Iterator [GoF]

Posted: July 7th, 2008 | Author: | Tags: , , , , | Comments Off

Lot’s of people like to write down things into lists, so that they can go through that list later and check whether everything was fine. Normally every recipe has a list ~ there’s always a list of ingredients at the beginning of the recipe.
This short example shows how such a list can be processed in PHP. So why would you want to have something else than a array to hold your objects? – My example still uses a array to hold the objects (uni- or bidirectional lists would also be possible) but it adds a kind of a facade to the array so that the common managements-tasks are handled within the List-Object. Everything you need for this example is present in PHP since version 5.0. The basic steps you need to do is to provide a “Object” and an “ObjectList” which implements the native “Iterator” interface and then you’re able to have very handy lists :)

class Incredient {

public $name,$amount;
public function __construct($name,$amount) {
$this->name = $name;
$this->amount = $amount;
}
}

class Recipe implements Iterator {
public $title;
private $ingredients;

public function __construct ($title) {
$this->title = $title;
}

public function addIncredient(Incredient $in) {
$this->ingredients[] = $in;
}

public function current ()  {   return current ($this->ingredients);    }
public function key ()      {   return key($this->ingredients);         }
public function valid ()    {   return current ($this->ingredients);    }
public function rewind ()   {   return reset ($this->ingredients);      }
public function next ()     {   return next ($this->ingredients);       }
}

$cookieRecipe = new Recipe(“Chocolate Cookie”);
$cookieRecipe->addIncredient(new Incredient(‘Flour’,’2.5 cups’));
$cookieRecipe->addIncredient(new Incredient(‘Baking soda’,’1 teaspoon’));
$cookieRecipe->addIncredient(new Incredient(‘Salt’,’0.5 teaspoon’));
$cookieRecipe->addIncredient(new Incredient(‘Butter’,’1 cup’));
$cookieRecipe->addIncredient(new Incredient(‘Sugar’,’1 cup’));
$cookieRecipe->addIncredient(new Incredient(‘Brown Sugar’,’0.5 cup’));
$cookieRecipe->addIncredient(new Incredient(‘Vanilla extract’,’1 teaspoon’));
$cookieRecipe->addIncredient(new Incredient(‘Egg’,’1-2′));
$cookieRecipe->addIncredient(new Incredient(‘Chocolate chips’,’2 cups’));

// process recipe:
foreach($cookieRecipe as $inc) {
echo $inc->name.“ => ”.$inc->amount.“<br/>”;
}

As you see it’s pretty easy to have lists of objects in PHP. You might also think that always creating to some list-object over and over again is very odd and you’re right. For the most common tasks like iterating through arrays, directory-lists and a few more task you can use objects which are shipped with the Standard PHP Library ,which is also part of PHP since version 5 and mandatory in 5.3. So the example shown above could also look like this:

$recipe = array();
$recipe[] = new Incredient(‘Flour’,’2.5 cups’);
$recipe[] = new Incredient(‘Baking soda’,’1 teaspoon’);
$recipe[] = new Incredient(‘Salt’,’0.5 teaspoon’);
$recipe[] = new Incredient(‘Butter’,’1 cup’);
$recipe[] = new Incredient(‘Sugar’,’1 cup’);
$recipe[] = new Incredient(‘Brown Sugar’,’0.5 cup’);
$recipe[] = new Incredient(‘Vanilla extract’,’1 teaspoon’);
$recipe[] = new Incredient(‘Egg’,’1-2′);
$recipe[] = new Incredient(‘Chocolate chips’,’2 cups’);

$recipeIncObj = new ArrayObject($recipe);
$ingredientsIt = $recipeIncObj->getIterator();

foreach($ingredientsIt as $inc) {
echo $inc->name.“ => ”.$inc->amount.“<br/>”;
}

As I said at the beginning, there are lots of situations where you might want to have a list for something and if you store that list in PHP the Iterator-pattern can keep your code clean and tasty :)