How to avoid validation of passed argument inside a method? Proper OOD - php

Question

Writing group of parsers that rely on one abstract class which implements shared methods and asks to implement addition method which contains per parser logic.
Abstract parser code:
<?
abstract class AbstractParser {
/*
* The only abstract method to implement. It contains unique logic of each feed passed to the function
*/
public abstract function parse($xmlObject);
/**
* #param $feed string
* #return SimpleXMLElement
* #throws Exception
*/
public function getFeedXml($feed) {
$xml = simplexml_load_file($feed);
return $xml;
}
/**
* #return array
*/
public function getParsedData() {
return $this->data;
}
/**
* #param SimpleXMLElement
* #return Array
*/
public function getAttributes($object) {
// implementation here
}
}
Concrete Parser class:
<?php
class FormulaDrivers extends AbstractParser {
private $data;
/**
* #param SimpleXMLElement object
* #return void
*/
public function parse($xmlObject) {
if (!$xmlObject) {
throw new \Exception('Unable to load remote XML feed');
}
foreach($xmlObject->drivers as $driver) {
$driverDetails = $this->getAttributes($driver);
var_dump($driver);
}
}
}
Instantiation:
$parser = new FormulaDrivers();
$parser->parse( $parser->getFeedXml('http://api.xmlfeeds.com/formula_drivers.xml') );
As you can see, I pass the result of getFeedXml method to parse method, basically delegating the validation of result of getFeedXml to parse method.
How can I avoid it, make sure it returns correct XML object before I pass it to parse method?
Increasing instantiation process and amount of called methods leads to the need of some factory method...
Anyway, how would you fix this small issue?
Thanks!

Answer 1

Make parse protected, so that only parse_xml_file calls it:
abstract class AbstractParser {
/*
* The only abstract method to implement. It contains unique logic of each feed passed to the function
*/
protected abstract function parse($xmlObject);
/**
* #param $feed string
* #return [whatever .parse returns]
* #throws Exception
*/
public function parseFile($feed) {
$xml = simplexml_load_file($feed);
if (!$xml) {
throw new \Exception('Unable to load remote XML feed');
}
return $this->parse($xml);
}
/**
* #return array
*/
public function getParsedData() {
return $this->data;
}
/**
* #param SimpleXMLElement
* #return Array
*/
public function getAttributes($object) {
// implementation here
}
}
$parser->parseFile('http://api.xmlfeeds.com/formula_drivers.xml');