An Objective C framework for mapping parsed JSON into domain objects automatically
Latest commit 8b00283 Jul 21, 2012 @mattiaslevin Update


ObjectMapper is an Objective-C framework that map parsed JSON objects (e.g. from JSONKit) into domain objects automatically.


Most JSON parsers in Objective-C will parse your JSON and delivery either NSDictionary or NSArray as the result. Turning these objects into domain objects often requires writing repetative boilerplate code, manually setting your properties from the NSDictionary and NSArray. Not any longer, ObjectMapper will solve this as long as you follow a few conventions when defining your domain objects.

How does it work?

Turning JSON into domain objects typically involves the following steps:

  1. Request your JSON with a http request
  2. Parse the response JSON using a JSON parser/decoder, e.g. JSONKit
  3. Manually map the parsed JSON into the domain objects properties
  4. Use your newly populated domain objects to store in core data or use as your model objects

The framework address step 3 above, mapping the parsed JSON into domain objects automatically, drastically reducing the need to write boilerplate code.

The framework will iterate over the parsed JSON objects (normally NSDictonary and NSArray) and figure out what domain objects to create and what properties to set using the runtime system and reflection.

Mapping conventions

The following conventions are used for mapping key/values into domain objects:

  1. Each key/value-pair in the NSDictionary will be mapped to a property with the same name in the domain object
  2. If the key is a reserved word in Objective-C, add “_” in front of the property name in the domain object to get around any compiler errors
  3. Use the macro MapKeyToPropert to map a key to an arbitrary property name
  4. If a matching property can not be found in the domain object at this point, it will be ignored

If the property is a class, a new instance of that class will be created and assigned to the property and the mapping process will continue.

If the property is an array, a new instance of NSArray will be created and assigned to the property. Since it is not possible to find out the type of the object to put in the array just by inspecting the property, the developer need to help the mapper by annotating the property with the MapClassToArray marco. The mapper will use this macro to figure out what class should go into the array. A new instance of this class is created for each item in the list and added to the array and the mapping proccess will continue.

The process steps described above will continue recursevly until the complete object graph has been created.


The framework use macros to help the mapper when the normal conversions are not enough.

  • MapKeyToPropert(key, property) - Map a key/value to an arbitrary property
  • MapClassToArray(clazz, property)- Tell the mapper what class to put in a NSArray property

Use these macros in the @implementation section of your domain objects just above the properties @synthesize statement.

@implementation Person

MapKeyToPropert(name, personName)
@synthesize personName;

MapClassToArray(NSString, phoneNumbers)
@synthesize phoneNumbers;


The example above will map the key name to the domain object property personName. It also tells the mapper that the NSArray property phoneNumbers should contain NSString.

Custom converters

The framwork support executing custom converter blocks before assigning a value to a property. This is useful for converting the value type into a different property type or performing calculations on the value before assigning it to the property.

The converter block must confirm to the definition id(^)(id), accepting a key value and returning a new value that will be assigned to the property.

Use the macro below to annotate your property with a converter block.

  • MapKeyToBlock(key, block)

Use this macro in the @implementation section of your domain objects just above the properties @synthesize statement.

@implementation Dates

MapKeyToBlock(dateFromUnixTimestamp, ^(NSNumber* timestamp) {
  return [NSDate dateWithTimeIntervalSince1970:timestamp];
@synthesize dateFromUnixTimestamp;


The example above will convert a unix timestamp into an NSDate before assigning it to the property.

Code examples

The code examples below demonstrate how to map incoming JSON to a Peron domain object.


@interface Person : NSObject

@property (retain) NSString *personName;
@property (retain) NSNumber *age;
@property BOOL male;
@property (retina) Address *address;    



@implementation Person

MapKeyToPropert(name, personName)
@synthesize personName;
@synthesize age;
@synthesize male;
@synthesize address;



@interface Address : NSObject

@property (retain) NSString *street;
@property (retain) NSString *city;



@implementation Address

@synthesize street;
@synthesize city;


Incoming JSON:


Parse JSON and map to domain objects:

//Get your JSON data
NSData *jsonData = ...

//Parse it using JSONKit
id parsedJSON = [jsonData objectFromJSONData];

//Automatically map to domain objects
JSONMapper *mapper = [JSONMapper mapper];
NSError *error;
Person *person = [mapper mapParsedJSON:parsedJSON toRootClass:[Person class] error:&error];

Finally an example how to map to a NSArray using annotation:


@interface MyContacts : NSObject 

@property (retain) NSArray *contacts;



@implementation MyContacts

MapClassToArray(Persons, contacts)
@synthesize contacts;


Incoming JSON:

              {"name":"mattias", "age":35, ... },
              {"name":"kalle", "age":27, ...}


The interface for performing the mapping is very simple:

// Get a mapper instance
+ (id)mapper;
// Map parsed JSON to domain objects with root class
- (id)mapObject:(id)source toClass:(id)clazz withError:(NSError **)error;

There is also a convenience NSDictionary and NSArray category:

// Map parsed JSON to domain objects with root class
- (id)mapToClass:(id)clazz withError:(NSError**)error;