迭代器模式

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。
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。

这个模式有两种实现的方式

方式一

 

<?php

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();
    }
}


class BookList implements Countable, Iterator
{
    private $_books = [];
    private $_currentIndex = 0;


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

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

        $this->_books = array_values($this->_books);
    }



    /* 以下是接口部分 */

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

    public function current()
    {
        return $this->_books[$this->_currentIndex];
    }

    public function key()
    {
        return $this->_currentIndex;
    }

    public function next()
    {
        $this->_currentIndex++;
    }

    public function rewind()
    {
        $this->_currentIndex = 0;
    }

    public function valid()
    {
        return isset($this->_books[$this->_currentIndex]);
    }
}




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

foreach ($bookList as $book) {
    echo "<br/>";
    echo $book->getAuthorAndTitle();
}

方式二

<?php


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();
    }
}


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 => $book) {
            if ($book->getAuthorAndTitle() === $bookToRemove->getAuthorAndTitle()) {
                unset($this->_books[$key]);
            }
        }
    }

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


class BookListIterator implements Iterator
{
    private $_bookList;
    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);
    }

    /**
     * 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++;
    }

    /**
     * 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;
    }

    /**
     * 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);
    }

    /**
     * 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;
    }
}







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

$iterator = new BookListIterator($bookList);

while ($iterator->valid()) {
    print "<br/>";
    print $iterator->current()->getAuthorAndTitle();
    $iterator->next();
}