Object Iteration

Object Iteration

PHP 5 provides a way for objects to be defined so it is possible to iterate through a list of items, with, for example a foreach statement. By default, all visible properties will be used for the iteration.

Example 19-21. Simple Object Iteration

<?php
class MyClass
{
    
public $var1 = 'value 1';
    
public $var2 = 'value 2';
    
public $var3 = 'value 3';

    
protected $protected = 'protected var';
    
private   $private   = 'private var';

    function
iterateVisible() {
       echo
"MyClass::iterateVisible:\n";
       foreach(
$this as $key => $value) {
           print
"$key => $value\n";
       }
    }
}

$class = new MyClass();

foreach(
$class as $key => $value) {
    print
"$key => $value\n";
}
echo
"\n";


$class->iterateVisible();

?>

The above example will output:

var1 => value 1
var2 => value 2
var3 => value 3

MyClass::iterateVisible:
var1 => value 1
var2 => value 2
var3 => value 3
protected => protected var
private => private var

As the output shows, the foreach iterated through all visible variables that can be accessed. To take it a step further you can implement one of PHP 5's internal interface named Iterator. This allows the object to decide what and how the object will be iterated.

Example 19-22. Object Iteration implementing Iterator

<?php
class MyIterator implements Iterator
{
    
private $var = array();

    
public function __construct($array)
    {
        if (
is_array($array)) {
            
$this->var = $array;
        }
    }

    
public function rewind() {
        echo
"rewinding\n";
        
reset($this->var);
    }

    
public function current() {
        
$var = current($this->var);
        echo
"current: $var\n";
        return
$var;
    }

    
public function key() {
        
$var = key($this->var);
        echo
"key: $var\n";
        return
$var;
    }

    
public function next() {
        
$var = next($this->var);
        echo
"next: $var\n";
        return
$var;
    }

    
public function valid() {
        
$var = $this->current() !== false;
        echo
"valid: {$var}\n";
        return
$var;
    }
}

$values = array(1,2,3);
$it = new MyIterator($values);

foreach (
$it as $a => $b) {
    print
"$a: $b\n";
}
?>

The above example will output:

rewinding
current: 1
valid: 1
current: 1
key: 0
0: 1
next: 2
current: 2
valid: 1
current: 2
key: 1
1: 2
next: 3
current: 3
valid: 1
current: 3
key: 2
2: 3
next:
current:
valid:

You can also define your class so that it doesn't have to define all the Iterator functions by simply implementing the PHP 5 IteratorAggregate interface.

Example 19-23. Object Iteration implementing IteratorAggregate

<?php
class MyCollection implements IteratorAggregate
{
    
private $items = array();
    
private $count = 0;

    
// Required definition of interface IteratorAggregate
    
public function getIterator() {
        return new
MyIterator($this->items);
    }

    
public function add($value) {
        
$this->items[$this->count++] = $value;
    }
}

$coll = new MyCollection();
$coll->add('value 1');
$coll->add('value 2');
$coll->add('value 3');

foreach (
$coll as $key => $val) {
    echo
"key/value: [$key -> $val]\n\n";
}
?>

The above example will output:

rewinding
current: value 1
valid: 1
current: value 1
key: 0
key/value: [0 -> value 1]

next: value 2
current: value 2
valid: 1
current: value 2
key: 1
key/value: [1 -> value 2]

next: value 3
current: value 3
valid: 1
current: value 3
key: 2
key/value: [2 -> value 3]

next:
current:
valid:

Note: For more examples of iterators, see the SPL Extension.

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