迭代器模式 （Iterator），又叫做游标（Cursor）模式。提供一种方法访问一个容器（Container）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。
<?phpnamespace 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(); }}<?phpnamespace 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); }}<?phpnamespace 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)<br/> * 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)<br/> * 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)<br/> * 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)<br/> * 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; }}<?phpnamespace 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)<br/> * 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)<br/> * 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)<br/> * 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)<br/> * 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; }}<?phpnamespace 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); }}
