判断字符串是否为空

  + (BOOL) isBlankString:(NSString *)string {
      if (string == nil || string == NULL) {
          return YES;
      }
      if ([string isKindOfClass:[NSNull class]]) {
          return YES;
      }
      if ([[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0) {
          return YES;
     }
     return NO;
 }