Question

I’m newer to Xcode development and am trying to save the state of my app which tracks multiple index sets, integers and strings. I’ve tried a lot of different code and haven’t been able to get it to work saving to a .plist. What is the best approach for saving the following data types, NSMutableIndexSets and NSUIntegers? Any direction would be great, Thanks.

Answer 1

The short answer to your question is that you can’t save index sets to a plist, or to user defaults. There is a very short list of object types that you can write to a plist. Look up the docs on the NSDictionary class in Xcode, and search for the string “property list object” That’s where they tell you which objects can be written to a properly list. The object types are NSString, NSData, NSDate, NSNumber, NSArray, or NSDictionary objects.

Omar Abdelhafith posted a very long and complex block of code to convert an index set to an array, which should work.

There is a much simpler way however. NSIndexSet conforms to the NSCoding protocol, which means that you can convert it to NSData with a single call:

NSData *setData = [NSKeyedArchiver archivedDataWithRootObject: mySet];

And to turn it back into an index set:

NSIndexSet *setFromData= [NSKeyedUnarchiver unarchiveObjectWithData: setData];
NSMutableIndexSet *mutableSet = [setFromData mutableCopy];

Note that with all of these approaches, if you start with a mutable object (set, array, dictionary, etc) then when you read it back, the object you get back will be an immutable version. You have to manually convert it to a mutable version. Most objects that have a mutable variant support the method mutableCopy.