- (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 (self.myTextField == textField)  //判断是否时我们想要限定的那个输入框

    {

        if ([toBeString length] > 20) { //如果输入框内容大于20则弹出警告

            textField.text = [toBeString substringToIndex:20];

            UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:nil message:@"超过最大字数不能输入了" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil] autorelease];

            [alert show];

            return NO;

        }

    }

    return YES;