Simple Object Iterators in PHP
If you've been coding in PHP for a while, you may be familiar with the foreach loop. It provides an easy way to analyze every item in an array, e.g.
$myArray = array(); $myArray[] = "First item"; $myArray[] = "Second item"; $myArray[] = "Third item"; foreach ($myArray as $i) { echo "$i
"; }As well as arrays, it's also possible loop through an object. If your object contains a collection of items, you can use a foreach loop to iterate over each of them. It doesn't matter what's in that collection or how it's retrieved, e.g.
- records from a database
- navigation links
- names of files within a directory
- lines of text read from a file
- product objects in a specific shop category
Iterators is a subject which strikes fear into the heart of many developers. They sound complex and are often explained with indecipherable abstract references. They're best explained with a simple example so we'll create a basic class which defines a list of web technologies:
class WebTechnologies { private $tech; // constructor public function __construct() { $this-tech = explode(',', 'PHP,html,Xhtml,CSS,JavaScript,XML,XSLT,ASP,C#,Ruby,Python') ; } }The private $tech array cannot be accessed outside of the class. You could make it public or return its contents, but iterators provide a more elegant alternative.
Assuming you already have the data in an array, the quickest way to make an object traversable is to implement the IteratorAggregate interface:
class WebTechnologies implements IteratorAggregate {...To complete the code, we must then define a public getIterator() function within our class. This must return something which can be iterated - such an ArrayIterator object with our $tech array passed to its constructor:
// return iterator public function getIterator() { return new ArrayIterator($this-tech); }We can now iterate over all items within the $tech array. The full code:
class WebTechnologies implements IteratorAggregate { private $tech; // constructor public function __construct() { $this-tech = explode(',', 'PHP,html,Xhtml,CSS,JavaScript,XML,XSLT,ASP,C#,Ruby,Python'); } // return iterator public function getIterator() { return new ArrayIterator($this-tech); } } // create object $wt = new WebTechnologies(); // iterate over collection foreach ($wt as $n = $t) { echo "Technology $n: $t
"; }Great. But what if our object's collection isn't an array? In that situation, we require more sophisticated iterators a€¦ watch out for a full tutorial coming soon.