static Myclass _instance;

+(id)shareInstance{ 
   @synchronized(self){
    if(_instance == nil)
       _instance = [MyClass alloc] init]; 
   }
   return _instance;
}

+(id)shareInstance{
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
   if(_instance == nil)
       _instance = [MyClass alloc] init]; 
  });
   return _instance;
}

//重写allocWithZone,里面实现跟方法一,方法二一致就行.
+(id)allocWithZone:(struct _NSZone *)zone{
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
   if(_instance == nil)
      _instance = [MyClass alloc] init]; 
  });
   return _instance;
} 

//保证copy时相同
-(id)copyWithZone:(NSZone *)zone{ 
  return _instance; 
}

//不需要计数器+1 
- (id)retain { 
  return self; 
} 

//不需要. 堆区的对象才需要 
- (id)autorelease { 
  return self; 
} 

//不需要 
- (oneway void)release { 
} 

//不需要计数器个数. 直接返回最大无符号整数 
- (NSUInteger)retainCount { 
  return UINT_MAX; //参照常量区字符串的retainCount 
}

+(void)attemptDealloc{
  [_instance release]; //mrc 需要释放,当然你就不能重写release的方法了.
  _instance = nil;
}

+(void)attempDealloc{
  onceToken = 0; 
  // 只有置成0,GCD才会认为它从未执行过.它默认为0.这样才能保证下次再次调用shareInstance的时候,再次创建对象.
  [_instance release];
  _instance = nil;
 }