说明：本文章主要讲述PHP的对象遍历(Iterator)知识点。由于Laravel框架中就在集合(Collection)中用到了对象遍历知识点，故记录并学习之。同时，作者会将开发过程中的一些截图和代码黏上去，提高阅读效率。
Laravel中在基础集合类IlluminateSupportCollection、路由类中IlluminateRoutingRouteCollection和分页类中IlluminatePaginationPaginator等，都用到了对象遍历这个小知识点，这些类都是实现了IteratorAggregate这个接口，这个接口定义getIterator()，返回的是迭代器对象。PHP标准扩展库中提供了很多默认迭代器实现类，比较常用的是数组迭代器对象ArrayIterator，参考官网：迭代器。
对象遍历(Iterator)
基本遍历
PHP5提供了遍历对象属性的方法，而且默认是可见属性，如代码中foreach遍历对象属性，默认的都是可见属性：
<?php
/**
* Created by PhpStorm.
* User: liuxiang
* Date: 16/7/20
* Time: 17:29
*/
class TestIterator {
/**
* @var string
*/
public $name = 'PHP';
/**
* @var string
*/
public $address = 'php.net';
/**
* @var string
*/
protected $sex = 'man';
/**
* @var int
*/
private $age = 20;
}
$testIterator = new TestIterator();
foreach ($testIterator as $key => $value) {
echo $key.':'.$value.PHP_EOL;
}
输出的是：
name:PHP
address:php.net如果需要遍历对象的不可见属性，则在对象内部定义一个遍历方法：
public function unAccessIterator()
{
echo 'Iterator the unaccess fields:'.PHP_EOL;
foreach ($this as $key => $value) {
echo $key.':'.$value.PHP_EOL;
}
}对象外部访问：
$testIterator->unAccessIterator();将可以遍历对象的不可见属性，输出结果：
Iterator the unaccess fields:
name:PHP
address:php.net
sex:man
age:20Iterator接口
PHP提供了Iterator接口，用来定义迭代器对象来自定义遍历，所以利用Iterator接口来构造迭代器，需要实现Iterator定义的几个方法：
<?php
/**
* Created by PhpStorm.
* User: liuxiang
* Date: 16/7/20
* Time: 17:29
*/
class TestIterator implements Iterator{
/**
* @var string
*/
public $name = 'PHP';
/**
* @var string
*/
public $address = 'php.net';
/**
* @var string
*/
protected $sex = 'man';
/**
* @var int
*/
private $age = 20;
/**
* @var array
*/
private $composerPackage;
public function __construct($composerPackage = [])
{
$this->composerPackage = $composerPackage;
}
public function unAccessIterator()
{
echo 'Iterator the unaccess fields:'.PHP_EOL;
foreach ($this as $key => $value) {
echo $key.':'.$value.PHP_EOL;
}
}
/**
* Return the current element
* @link http://php.net/manual/en/iterator.current.php
* @return mixed Can return any type.
* @since 5.0.0
*/
public function current()
{
// TODO: Implement current() method.
echo 'Return the current element:'.PHP_EOL;
return current($this->composerPackage);
}
/**
* Move forward to next element
* @link http://php.net/manual/en/iterator.next.php
* @return void Any returned value is ignored.
* @since 5.0.0
*/
public function next()
{
// TODO: Implement next() method.
echo 'Move forward to next element:'.PHP_EOL;
return next($this->composerPackage);
}
/**
* Return the key of the current element
* @link http://php.net/manual/en/iterator.key.php
* @return mixed scalar on success, or null on failure.
* @since 5.0.0
*/
public function key()
{
// TODO: Implement key() method.
echo 'Return the key of the current element:'.PHP_EOL;
return key($this->composerPackage);
}
/**
* Checks if current position is valid
* @link http://php.net/manual/en/iterator.valid.php
* @return boolean The return value will be casted to boolean and then evaluated.
* Returns true on success or false on failure.
* @since 5.0.0
*/
public function valid()
{
// TODO: Implement valid() method.
echo 'Checks if current position is valid:'.PHP_EOL;
return current($this->composerPackage) !== false;
}
/**
* Rewind the Iterator to the first element
* @link http://php.net/manual/en/iterator.rewind.php
* @return void Any returned value is ignored.
* @since 5.0.0
*/
public function rewind()
{
// TODO: Implement rewind() method.
echo 'Rewind the Iterator to the first element:'.PHP_EOL;
reset($this->composerPackage);
}
}
/*
$testIterator = new TestIterator();
foreach ($testIterator as $key => $value) {
echo $key.':'.$value.PHP_EOL;
}
$testIterator->unAccessIterator();*/
$testIterator = new TestIterator([
'symfony/http-foundation',
'symfony/http-kernel',
'guzzle/guzzle',
'monolog/monolog'
]);
foreach ($testIterator as $key => $value) {
echo $key.':'.$value.PHP_EOL;
}
成员变量$composerPackage是不可见的，通过实现Iterator接口，同样可以遍历自定义的可不见属性，输出结果如下：
Rewind the Iterator to the first element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
0:symfony/http-foundation
Move forward to next element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
1:symfony/http-kernel
Move forward to next element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
2:guzzle/guzzle
Move forward to next element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
3:monolog/monolog
Move forward to next element:
Checks if current position is valid:IteratorAggregate接口
PHP真心为程序员考虑了很多，实现IteratorAggragate接口后只需实现getIterator()方法直接返回迭代器对象，就不需要实现Iterator接口需要的一些方法来创建一些迭代器对象，因为PHP已经提供了很多迭代器对象如ArrayIterator对象。所以再重构下上面代码：
class TestCollection implements IteratorAggregate{
...
/**
* @var array
*/
private $composerPackage;
...
/**
* Retrieve an external iterator
* @link http://php.net/manual/en/iteratoraggregate.getiterator.php
* @return Traversable An instance of an object implementing <b>Iterator</b> or
* <b>Traversable</b>
* @since 5.0.0
*/
public function getIterator()
{
// TODO: Implement getIterator() method.
return new ArrayIterator($this->composerPackage);
}
}
$testCollection = new TestCollection([
'symfony/http-foundation',
'symfony/http-kernel',
'guzzle/guzzle',
'monolog/monolog'
]);
foreach ($testCollection as $key => $value) {
echo $key.':'.$value.PHP_EOL;
}同样的，能遍历$testCollection对象的不可见属性$composerPackage，输出结果：
0:symfony/http-foundation
1:symfony/http-kernel
2:guzzle/guzzle
3:monolog/monolog文章开头聊到Laravel中就用到了IteratorAggragate这个接口，可以看看文件的源码。
总结：PHP提供的对象遍历特性功能还是很有用处的，下一篇准备看下generator生成器知识点，generator提供了另外一种方式定义Iterator。多多使用Laravel，研究Laravel源码并模仿之，也不错哦。
