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Cocoa Singletons and Protocols
[NSUserDefaults standardUserDefaults]
[NSNotificationCenter defaultCenter]

@protocol ConfigurationManager <NSObject>

//...

@interface ConfigurationManagerFactory : NSObject

+ (id<ConfigurationManager>)sharedConfiguration;

@end

// ...

id<ConfigurationManger> config = [ConfigurationManagerFactory sharedConfiguration];
[config ...];

@protocol ConfigurationManager <NSObject> ...

@interface ConfigurationManager : NSObject <ConfigurationManager>
+(ConfigurationManager *) defaultConfigurationManager;
...