[aerogear-dev] iOS/CoreData - API proposal - AGEntityMapper

[Matthias Wessendorf]

CoreData: API proposal - AGEntityMapperBackground

Mapping JSON representations to a "rich" domain model in ObjC is a bit
cumbersome <http://matthiaswessendorf.wordpress.com/2012/11/19/objc-json/>.
Similar is true when mapping a JSON data/response to a NSMangedObject (and
vise versa). There is a base framework for "remote persistence", via the
CoreData API, by leveraging a custom NSIncrementalStore ->
AFIncrementalStore <https://github.com/AFNetworking/AFIncrementalStore>.

For that,... a similar *two-way-mapping* is required, as described
here<http://matthiaswessendorf.wordpress.com/2013/01/02/coredata-data-mapping-with-afincrementalstore/>
.
AeroGear and CoreData

Our CoreData offerings are leveraging the AFIncrementalStore, therefore (as
with other frameworks/libraries) we need a mapping as well. We need to know
the name of the entity and we need a NSDictionary that covers the actual
JSON/property mapping. It could be done with the vanialla ObjC classes:

// some mappers:NSDictionary *task_mapper =
  [NSDictionary
dictionaryWithObjectsAndKeys:@"description",@"desc",@"id",@"myId",
nil];NSDictionary *project_mapper =
  [NSDictionary dictionaryWithObjectsAndKeys:@"id",@"myId", nil];
// create a schema out of the mappers:NSDictionary *schema =
  [NSDictionary dictionaryWithObjectsAndKeys:task_mapper, @"Task",
project_mapper, @"Project", nil];
// pass the schema to the AGCoreDataHelper class, when doing the init...

Proposal for a custom API

I'd like to introduce a new wrapper type, called AGEntityMapper:

@interface AGEnityMapper : NSObject
@property NSString *name;@property NSDictionary *mapper;
-(id) initWithName:(NSString *) name mapper:(NSDictionary *) mapper;@end

This AGEntityMapper would be applied onto the AGCoreDataConfig:

@protocol AGCoreDataConfig <NSObject>
@property (strong, nonatomic) NSManagedObjectModel
*managedObjectModel;@property (strong, nonatomic) NSURL *baseURL;
-(void)applyEntityMappers:(AGEnityMapper *)firstObject, ...
NS_REQUIRES_NIL_TERMINATION;
@end

The actual code, would look like this:

EnityMapper *taskMapper =
  [[EnityMapper alloc] initWithName:@"Task"
       // mapping the properties on the "entity" (NSManagedObject)
       // to the external representation (e.g. JSON)
                             mapper:@{ @"desc": @"description",
@"myId": @"id"}];
EnityMapper *projectMapper =
  [[EnityMapper alloc] initWithName:@"Project"
       // mapping the properties on the "entity" (NSManagedObject)
       // to the external representation (e.g. JSON)
                             mapper:@{ @"myId": @"id"}];
AGCoreDataHelper *helper = [[AGCoreDataHelper alloc]
initWithConfig:^(id<AGCoreDataConfig> config) {

  [config setBaseURL:[NSURL URLWithString:@"http://server.com"]];

  [config applyEntityMappers:
    taskMapper,
    projectMapper,
    nil // terminate the varargs.
  ];}];


-- 
Matthias Wessendorf

blog: http://matthiaswessendorf.wordpress.com/
sessions: http://www.slideshare.net/mwessendorf
twitter: http://twitter.com/mwessendorf
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