迭代器模式提供一種訪問一個容器對象中的各個元素，而又不暴露其內部細節的方法。在應用中，我們時常會遇到各種變量代碼，foreach在很多代碼處都可以見到，使用迭代器可以對不同數據結構的集合封裝，外部只需調用迭代器提供的接口即可，提高了應用的可擴展性。
迭代器UML類圖
PHP SPL中提供了迭代器接口Iterator和容器接口IteatorAggragate
YII代碼實例：
class CListIterator implements Iterator
{
/**
* @var array the data to be iterated through
*/
private $_d;
/**
* @var integer index of the current item
*/
private $_i;
/**
* @var integer count of the data items
*/
private $_c;
/**
* Constructor.
* @param array $data the data to be iterated through
*/
public function __construct(&$data)//數組引用傳遞
{
$this->_d=&$data;
$this->_i=0;
$this->_c=count($this->_d);
}
/**
* Rewinds internal array pointer.
* This method is required by the interface Iterator.
*/
public function rewind()
{
$this->_i=0;
}
/**
* Returns the key of the current array item.
* This method is required by the interface Iterator.
* @return integer the key of the current array item
*/
public function key()
{
return $this->_i;
}
/**
* Returns the current array item.
* This method is required by the interface Iterator.
* @return mixed the current array item
*/
public function current()
{
return $this->_d[$this->_i];
}
/**
* Moves the internal pointer to the next array item.
* This method is required by the interface Iterator.
*/
public function next()
{
$this->_i++;
}
/**
* Returns whether there is an item at current position.
* This method is required by the interface Iterator.
* @return boolean
*/
public function valid()
{
return $this->_i<$this->_c;
}
}
class CList extends CComponent implements IteratorAggregate,ArrayAccess,Countable
{
/**
* @var array internal data storage
*/
private $_d=array();
/**
* @var integer number of items
*/
private $_c=0;
/**
* @var boolean whether this list is read-only
*/
private $_r=false;
/**
* Constructor.
* Initializes the list with an array or an iterable object.
* @param array $data the initial data. Default is null, meaning no initialization.
* @param boolean $readOnly whether the list is read-only
* @throws CException If data is not null and neither an array nor an iterator.
*/
public function __construct($data=null,$readOnly=false)
{
if($data!==null)
$this->copyFrom($data);
$this->setReadOnly($readOnly);
}
/**
* @return boolean whether this list is read-only or not. Defaults to false.
*/
public function getReadOnly()
{
return $this->_r;
}
/**
* @param boolean $value whether this list is read-only or not
*/
protected function setReadOnly($value)
{
$this->_r=$value;
}
/**
* Returns an iterator for traversing the items in the list.
* This method is required by the interface IteratorAggregate.
* @return Iterator an iterator for traversing the items in the list.
*/
public function getIterator()
{
return new CListIterator($this->_d);
}
/**
* Returns the number of items in the list.
* This method is required by Countable interface.
* @return integer number of items in the list.
*/
public function count()
{
return $this->getCount();
}
/**
* Returns the number of items in the list.
* @return integer the number of items in the list
*/
public function getCount()
{
return $this->_c;
}
/**
* Returns the item at the specified offset.
* This method is exactly the same as {@link offsetGet}.
* @param integer $index the index of the item
* @return mixed the item at the index
* @throws CException if the index is out of the range
*/
public function itemAt($index)
{
if(isset($this->_d[$index]))
return $this->_d[$index];
else if($index>=0 && $index<$this->_c) // in case the value is null
return $this->_d[$index];
else
throw new CException(Yii::t('yii','List index "{index}" is out of bound.',
array('{index}'=>$index)));
}
/**
* Appends an item at the end of the list.
* @param mixed $item new item
* @return integer the zero-based index at which the item is added
*/
public function add($item)
{
$this->insertAt($this->_c,$item);
return $this->_c-1;
}
/**
* Inserts an item at the specified position.
* Original item at the position and the next items
* will be moved one step towards the end.
* @param integer $index the specified position.
