- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;
{  //string就是此时输入的那个字符textField就是此时正在输入的那个输入框返回YES就是可以改变输入框的值NO相反

   if ([string isEqualToString:@"\n"])  //按会车可以改变
    {
        return YES;
    }

    NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; //得到输入框的内容

    if (_phoneTextField == textField)  //判断是否时我们想要限定的那个输入框
    {
        if ([toBeString length] > 11) { //如果输入框内容大于11则弹出警告
            textField.text = [toBeString substringToIndex:11];//将输入框文字换成截止到11位的string
            [MBProgressHUD bwm_showTitle:@"手机号最多11位" toView:self.view hideAfter:2];
        return NO;
    }
}
return YES;
}