static AccountManager *DefaultManager = nil;  

 + (AccountManager *)defaultManager {

     if (!DefaultManager) DefaultManager = [[self allocWithZone:NULL] init];

     return DefaultManager;

 } 

 + (AccountManager *)sharedManager

 {

         static AccountManager *sharedAccountManagerInstance = nil;

         static dispatch_once_t predicate;

         dispatch_once(&predicate, ^{

                 sharedAccountManagerInstance = [[self alloc] init];

         });

     return sharedAccountManagerInstance;

 }