Mantle is my number 1 choice for the model class. It has a lot of predefined value transformers, you can take a look at NSValueTransformer+MTLInversionAdditions and NSValueTransformer+MTLPredefinedTransformerAdditions.

Predefined value transformer

Currently Mantle 1.5 supports these value transformers

mtl_invertedTransformer
mtl_JSONDictionaryTransformerWithModelClass
mtl_JSONArrayTransformerWithModelClass
mtl_valueMappingTransformerWithDictionary
URLValueTransformer
booleanValueTransformer

The transformer without parameters (like URLValueTransformer and booleanValueTransformer are registered with NSValueTransformer in the “load” method

[code language=”objc”]

• (void)load {
  @autoreleasepool {

  MTLValueTransformer *URLValueTransformer = [MTLValueTransformer
      reversibleTransformerWithForwardBlock:^ id (NSString *str) {
          if (![str isKindOfClass:NSString.class]) return nil;
          return [NSURL URLWithString:str];
      }
      reverseBlock:^ id (NSURL *URL) {
          if (![URL isKindOfClass:NSURL.class]) return nil;
          return URL.absoluteString;
      }];
  
  [NSValueTransformer setValueTransformer:URLValueTransformer forName:MTLURLValueTransformerName];
  
  MTLValueTransformer *booleanValueTransformer = [MTLValueTransformer
      reversibleTransformerWithBlock:^ id (NSNumber *boolean) {
          if (![boolean isKindOfClass:NSNumber.class]) return nil;
          return (NSNumber *)(boolean.boolValue ? kCFBooleanTrue : kCFBooleanFalse);
      }];
  
  [NSValueTransformer setValueTransformer:booleanValueTransformer forName:MTLBooleanValueTransformerName];
  

  }
  }
  [/code]

But eventually, those transformers end up calling reversibleTransformerWithForwardBlock:reverseBlock: and the real converter is the MTLJSONAdapter

Custom value transformer

If those value transformer do not suit your need. You can learn from what Mantle already does, and build your own value transformer

For example, this is a custom value transformer from my FTGValueTransformer that converts the first object in a JSON array to a model

[code language=”objc”]

• (NSValueTransformer )ftg_JSONFirstObjectInArrayTransformerWithModelClass:(Class)modelClass {
  NSValueTransformer dictionaryTransformer = [self mtl_JSONDictionaryTransformerWithModelClass:modelClass];

  return [MTLValueTransformer

  reversibleTransformerWithForwardBlock:^ id (NSArray *dictionaries) {
      if (dictionaries == nil || dictionaries.count == 0) {
          return nil;
      }
  
      NSAssert([dictionaries isKindOfClass:NSArray.class], @"Expected a array of dictionaries, got: %@", dictionaries);
  
      NSDictionary *firstDictionary = [dictionaries firstObject];
  
      NSAssert([firstDictionary isKindOfClass:NSDictionary.class], @"Expected a dictionary or an NSNull, got: %@", firstDictionary);
  
      id model = [dictionaryTransformer transformedValue:firstDictionary];
  
      return model;
  }
  reverseBlock:^ id (id model) {
      if (model == nil) {
          return nil;
      }
  
      NSAssert([model isKindOfClass:MTLModel.class], @"Expected an MTLModel or an NSNull, got: %@", model);
  
      NSDictionary *dict = [dictionaryTransformer reverseTransformedValue:model];
  
      if (dict) {
          return @[ dict ];
      }
  
      return @[];
  }];
  

  }
  [/code]

reversibleTransformerWithForwardBlock:reverseBlock:

The reversibleTransformerWithForwardBlock is called to convert from JSON to model, and reverseBlock is called to convert model back to JSON

Take a look at this example, it is a value transformer that convert NSNumber to NSString

[code language=”objc”]

• (NSValueTransformer )ftg_stringValueTransformer {
  NSValueTransformer stringValueTransformer = [MTLValueTransformer

  reversibleTransformerWithBlock:^ id (NSNumber *number) {
      if (![number isKindOfClass:NSNumber.class]) {
          return  nil;
      }
  
      return number.stringValue;
  }];
  

  return stringValueTransformer;
  }
  [/code]

In the reversibleTransformerWithBlock, the parameter is of kind NSNumber (in JSON dictionary) and the return value is of kind NSString (the property you declare on your model)

transformerWithBlock:

If you just need to convert from JSON to model, you can just use transformerWithBlock, which just hav reverseBlock set to nil

How to hook into Mantle

If for some reason, you don’t want to use those value transformer, you can override initWithDictionary method (remember to call super).

[code language=”objc”]

• (instancetype)initWithDictionary:(NSDictionary )dictionaryValue error:(NSError *)error {
  self = [super initWithDictionary:dictionaryValue error:error];
  if (self == nil) return nil;

  // Store a value that needs to be determined locally upon initialization.
  _retrievedAt = [NSDate date];

  return self;
  }
  [/code]

Reference

1. Mantle

1. Possible to access first object in nested array ?

1. FTGValueTransformer

1. How to hook into Mantle