adding subscribers getter from dynamic segments

lib/Subscribers/ImportExport/Export/DynamicSubscribersGetter.php

@@ -0,0 +1,65 @@
+<?php
+
+namespace MailPoet\Subscribers\ImportExport\Export;
+
+use MailPoet\Models\Segment;
+use MailPoet\Models\Subscriber;
+use MailPoet\WP\Hooks;
+
+/**
+ * Gets batches of subscribers from dynamic segments.
+ */
+class DynamicSubscribersGetter extends SubscribersGetter {
+
+  protected $segment_index;
+
+  public function reset() {
+    parent::reset();
+    $this->segment_index = 0;
+  }
+
+  protected function filter($subscribers) {
+    $segment_id = $this->segments_ids[$this->segment_index];
+
+    $filters = Hooks::applyFilters(
+      'mailpoet_add_segment_filters', 
+      $segment_id
+    );
+    
+    if(!is_array($filters) || empty($filters)) {
+      return array();
+    }
+
+    $name = Segment::findOne($segment_id)->name;
+
+    foreach($filters as $filter) {
+      $subscribers = $filter->toSql($subscribers);
+    }
+
+    return $subscribers
+      ->selectMany(array(
+        'list_status' => Subscriber::$_table . '.status'
+      ))
+      ->selectExpr("'{$name}' AS segment_name")
+      ->offset($this->offset)
+      ->limit($this->batch_size)
+      ->findArray();
+  }
+
+  public function get() {
+    if($this->segment_index >= count($this->segments_ids)) {
+      $this->finished = true;
+    }
+
+    $subscribers = parent::get();
+
+    if($subscribers !== false && count($subscribers) < $this->batch_size) {
+      $this->segment_index ++;
+      $this->offset = 0;
+      $this->finished = false;
+    }
+
+    return $subscribers;
+  }
+
+}