php - 使用 PHP 迭代器

Question

friend 。我知道，这里已经有很多关于这些迭代器的问题。 我读过一些东西，我不是初学者......但我的想法有点停留在这个上面。请帮助我理解如何在实践中使用迭代器。

假设，我有一个可以从数据库中选择实例的 ORM 对象。一个实例包含字段，可以像往常一样插入、更新等。 我想遍历一个类型的所有对象，但由于它们可能很多，所以我更喜欢按“页面”选择它们。我的代码:

$limit = 100;
$offset = 0;
do
{
  $recs = $orm->select($filter, $sorting, $limit , $offset);
  $offset += $limit;
  foreach ($recs as $rec)
  {
    // doing something with record
  }
}
while (count($recs) == $limit);

我觉得迭代器范式适合这里，但在这种情况下哪种接口(interface)更适合实现，或者某些基础 SPL 类？

更新 理想情况下，上面带有迭代器的代码可能如下所示:

$iterator = new ORMPagedIterator($ormobject, $filter, $sorting);
foreach ($iterator as $rec)
{
  // do something with record
}

例如所有逐页行为都在迭代器中。

Answer 1

我会使用一个迭代器来迭代另一个迭代器，并在它到达前一个迭代器的末尾时请求下一个迭代器……好吧，听起来比实际情况要复杂:

<?php
$limit = 100;
$offset = 0;

$iter = new NextIteratorCallbackIterator(function($i) use ($orm, $limit, &$offset) {
    printf("selecting next bunch at offset %d\n", $offset);
    $recs = $orm->select($filter, $sorting, $limit , $offset);
    $offset += $limit;
    if ($recs) {
        return new ArrayIterator($recs);
    }
    return null; // end reached
});

foreach ($iter as $rec) {
    // do something with record
}
?>

下面是 NextIteratorCallbackIterator 的示例实现:

<?php
class NextIteratorCallbackIterator implements Iterator {
    private $_iterator = null;
    private $_count = 0;
    private $_callback;

    public function __construct($callback) {
        if (!is_callable($callback)) {
            throw new Exception(__CLASS__.": callback must be callable");
        }
        $this->_callback = $callback;
    }

    public function current() {
        return $this->_iterator !== null ? $this->_iterator->current() : null;
    }

    public function key() {
        return $this->_iterator !== null ? $this->_iterator->key() : null;
    }

    public function next() {
        $tryNext = ($this->_iterator === null);
        do {
            if ($tryNext) {
                $tryNext = false;
                $this->_iterator = call_user_func($this->_callback, ++$this->_count);
            }
            elseif ($this->_iterator !== null) {
                $this->_iterator->next();
                if ($this->_iterator->valid() == false) {
                    $tryNext = true;
                }
            }
        } while ($tryNext);
    }

    public function rewind() {
        $this->_iterator = call_user_func($this->_callback, $this->_count = 0);
    }

    public function valid () {
        return $this->_iterator !== null;
    }
}
?>

更新:您的 ORMPagedIterator 可以使用 NextIteratorCallbackIterator 轻松实现:

<?php
class ORMPagedIterator implements IteratorAggregate {
    function __construct($orm, $filter, $sorting, $chunksize = 100) {
        $this->orm = $orm;
        $this->filter = $filter;
        $this->sorting = $sorting;
        $this->chunksize = $chunksize;
    }

    function iteratorNext($i) {
        $offset = $this->chunksize * $i;
        $recs = $this->orm->select($this->filter, $this->sorting, $this->chunksize, $offset);
        if ($recs) {
            return new ArrayIterator($recs);
        }
        return null; // end reached
    }

    function getIterator() {
        return new NextIteratorCallbackIterator(array($this,"iteratorNext"));
    }
}
?>