textField 判断输入长度限制

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

    if ([string isEqualToString:@"n"])
    {
        return YES;
    }
    NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; //得到输入框的内容
    if (newPwd == textField || currPwd == 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
            textField.text = [toBeString substringToIndex:20];
            UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message:@"密码最多20位" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil];
            [alert show];
            return NO;
        }

    }
    return YES;
}