singleton class

#import "classname.h"

@interface classname ()

@end

@implementation classname

+(classname *)instance {                    
    static dispatch_once_t onetime;         
    static classname *shared = nil;         
    dispatch_once(&onetime, ^{              
        shared = [[classname alloc] init];  
    });                                     
    return shared;                          
}

@end