行为型设计模式-迭代器 Iterator

简介

提供一种方法顺序访问一个聚合对象中的各个元素，而又不需要暴露该对象的内部表示。

角色

• Iterator抽象迭代器
• Collection抽象集合
• ConcreteIterator具体迭代器
• ConcreteCollection具体集合

类图

如图，Iterator和IterableCollection定义了操作接口，ConcreteIterator实现类持有ConcreteCollection的引用，默默实现迭代的逻辑。

代码

use Iterator;

class AlphabeticalOrderIterator implements \Iterator
{
    private $collection;

    private $position = 0;

    private $reverse = false;

    public function __construct($collection, $reverse = false)
    {
        $this->collection = $collection;
        $this->reverse = $reverse;
    }

    public function rewind()
    {
        $this->position = $this->reverse ?
            count($this->collection->getItems()) - 1 : 0;
    }

    public function current()
    {
        return $this->collection->getItems()[$this->position];
    }

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

    public function next()
    {
        $this->position = $this->position + ($this->reverse ? -1 : 1);
    }

    public function valid()
    {
        return isset($this->collection->getItems()[$this->position]);
    }
}

class WordsCollection implements \IteratorAggregate
{
    private $items = [];

    public function getItems()
    {
        return $this->items;
    }

    public function addItem($item)
    {
        $this->items[] = $item;
    }

    public function getIterator(): Iterator
    {
        return new AlphabeticalOrderIterator($this);
    }

    public function getReverseIterator(): Iterator
    {
        return new AlphabeticalOrderIterator($this, true);
    }
}

$collection = new WordsCollection();
$collection->addItem("First");
$collection->addItem("Second");
$collection->addItem("Third");

echo "Straight traversal:\n";
foreach ($collection->getIterator() as $item) {
    echo $item . "\n";
}

echo "\n";
echo "Reverse traversal:\n";
foreach ($collection->getReverseIterator() as $item) {
    echo $item . "\n";
}

output:

Straight traversal:
First
Second
Third

Reverse traversal:
Third
Second
First

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