--- /dev/null
+<?php
+
+namespace Drupal\Core\Database\Query;
+
+use Drupal\Core\Database\Connection;
+
+/**
+ * Query extender for pager queries.
+ *
+ * This is the "default" pager mechanism. It creates a paged query with a fixed
+ * number of entries per page.
+ *
+ * When adding this extender along with other extenders, be sure to add
+ * PagerSelectExtender last, so that its range and count are based on the full
+ * query.
+ */
+class PagerSelectExtender extends SelectExtender {
+
+ /**
+ * The highest element we've autogenerated so far.
+ *
+ * @var int
+ */
+ public static $maxElement = 0;
+
+ /**
+ * The number of elements per page to allow.
+ *
+ * @var int
+ */
+ protected $limit = 10;
+
+ /**
+ * The unique ID of this pager on this page.
+ *
+ * @var int
+ */
+ protected $element = NULL;
+
+ /**
+ * The count query that will be used for this pager.
+ *
+ * @var \Drupal\Core\Database\Query\SelectInterface
+ */
+ protected $customCountQuery = FALSE;
+
+ public function __construct(SelectInterface $query, Connection $connection) {
+ parent::__construct($query, $connection);
+
+ // Add pager tag. Do this here to ensure that it is always added before
+ // preExecute() is called.
+ $this->addTag('pager');
+ }
+
+ /**
+ * Override the execute method.
+ *
+ * Before we run the query, we need to add pager-based range() instructions
+ * to it.
+ */
+ public function execute() {
+ // By calling preExecute() here, we force it to preprocess the extender
+ // object rather than just the base query object. That means
+ // hook_query_alter() gets access to the extended object.
+ if (!$this->preExecute($this)) {
+ return NULL;
+ }
+
+ // A NULL limit is the "kill switch" for pager queries.
+ if (empty($this->limit)) {
+ return;
+ }
+ $this->ensureElement();
+
+ $total_items = $this->getCountQuery()->execute()->fetchField();
+ $current_page = pager_default_initialize($total_items, $this->limit, $this->element);
+ $this->range($current_page * $this->limit, $this->limit);
+
+ // Now that we've added our pager-based range instructions, run the query normally.
+ return $this->query->execute();
+ }
+
+ /**
+ * Ensure that there is an element associated with this query.
+ * If an element was not specified previously, then the value of the
+ * $maxElement counter is taken, after which the counter is incremented.
+ *
+ * After running this method, access $this->element to get the element for this
+ * query.
+ */
+ protected function ensureElement() {
+ if (!isset($this->element)) {
+ $this->element = self::$maxElement++;
+ }
+ }
+
+ /**
+ * Specify the count query object to use for this pager.
+ *
+ * You will rarely need to specify a count query directly. If not specified,
+ * one is generated off of the pager query itself.
+ *
+ * @param \Drupal\Core\Database\Query\SelectInterface $query
+ * The count query object. It must return a single row with a single column,
+ * which is the total number of records.
+ */
+ public function setCountQuery(SelectInterface $query) {
+ $this->customCountQuery = $query;
+ }
+
+ /**
+ * Retrieve the count query for this pager.
+ *
+ * The count query may be specified manually or, by default, taken from the
+ * query we are extending.
+ *
+ * @return \Drupal\Core\Database\Query\SelectInterface
+ * A count query object.
+ */
+ public function getCountQuery() {
+ if ($this->customCountQuery) {
+ return $this->customCountQuery;
+ }
+ else {
+ return $this->query->countQuery();
+ }
+ }
+
+ /**
+ * Specify the maximum number of elements per page for this query.
+ *
+ * The default if not specified is 10 items per page.
+ *
+ * @param int|false $limit
+ * An integer specifying the number of elements per page. If passed a false
+ * value (FALSE, 0, NULL), the pager is disabled.
+ */
+ public function limit($limit = 10) {
+ $this->limit = $limit;
+ return $this;
+ }
+
+ /**
+ * Specify the element ID for this pager query.
+ *
+ * The element is used to differentiate different pager queries on the same
+ * page so that they may be operated independently. If you do not specify an
+ * element, every pager query on the page will get a unique element. If for
+ * whatever reason you want to explicitly define an element for a given query,
+ * you may do so here.
+ *
+ * Setting the element here also increments the static $maxElement counter,
+ * which is used for determining the $element when there's none specified.
+ *
+ * Note that no collision detection is done when setting an element ID
+ * explicitly, so it is possible for two pagers to end up using the same ID
+ * if both are set explicitly.
+ *
+ * @param $element
+ * Element ID that is used to differentiate different pager queries.
+ */
+ public function element($element) {
+ $this->element = $element;
+ if ($element >= self::$maxElement) {
+ self::$maxElement = $element + 1;
+ }
+ return $this;
+ }
+
+}
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