设计模式

#import <Foundation/Foundation.h>  
  
@interface Singleton : NSObject  
  
+(instancetype) shareInstance ;  
  
@end  

#import "Singleton.h"  
  
@implementation Singleton  
  
static Singleton* _instance = nil;  
  
+(instancetype) shareInstance  
{  
    static dispatch_once_t onceToken ;  
    dispatch_once(&onceToken, ^{  
        _instance = [[super allocWithZone:NULL] init] ;  
    }) ;  
    //GCD方式实现单次访问
    return _instance ;  
}  
  
+(id) allocWithZone:(struct _NSZone *)zone  
{  
    return [Singleton shareInstance] ;  
}  
  
-(id) copyWithZone:(struct _NSZone *)zone  
{  
    return [Singleton shareInstance] ;  
}  
  
@end