摘要：中在基礎集合類路由類中和分頁類中等，都用到了對象遍歷這個小知識點，這些類都是實現了這個接口，這個接口定義，返回的是迭代器對象。標準擴展庫中提供了很多默認迭代器實現類，比較常用的是數組迭代器對象，參考官網迭代器。

說明：本文章主要講述PHP的對象遍歷(Iterator)知識點。由于Laravel框架中就在集合(Collection)中用到了對象遍歷知識點，故記錄并學習之。同時，作者會將開發過程中的一些截圖和代碼黏上去，提高閱讀效率。

Laravel中在基礎集合類IlluminateSupportCollection、路由類中IlluminateRoutingRouteCollection和分頁類中IlluminatePaginationPaginator等，都用到了對象遍歷這個小知識點，這些類都是實現了IteratorAggregate這個接口，這個接口定義getIterator()，返回的是迭代器對象。PHP標準擴展庫中提供了很多默認迭代器實現類，比較常用的是數組迭代器對象ArrayIterator，參考官網：迭代器。

PHP5提供了遍歷對象屬性的方法，而且默認是可見屬性，如代碼中foreach遍歷對象屬性，默認的都是可見屬性：

 $value) {
    echo $key.":".$value.PHP_EOL;
}

輸出的是：

name:PHP
address:php.net

如果需要遍歷對象的不可見屬性，則在對象內部定義一個遍歷方法：

public function unAccessIterator()
    {
        echo "Iterator the unaccess fields:".PHP_EOL;
        foreach ($this as $key => $value) {
            echo $key.":".$value.PHP_EOL;
        }
    }

對象外部訪問：

$testIterator->unAccessIterator();

將可以遍歷對象的不可見屬性，輸出結果：

Iterator the unaccess fields:
name:PHP
address:php.net
sex:man
age:20

PHP提供了Iterator接口，用來定義迭代器對象來自定義遍歷，所以利用Iterator接口來構造迭代器，需要實現Iterator定義的幾個方法：

composerPackage = $composerPackage;
    }

    public function unAccessIterator()
    {
        echo "Iterator the unaccess fields:".PHP_EOL;
        foreach ($this as $key => $value) {
            echo $key.":".$value.PHP_EOL;
        }
    }

    /**
     * Return the current element
     * @link http://php.net/manual/en/iterator.current.php
     * @return mixed Can return any type.
     * @since 5.0.0
     */
    public function current()
    {
        // TODO: Implement current() method.
        echo "Return the current element:".PHP_EOL;
        return current($this->composerPackage);
    }

    /**
     * Move forward to next element
     * @link http://php.net/manual/en/iterator.next.php
     * @return void Any returned value is ignored.
     * @since 5.0.0
     */
    public function next()
    {
        // TODO: Implement next() method.
        echo "Move forward to next element:".PHP_EOL;
        return next($this->composerPackage);
    }

    /**
     * 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.
     * @since 5.0.0
     */
    public function key()
    {
        // TODO: Implement key() method.
        echo "Return the key of the current element:".PHP_EOL;
        return key($this->composerPackage);
    }

    /**
     * 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.
     * @since 5.0.0
     */
    public function valid()
    {
        // TODO: Implement valid() method.
        echo "Checks if current position is valid:".PHP_EOL;
        return current($this->composerPackage) !== false;
    }

    /**
     * Rewind the Iterator to the first element
     * @link http://php.net/manual/en/iterator.rewind.php
     * @return void Any returned value is ignored.
     * @since 5.0.0
     */
    public function rewind()
    {
        // TODO: Implement rewind() method.
        echo "Rewind the Iterator to the first element:".PHP_EOL;
        reset($this->composerPackage);
    }
}

/*
$testIterator = new TestIterator();
foreach ($testIterator as $key => $value) {
    echo $key.":".$value.PHP_EOL;
}
$testIterator->unAccessIterator();*/

$testIterator = new TestIterator([
    "symfony/http-foundation",
    "symfony/http-kernel",
    "guzzle/guzzle",
    "monolog/monolog"
]);
foreach ($testIterator as $key => $value) {
    echo $key.":".$value.PHP_EOL;
}

成員變量$composerPackage是不可見的，通過實現Iterator接口，同樣可以遍歷自定義的可不見屬性，輸出結果如下：

Rewind the Iterator to the first element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
0:symfony/http-foundation
Move forward to next element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
1:symfony/http-kernel
Move forward to next element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
2:guzzle/guzzle
Move forward to next element:
Checks if current position is valid:
Return the current element:
Return the key of the current element:
3:monolog/monolog
Move forward to next element:
Checks if current position is valid:

PHP真心為程序員考慮了很多，實現IteratorAggragate接口后只需實現getIterator()方法直接返回迭代器對象，就不需要實現Iterator接口需要的一些方法來創建一些迭代器對象，因為PHP已經提供了很多迭代器對象如ArrayIterator對象。所以再重構下上面代碼：

class TestCollection implements IteratorAggregate{
    ...
    /**
     * @var array
     */
    private $composerPackage;
    
    ...
    
    /**
     * Retrieve an external iterator
     * @link http://php.net/manual/en/iteratoraggregate.getiterator.php
     * @return Traversable An instance of an object implementing Iterator or
     * Traversable
     * @since 5.0.0
     */
    public function getIterator()
    {
        // TODO: Implement getIterator() method.
        return new ArrayIterator($this->composerPackage);
    }
   
}

$testCollection = new TestCollection([
    "symfony/http-foundation",
    "symfony/http-kernel",
    "guzzle/guzzle",
    "monolog/monolog"
]);
foreach ($testCollection as $key => $value) {
    echo $key.":".$value.PHP_EOL;
}

同樣的，能遍歷$testCollection對象的不可見屬性$composerPackage，輸出結果：

0:symfony/http-foundation
1:symfony/http-kernel
2:guzzle/guzzle
3:monolog/monolog

文章開頭聊到Laravel中就用到了IteratorAggragate這個接口，可以看看文件的源碼。

總結：PHP提供的對象遍歷特性功能還是很有用處的，下一篇準備看下generator生成器知識點，generator提供了另外一種方式定義Iterator。多多使用Laravel，研究Laravel源碼并模仿之，也不錯哦。

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