判断对象是否为空 - iOS

//判断对象是否为空
- (BOOL)isNull
{
    if ([self isEqual:[NSNull null]])
    {
        return YES;
    }else
    {
        if ([self isKindOfClass:[NSNull class]])
        {
            return YES;
        }else
        {
            if (self==nil)
            {
                return YES;
            }
        }
    }
    if ([self isKindOfClass:[NSString class]]) {
        if ([((NSString *)self) isEqualToString:@"(null)"]) {
            return YES;
        }
    }
        return NO;
}