* @param mixed $item new item
* @throws CException If the index specified exceeds the bound or the list is read-only
*/
public function insertAt($index,$item)
{
if(!$this->_r)
{
if($index===$this->_c)
$this->_d[$this->_c++]=$item;
else if($index>=0 && $index<$this->_c)
{
array_splice($this->_d,$index,0,array($item));
$this->_c++;
}
else
throw new CException(Yii::t('yii','List index "{index}" is out of bound.',
array('{index}'=>$index)));
}
else
throw new CException(Yii::t('yii','The list is read only.'));
}
/**
* Removes an item from the list.
* The list will first search for the item.
* The first item found will be removed from the list.
* @param mixed $item the item to be removed.
* @return integer the index at which the item is being removed
* @throws CException If the item does not exist
*/
public function remove($item)
{
if(($index=$this->indexOf($item))>=0)
{
$this->removeAt($index);
return $index;
}
else
return false;
}
/**
* Removes an item at the specified position.
* @param integer $index the index of the item to be removed.
* @return mixed the removed item.
* @throws CException If the index specified exceeds the bound or the list is read-only
*/
public function removeAt($index)
{
if(!$this->_r)
{
if($index>=0 && $index<$this->_c)
{
$this->_c--;
if($index===$this->_c)
return array_pop($this->_d);
else
{
$item=$this->_d[$index];
array_splice($this->_d,$index,1);
return $item;
}
}
else
throw new CException(Yii::t('yii','List index "{index}" is out of bound.',
array('{index}'=>$index)));
}
else
throw new CException(Yii::t('yii','The list is read only.'));
}
/**
* Removes all items in the list.
*/
public function clear()
{
for($i=$this->_c-1;$i>=0;--$i)
$this->removeAt($i);
}
/**
* @param mixed $item the item
* @return boolean whether the list contains the item
*/
public function contains($item)
{
return $this->indexOf($item)>=0;
}
/**
* @param mixed $item the item
* @return integer the index of the item in the list (0 based), -1 if not found.
*/
public function indexOf($item)
{
if(($index=array_search($item,$this->_d,true))!==false)
return $index;
else
return -1;
}
/**
* @return array the list of items in array
*/
public function toArray()
{
return $this->_d;
}
/**
* Copies iterable data into the list.
* Note, existing data in the list will be cleared first.
* @param mixed $data the data to be copied from, must be an array or object implementing Traversable
* @throws CException If data is neither an array nor a Traversable.
*/
public function copyFrom($data)
{
if(is_array($data) || ($data instanceof Traversable))
{
if($this->_c>0)
$this->clear();
if($data instanceof CList)
$data=$data->_d;
foreach($data as $item)
$this->add($item);
}
else if($data!==null)
throw new CException(Yii::t('yii','List data must be an array or an object implementing Traversable.'));
}
/**
* Merges iterable data into the map.
* New data will be appended to the end of the existing data.
* @param mixed $data the data to be merged with, must be an array or object implementing Traversable
* @throws CException If data is neither an array nor an iterator.
*/
public function mergeWith($data)
{
if(is_array($data) || ($data instanceof Traversable))
{
if($data instanceof CList)
$data=$data->_d;
foreach($data as $item)
$this->add($item);
}
else if($data!==null)
throw new CException(Yii::t('yii','List data must be an array or an object implementing Traversable.'));
}
/**
* Returns whether there is an item at the specified offset.
* This method is required by the interface ArrayAccess.
* @param integer $offset the offset to check on
* @return boolean
*/
public function offsetExists($offset)
{
return ($offset>=0 && $offset<$this->_c);
}
/**
* Returns the item at the specified offset.
* This method is required by the interface ArrayAccess.
* @param integer $offset the offset to retrieve item.
* @return mixed the item at the offset
* @throws CException if the offset is invalid
*/
public function offsetGet($offset)
{
return $this->itemAt($offset);
}
/**
* Sets the item at the specified offset.
* This method is required by the interface ArrayAccess.
* @param integer $offset the offset to set item
* @param mixed $item the item value
*/
public function offsetSet($offset,$item)
{
if($offset===null || $offset===$this->_c)
$this->insertAt($this->_c,$item);
else
{
$this->removeAt($offset);
$this->insertAt($offset,$item);
}
}
/**
* Unsets the item at the specified offset.
* This method is required by the interface ArrayAccess.
* @param integer $offset the offset to unset item
*/
public function offsetUnset($offset)
{
$this->removeAt($offset);
}
}