JSONModel iOS and Polymorphism

Question

Using the following models as examples, what are the best practices of handling polymorphism within JSONModel?

@interface GameModel : JSONModel
@property (nonatomic, assign) long id;
@property (nonatomic, assign) NSArray<GameEventModel> *events;
/*
  ...
*/
@end

@interface GameEventModel : JSONModel
@property (nonatomic, assign) long long timestamp;
/*
  ...
*/
@end

@interface GameTouchEventModel : GameEventModel
@property (nonatomic, assign) CGPoint point;
/*
  ...
*/
@end

When GameModel is initiated with a JSON string of {id:1, events:[{point:{x:1, y:1}, timestamp:...}]}

JSONModel will use the GameEventModel and ignore the point property.

Would it be better to use a generic GameEventModel which contains a type property and info property such as...

@interface GameTouchEventModel : GameEventModel
@property (nonatomic, strong) NSString *type;
@property (nonatomic, strong) NSDictionary *info;
@end

And therefore the model could accept JSON as {id:1, events:[{ type:"GameTouchEventModel", info:{ point:{x:1, y:1}, timestamp:... } }]}

The problem with this approach is harder to read code and no compiler warnings/errors amongst others.

Is there no way to use polymorphic models in JSONModel?

Answer 1

We solved this with 2 minor alterations to JSONModel.m, introducing a new special JSON property __subclass which is picked up by the JSONModel parser and uses the value as the object type. __subclass is required to be a reserved keyword (therefore no models can use __subclass as a property name).

Alterations to JSONModel.m

// ...
-(id)initWithDictionary:(NSDictionary*)dict error:(NSError**)err
{
      // ...
      if ([self __isJSONModelSubClass:property.type]) {

            //initialize the property's model, store it
            JSONModelError* initErr = nil;

            -- id value = [[property.type alloc] initWithDictionary: jsonValue error:&initErr];

            ++ id value;
            ++ if([jsonValue valueForKey:@"subclass"] != NULL)
            ++ {
            ++       Class jsonSubclass = NSClassFromString([d valueForKey:@"subclass"]);
            ++       if(jsonSubclass)
            ++             obj = [[jsonSubclass alloc] initWithDictionary:d error:&initErr];
            ++ }
            ++ else
            ++     value = [[property.type alloc] initWithDictionary: jsonValue error:&initErr];
       //...
//...
+(NSMutableArray*)arrayOfModelsFromDictionaries:(NSArray*)array error:(NSError**)err
{
      // ...
      for (NSDictionary* d in array) {
           JSONModelError* initErr = nil;

           -- id obj = [[self alloc] initWithDictionary:d error:&initErr];

           ++ id obj;
           ++ if([d valueForKey:@"subclass"] != NULL)
           ++ {
           ++       Class jsonSubclass = NSClassFromString([d valueForKey:@"subclass"]);
           ++       if(jsonSubclass)
           ++             obj = [[jsonSubclass alloc] initWithDictionary:d error:&initErr];
           ++ }
           ++ else
           ++      obj = [[self alloc] initWithDictionary:d error:&initErr];
       // ...
 // ...

NOTE: If the _subclass'ed JSON model class doesn't exist, then the model will fallback to the superclass.

This will then work with the following models

@interface GameModel : JSONModel
@property (nonatomic, assign) long id;
@property (nonatomic, assign) NSArray<GameEventModel> *events;
@end

@protocol GameEventModel
@end

@interface GameEventModel : JSONModel
@property (nonatomic, assign) long long timestamp;
@end

@interface GameTouchEventModel : GameEventModel
@property (nonatomic, strong) NSArray *point;
@end

When passed the JSON string {id:1, events:[ { __subclass:'GameTouchEventModel', timestamp:1, point: [0,0] } ] }