adding subscribers getter from dynamic segments

lib/Subscribers/ImportExport/Export/SubscribersGetter.php

@@ -0,0 +1,74 @@
+<?php
+
+namespace MailPoet\Subscribers\ImportExport\Export;
+
+use MailPoet\Models\Subscriber;
+
+/**
+ * Gets batches of subscribers for export.
+ */
+abstract class SubscribersGetter {
+
+  protected $segments_ids;
+  protected $batch_size;
+  protected $offset;
+  protected $finished;
+
+  public function __construct($segments_ids, $batch_size) {
+    $this->segments_ids = $segments_ids;
+    $this->batch_size = $batch_size;
+    $this->reset();
+  }
+
+  public function reset() {
+    $this->offset = 0;
+    $this->finished = false;
+  }
+
+  /**
+   * Initialize the query by selecting fields and ignoring trashed subscribers.
+   * 
+   * @return \ORM
+   */
+  protected function select() {
+    return Subscriber::selectMany(
+      'first_name',
+      'last_name',
+      'email',
+      'subscribed_ip',
+      array(
+        'global_status' => Subscriber::$_table . '.status'
+      )
+    )
+    ->filter('filterWithCustomFieldsForExport')
+    ->whereNull(Subscriber::$_table . '.deleted_at');
+  }
+
+  /**
+   * Filters the subscribers query based on the segments, offset and batch size.
+   * 
+   * @param  \ORM $subscribers
+   * @return array
+   */
+  abstract protected function filter($subscribers);
+
+  /**
+   * Gets the next batch of subscribers or `false` if no more!
+   */
+  public function get() {
+    if($this->finished) {
+      return false;
+    }
+
+    $subscribers = $this->select();
+    $subscribers = $this->filter($subscribers);
+
+    $this->offset += $this->batch_size;
+
+    if(count($subscribers) < $this->batch_size) {
+      $this->finished = true;
+    }
+
+    return $subscribers;
+  }
+}