【一】模式定义

    迭代器模式又叫做游标模式。提供一种方法访问一个容器对象中各个元素，而又不需暴露该对象的内部细节。

    当你需要访问一个聚合对象，而且不管这些对象是什么都需要遍历的时候，就应该考虑使用迭代器模式。另外，当需要对聚集有多种方式遍历时，可以考虑去使用迭代器模式。迭代器模式为遍历不同的聚集结构提供如开始、下一个、是否结束、当前哪一项等统一的接口。

    PHP标准库（SPL）中提供了迭代器接口 Iterator，要实现迭代器模式，实现该接口即可。

【二】UML类图

【三】示例代码

Book.php

<?php

namespace DesignPatterns\Behavioral\Iterator;

class Book
{

    private $author;

    private $title;

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

    public function getAuthor()
    {
        return $this->author;
    }

    public function getTitle()
    {
        return $this->title;
    }

    public function getAuthorAndTitle()
    {
        return $this->getTitle() . ' by ' . $this->getAuthor();
    }
}

BookList.php

<?php

namespace DesignPatterns\Behavioral\Iterator;

class BookList implements \Countable
{

    private $books;

    public function getBook($bookNumberToGet)
    {
        if (isset($this->books[$bookNumberToGet])) {
            return $this->books[$bookNumberToGet];
        }

        return null;
    }

    public function addBook(Book $book)
    {
        $this->books[] = $book;
    }

    public function removeBook(Book $bookToRemove)
    {
        foreach ($this->books as $key => $book) {
            /** @var Book $book */
            if ($book->getAuthorAndTitle() === $bookToRemove->getAuthorAndTitle()) {
                unset($this->books[$key]);
            }
        }
    }

    public function count()
    {
        return count($this->books);
    }
}

BookListIterator.php

<?php

namespace DesignPatterns\Behavioral\Iterator;

class BookListIterator implements \Iterator
{

    /**
     * @var BookList
     */
    private $bookList;

    /**
     * @var int
     */
    protected $currentBook = 0;

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

    /**
     * Return the current book
     * @link http://php.net/manual/en/iterator.current.php
     * @return Book Can return any type.
     */
    public function current()
    {
        return $this->bookList->getBook($this->currentBook);
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * Move forward to next element
     * @link http://php.net/manual/en/iterator.next.php
     * @return void Any returned value is ignored.
     */
    public function next()
    {
        $this->currentBook++;
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * 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.
     */
    public function key()
    {
        return $this->currentBook;
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * 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.
     */
    public function valid()
    {
        return null !== $this->bookList->getBook($this->currentBook);
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * Rewind the Iterator to the first element
     * @link http://php.net/manual/en/iterator.rewind.php
     * @return void Any returned value is ignored.
     */
    public function rewind()
    {
        $this->currentBook = 0;
    }
}

BookListReverseIterator.php

<?php

namespace DesignPatterns\Behavioral\Iterator;

class BookListReverseIterator implements \Iterator
{

    /**
     * @var BookList
     */
    private $bookList;

    /**
     * @var int
     */
    protected $currentBook = 0;

    public function __construct(BookList $bookList)
    {
        $this->bookList = $bookList;
        $this->currentBook = $this->bookList->count() - 1;
    }

    /**
     * Return the current book
     * @link http://php.net/manual/en/iterator.current.php
     * @return Book Can return any type.
     */
    public function current()
    {
        return $this->bookList->getBook($this->currentBook);
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * Move forward to next element
     * @link http://php.net/manual/en/iterator.next.php
     * @return void Any returned value is ignored.
     */
    public function next()
    {
        $this->currentBook--;
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * 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.
     */
    public function key()
    {
        return $this->currentBook;
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * 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.
     */
    public function valid()
    {
        return null !== $this->bookList->getBook($this->currentBook);
    }

    /**
     * (PHP 5 >= 5.0.0)
     *
     * Rewind the Iterator to the first element
     * @link http://php.net/manual/en/iterator.rewind.php
     * @return void Any returned value is ignored.
     */
    public function rewind()
    {
        $this->currentBook = $this->bookList->count() - 1;
    }
}

【四】测试代码

Tests/IteratorTest.php

<?php

namespace DesignPatterns\Behavioral\Iterator\Tests;

use DesignPatterns\Behavioral\Iterator\Book;
use DesignPatterns\Behavioral\Iterator\BookList;
use DesignPatterns\Behavioral\Iterator\BookListIterator;
use DesignPatterns\Behavioral\Iterator\BookListReverseIterator;

class IteratorTest extends \PHPUnit_Framework_TestCase
{

    /**
     * @var BookList
     */
    protected $bookList;

    protected function setUp()
    {
        $this->bookList = new BookList();
        $this->bookList->addBook(new Book('Learning PHP Design Patterns', 'William Sanders'));
        $this->bookList->addBook(new Book('Professional Php Design Patterns', 'Aaron Saray'));
        $this->bookList->addBook(new Book('Clean Code', 'Robert C. Martin'));
    }

    public function expectedAuthors()
    {
        return array(
            array(
                array(
                    'Learning PHP Design Patterns by William Sanders',
                    'Professional Php Design Patterns by Aaron Saray',
                    'Clean Code by Robert C. Martin'
                )
            ),
        );
    }

    /**
     * @dataProvider expectedAuthors
     */
    public function testUseAIteratorAndValidateAuthors($expected)
    {
        $iterator = new BookListIterator($this->bookList);

        while ($iterator->valid()) {
            $expectedBook = array_shift($expected);
            $this->assertEquals($expectedBook, $iterator->current()->getAuthorAndTitle());
            $iterator->next();
        }
    }

    /**
     * @dataProvider expectedAuthors
     */
    public function testUseAReverseIteratorAndValidateAuthors($expected)
    {
        $iterator = new BookListReverseIterator($this->bookList);

        while ($iterator->valid()) {
            $expectedBook = array_pop($expected);
            $this->assertEquals($expectedBook, $iterator->current()->getAuthorAndTitle());
            $iterator->next();
        }
    }

    /**
     * Test BookList Remove
     */
    public function testBookRemove()
    {
        $this->bookList->removeBook($this->bookList->getBook(0));
        $this->assertEquals($this->bookList->count(), 2);
    }
}