Iterator Design Pattern in PHP

In the Iterator pattern a class will be able to traverse the elements of another class.

In this example, the BookList class will have stored zero to many elements of the Book class. The BookListIterator can return all of the Books in the BookList one by one and in the sequential order that the Book elements were added to the BookList.

I also show a BookListReverseIterator. The BookListReverseIterator can return all of the Books in the BookList one by one and in the reverse sequential order that the Book elements were added to the BookList. This is added to show that custom iterators can be quite useful when you need to process a list in an sequence other than sequentially first to last. While this Iterator is reverse, I could have also shown an iterator that returned, for example, only odd numbered elements.

Note that the Iterators shown are not “robust”, and would yield unpredictable results if the List the Iterator is processing has elements removed while the Iterator is traversing the List.

<?php

class Book {
    private $author;
    private $title;
    function __construct($title_in, $author_in) {
      $this->author = $author_in;
      $this->title  = $title_in;
    }
    function getAuthor() {return $this->author;}
    function getTitle() {return $this->title;}
    function getAuthorAndTitle() {
      return $this->getTitle() . ' by ' . $this->getAuthor();
    }
}

class BookList {
    private $books = array();
	private $bookCount = 0;
    public function __construct() {
    }
    public function getBookCount() {
      return $this->bookCount;
    }
    private function setBookCount($newCount) {
      $this->bookCount = $newCount;
    }
    public function getBook($bookNumberToGet) {
	  if ( (is_numeric($bookNumberToGet)) && 
           ($bookNumberToGet <= $this->getBookCount())) {
           return $this->books[$bookNumberToGet];
         } else {
           return NULL;
         }
	}
    public function addBook(Book $book_in) {
      $this->setBookCount($this->getBookCount() + 1);
      $this->books[$this->getBookCount()] = $book_in;
      return $this->getBookCount();
    }
    public function removeBook(Book $book_in) {
      $counter = 0;
      while (++$counter <= $this->getBookCount()) {
        if ($book_in->getAuthorAndTitle() == 
          $this->books[$counter]->getAuthorAndTitle())
		  {
            for ($x = $counter; $x < $this->getBookCount(); $x++) {
              $this->books[$x] = $this->books[$x + 1];
          }
          $this->setBookCount($this->getBookCount() - 1);
        }
      }
      return $this->getBookCount();
    }
}

class BookListIterator {
    protected $bookList;
    protected $currentBook = 0;

    public function __construct(BookList $bookList_in) {
      $this->bookList = $bookList_in;
    }
    public function getCurrentBook() {
      if (($this->currentBook > 0) && 
          ($this->bookList->getBookCount() >= $this->currentBook)) {
        return $this->bookList->getBook($this->currentBook);
      }
    }
    public function getNextBook() {
	  if ($this->hasNextBook()) {
        return $this->bookList->getBook(++$this->currentBook);
      } else {
        return NULL;
      }
    }
    public function hasNextBook() {
      if ($this->bookList->getBookCount() > $this->currentBook) {
	    return TRUE;
	  } else {
        return FALSE;
	  }
    }
}

class BookListReverseIterator extends BookListIterator {
    public function __construct(BookList $bookList_in) {
      $this->bookList = $bookList_in;
	  $this->currentBook = $this->bookList->getBookCount() + 1;
    }
    public function getNextBook() {
	  if ($this->hasNextBook()) {
        return $this->bookList->getBook(--$this->currentBook);
      } else {
        return NULL;
      }
    }
    public function hasNextBook() {
      if (1 < $this->currentBook) {
	    return TRUE;
	  } else {
        return FALSE;
	  }
    }
}


  writeln('BEGIN TESTING ITERATOR PATTERN');
  writeln('');
 
  $firstBook = new Book('Core PHP Programming, Third Edition', 'Atkinson and Suraski');
  $secondBook = new Book('PHP Bible', 'Converse and Park');
  $thirdBook = new Book('Design Patterns', 'Gamma, Helm, Johnson, and Vlissides');

  $books = new BookList();
  $books->addBook($firstBook);
  $books->addBook($secondBook);
  $books->addBook($thirdBook);
 
  writeln('Testing the Iterator');
 
  $booksIterator = new BookListIterator($books);

  while ($booksIterator->hasNextBook()) {
    $book = $booksIterator->getNextBook();
    writeln('getting next book with iterator :');
    writeln($book->getAuthorAndTitle());
    writeln('');
  }
 
  $book = $booksIterator->getCurrentBook();
  writeln('getting current book with iterator :');
  writeln($book->getAuthorAndTitle());
  writeln('');  

  writeln('Testing the Reverse Iterator');

  $booksReverseIterator = new BookListReverseIterator($books);

  while ($booksReverseIterator->hasNextBook()) {
    $book = $booksReverseIterator->getNextBook();
    writeln('getting next book with reverse iterator :');
    writeln($book->getAuthorAndTitle());
    writeln('');
  }
 
  $book = $booksReverseIterator->getCurrentBook();
  writeln('getting current book with reverse iterator :');
  writeln($book->getAuthorAndTitle());
  writeln('');

  writeln('END TESTING ITERATOR PATTERN');
 
  function writeln($line_in) {
    echo $line_in."<br/>";
  }

?>

Output

BEGIN TESTING ITERATOR PATTERN

Testing the Iterator
getting next book with iterator : 
Core PHP Programming, Third Edition by Atkinson and Suraski

getting next book with iterator : 
PHP Bible by Converse and Park

getting next book with iterator : 
Design Patterns by Gamma, Helm, Johnson, and Vlissides

getting current book with iterator : 
Design Patterns by Gamma, Helm, Johnson, and Vlissides

Testing the Reverse Iterator
getting next book with reverse iterator : 
Design Patterns by Gamma, Helm, Johnson, and Vlissides

getting next book with reverse iterator : 
PHP Bible by Converse and Park

getting next book with reverse iterator : 
Core PHP Programming, Third Edition by Atkinson and Suraski

getting current book with reverse iterator : 
Core PHP Programming, Third Edition by Atkinson and Suraski

END TESTING ITERATOR PATTERN

Code examples