Dependency Injection / Polymorphism in Objective C

Question

Can I use Objective C protocols the same way I would use Java interfaces for Dependency Injection or Polymorphism?

It seems like I cannot. Note that in (1) below I have to type the property as the implementation rather than as the protocol. I get a compiler error if I use the protocol as the property type.

(1) The object to have dependencies injected into:

#import <Foundation/Foundation.h>
#import "FOO_AccountService.h"
#import "FOO_BasicAccountService.h"

@interface FOO_ServiceManager : NSObject

@property (nonatomic, strong) FOO_BasicAccountService <FOO_AccountService> *accountService;

...

@end

(2) The protocol:

#import <Foundation/Foundation.h>
#import "FOO_Service.h"

@protocol FOO_AccountService <FOO_Service>

...

@end

(3) The implementation:

#import <Foundation/Foundation.h>
#import "FOO_BasicService.h"
#import "FOO_AccountService.h"

@interface FOO_BasicAccountService : FOO_BasicService <FOO_AccountService>

...    

@end

Answer 1

All you need is:

@property (nonatomic, strong) id<FOO_AccountService> accountService;

Now your property will work with any object type that conforms to the FOO_AccountService protocol.