KVMapper 0.4.0

KVMapper 0.4.0

LangLanguage Obj-CObjective C
License MIT
ReleasedLast Release Dec 2014

Maintained by Unclaimed.

KVMapper 0.4.0

  • By
  • Daniel Haight

A Library to map one NSDictionary to another that can transform BOTH keys and values with a single map. It also allows you to decouple any formatting you may have to do from one representation of data to another (eg. rest api -> internal model representation).


In trying to allow for as much customisation as possible, this tiny library makes very little assumptions which means you must be somewhat verbose. But see convenience methods that should greatly reduce code for the simpler cases.



(pro-tip use cocoapods)

Drag contents of KVMapper folder to Xcode your favourite IDE.


instant attention grabbing example

You have a dictionary from a JSON Rest API that gives you a key-value store of cat names and their ages: the NSDictionary representaion looks like this:

NSDictionary *[email protected]{
                            @"maggy_the_ninja": @5,
                            @"magrat_the_rockstar": @6,
                            @"mable_the_wizard": @2,
                            … 15 or so more cats

you want this in an NSDictionary which looks like this:

//ages are now in human years (sort of)
NSDictionary *[email protected]{
                            @"maggyTheNinja": @30,
                            @"magratTheRockstar": @36,
                            @"mableTheWizard": @12,
                            … 15 or so more cats

here's how you can do that:

 #import "KVMapper.h"
 #import "Transformers.h" //see apendix, used to do snake_case to llameCase transformation

NSDictionary *catsMappingDict=[KVMapper mappingDictionaryForKeys:catsFromAPI.allKeys
                              defaultKeyTransformer:^NSString *(NSString *catName) {
                                  return [Transformers snakeToLlamaCase:catName];
                            defaultValueTransformer:^id(NSNumber *catAge) {
                                float humanAge=catAge.floatValue+5;
                                return [NSNumber numberWithFloat:humanAge];

NSDictionary *catsForMyModel=[KVMapper mappedDictionaryWithInputDictionary:catsFromAPI mappingDictionary:catsMappingDict];

//catsForMyModel is now as in the form @{@"maggyTheNinja": @30, …… etc.}

Creating a dictionary

To create a dictionary from a dictionary, you need mappedDictionaryWithInputDictionary:mappingDictionary:


+(NSDictionary *)mappedDictionaryWithInputDictionary:(NSDictionary *)inputDict mappingDictionary:(NSDictionary *)mappingDict;
NSDictionary *myMappedDictionary=[KVMapper mappedDictionaryWithInputDictionary:inputDict mappingDictionary:mappingDict];

This takes your input dictionary and a mapping dictionary.

mapping dictionary

A mapping dictionary is an NSDictionary whose keys correspond to the keys of the input dictionary and the values are instances of KVMap.


the KVMap object is just an NSObject with two properties:

  1. keyTransformationBlock
  2. valueTransformationBlock

The former receives and returns an NSString (the key) the latter receives and return id ie any object you want (the value).

creating a KVMap

making a KVMap is a simple as setting the key and value transformation blocks:

KVMap *myKVMap=[[KVMap alloc] init];

myKVMap.keyTransformationBlock=^(NSString *inputString){
    return [inputString lowercaseString];

myKVMap.valueTransformationBlock=^(id *input){
    return @5;

there is an initaliser initWithKeyTransformationBlock:valueTransformationBlock: to create one in a single step.

creating a mapping dictionary

To create a mapping dictionary you must create the various KVMaps you need to map the input dictionary's key-value pairs into the form you want. Then set these as values to the key correspondong to the input dictionary's key:

// the input dictionary
NSDictionary *[email protected]{

//set up maps (you can use the same map if it applies to multiple KV pairs)
KVMap *myKeyOneAndTwoKVMap=[[KVMap alloc] init];
myKeyOneAndTwoKVMap.keyTransformationBlock=^(NSString *inputString){
    return [NSSString stringWithFormat:@"my%@",[inputSting lowerCaseString]];
myKeyOneAndTwoKVMap.valueTransformationBlock=^(id *input){
    return [NSString stringWithFormat:@"%i",input];

KVMap *myKeyThreeKVMap=[[KVMap alloc] init];
myKeyThreeKVMap.keyTransformationBlock=^(NSString *inputString){
    return [inputSting lowerCaseString];

//create mapping dict
NSDictionary *[email protected]{

// [KVMapper mappedDictionaryWithInputDictionary:inputDict mappingDictionary:mappingDict];
// will return:
//    @{
//      @"mykeyone":@"1",
//      @"mykeytwo":@"2",
//      @"mykeythree":@"3"
//      };

Convenience Methods

Writing out all the mapping logic can quickly become tedious, especially when we often have to do the same transformation for a key, value or both.

so KVMapper has: KVMapsForKeys:defaultKeyTransformer: and KVMapsForKeys:defaultKeyTransformer:defaultValueTransformer:

so suppose we want to create a mappingDictionary for where all the keys are transformed to lowerCase and all the values (which we get as strings) are NSURLs.

        NSDictionary *mappingDict=[KVMapper mappingDictionaryForKeys:testDict.allKeys
                                               defaultKeyTransformer:^(NSString *inKey){
                                                   return [Transformers snakeToLlamaCase:inKey];
                                             defaultValueTransformer:^(id inValue){
                                                 NSURL *url=[NSURL URLWithString:inValue];
                                                 return url;

This mappingDict can now be used in


The random transformer class

#import <Foundation/Foundation.h>

@interface Transformers : NSObject
+(NSString *)snakeToLlamaCase:(NSString *)inKey;


#import "Transformers.h"

@implementation Transformers

#pragma mark -string transformers

-(void)setValue:(id)value forUndefinedKey:(NSString *)key{
    NSLog(@"couldn't set value: %@ for undefkey:%@",[value description],key);

+(NSString *)snakeToLlamaCase:(NSString *)inKey{

    NSArray *keyComponents=[inKey componentsSeparatedByString:@"_"];

    return [self llamaCasFromComponents:keyComponents];

+(NSString *)llamaCasFromComponents:(NSArray *)components{
    NSMutableString *llamaCaseString=[NSMutableString string];
    for (NSString *component in components) {
        NSString *firstLetterUpper=[[component substringToIndex:1] uppercaseString];
        NSString *restOfComponent=[component substringFromIndex:1];
        NSString *newComponent=[NSString stringWithFormat:@"%@%@",firstLetterUpper,restOfComponent];
        [llamaCaseString appendString:newComponent];
    NSString *firstLetterLower=[[llamaCaseString substringToIndex:1] lowercaseString];
    [llamaCaseString replaceCharactersInRange:NSMakeRange(0, 1) withString:firstLetterLower];

    return llamaCaseString;



  • methods to take in and return different types of key-value stores eg NSObjects and JSON strings.
  • better docs? any advice or questions are appriciated.
  • include some sensible string/ value transformations. (you should out this TransformerKit by @mattt)




Is welcome!!

  • start a conversation in issues / make a pull request if you have code
  • KVMapper is tested with Kiwi so adding tests to your code would be nice, (also feel free to use OCUnit if you prefer)

Eternal thanks