限制textField长度

• (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {

  if (textField == self.liveThemeTextField) {
  //这里的if时候为了获取删除操作,如果没有次if会造成当达到字数限制后删除键也不能使用的后果.
  if (range.length == 1 && string.length == 0) {
  return YES;
  }
  //so easy
  else if (self.liveThemeTextField.text.length >= 30) {
  self.liveThemeTextField.text = [textField.text substringToIndex:30];
  return NO;
  }
  }
  return YES;
  }