no
-[NSKeyedUnarchiver decodeObject]	Format 1	no
+[NSKeyedArchiver archivedDataWithRootObject:]	Format 2	no
+[NSKeyedUnarchiver unarchiveObjectWithData:]	Format 2	no
-[NSKeyedArchiver encodeObject:forKey:NSKeyedArchiveRootObjectKey]	Format 2	yes
-[NSKeyedUnarchiver decodeObjectForKey:NSKeyedArchiveRootObjectKey]	Format 2	yes

The Bad News is that you must use -[NSKeyedUnarchiver decodeObject] to decode what -[NSKeyedArchiver encodeRootObject:] is cooking, and -decodeObject isn't compatible with NSSecureCoding (you need to use -decodeObjectOfClasses:forKey:).

tl;dr: don't use -[NSKeyedArchiver encodeRootObject:] or -[NSKeyedUnarchiver decodeObject] in new code unless you need compatibility with Format 1 archives.

Since +[NSKeyedArchiver archivedDataWithRootObject:] and +[NSKeyedUnarchiver unarchiveObjectWithData:] don't give you an opportunity to call -setRequiresSecureCoding:YES, they're out of the party as well.

That leaves us with -[NSKeyedArchiver encodeObject:forKey:NSKeyedArchiveRootObjectKey] and -[NSKeyedUnarchiver decodeObjectForKey:NSKeyedArchiveRootObjectKey]. Which is what NSKeyedArchiver+butWithNSError uses.

NSError**

I could make a case for NSKeyedArchiver throwing exceptions since as the programmer you can know what you're passing it. But it seems wrong that NSKeyedUnarchiver throws exceptions.

Here is a class responsible for decoding potentially hostile data and it blows up your entire process if things aren't exactly to its liking?! That's just nuts.

NSKeyedArchiver+butWithNSError catches exceptions, unravels them and transmogrifies them into NSErrors for you so you can handle them normally (I'm biased, but I recommend JRErr).

TODO

Promote to v1.0 when I ship.

Version History

v1.0b1: Mar 28 2014

• Initial release.