Transform String to Core Data by NSValueTransformer - iphone

Question

I'm trying to use Core Data NSValueTransformer to transform NSArray to NSString. I'm not sure whether it can be archived, but I saw apple's official doc show a code snippet:
#interface ClassNameTransformer: NSValueTransformer {}
#end
#implementation ClassNameTransformer
+ (Class)transformedValueClass { return [NSString class]; }
+ (BOOL)allowsReverseTransformation { return NO; }
- (id)transformedValue:(id)value {
return (value == nil) ? nil : NSStringFromClass([value class]);
}
#end
It seems it can store data into NSString (maybe I misunderstood..), so I tried like below:
#implementation ArrayToStringTransformer
+ (BOOL)allowsReverseTransformation {
return YES;
}
+ (Class)transformedValueClass {
return [NSString class];
}
- (id)transformedValue:(id)value {
// return NSStringFromClass([value class]);
// return NSStringFromClass([#"11" class]);
NSLog(#"!!!!!! %#, %#", NSStringFromClass([value class]), value);
if ([value isKindOfClass:[NSString class]])
return NSStringFromClass([value class]);
NSMutableString * string = [NSMutableString string];
for (NSNumber * number in value)
[string appendString:[NSString stringWithFormat:#"%#,", number]];
return string;
}
- (id)reverseTransformedValue:(id)value {
NSArray * array = [value componentsSeparatedByString:#","];
return array;
}
#end
However, it crashed with the error below (it includes the NSLog):
[26364:11903] !!!!!! __NSCFString, 0,0
[26364:11903] -[__NSCFString bytes]: unrecognized selector sent to instance 0x905f620
[26364:11903] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString bytes]: unrecognized selector sent to instance 0x905f620'
Any idea? Please!!
EDIT:
Well, I just transform NSArray to NSData, and if it is NSString type, transform it to NSArray first, then to NSData, and it works:
+ (Class)transformedValueClass {
return [NSArray class];
}
- (id)transformedValue:(id)value {
if (! [value isKindOfClass:[NSString class]])
return [NSKeyedArchiver archivedDataWithRootObject:value];
NSLog(#"!!! Convert NSString to NSArray");
NSArray * array = [value componentsSeparatedByString:#","];
NSData * data = [NSKeyedArchiver archivedDataWithRootObject:array];
return data;
}
- (id)reverseTransformedValue:(id)value {
return [NSKeyedUnarchiver unarchiveObjectWithData:value];
}
!!! Attention Here
But I wonder whether it is a right way? As you can see, I return [NSArray class] in transformedValueClass method, but actually return NSData type data value in transformedValue:.
The apple DOC said:
An NSData object containing the encoded form of the object graph whose root object is rootObject
I'm totally confused...

Answer 1

Currently Core Data can only transform to NSData. The default transformer can handle transforming an array of strings to NSData by setting the attribute type to transformable and that's the only configuration required.
Personally I would like Core Data in the future to allow transformation to a string, so for example when using a SQLite browser the record field is human-readable instead of being a binary plist.
One theory why they might not have included this feature, is if you are trying to include an array of strings, then you would likely be better off using a many-to-many relation for implementing what you are trying to store instead.