- Adds new task scheduler configuration class

- Introduces method to start cron on demand when there are
  scheduled newsletters or queues in progress

lib/Config/TaskScheduler.php

@@ -0,0 +1,65 @@
+<?php
+namespace MailPoet\Config;
+
+use MailPoet\Cron\Supervisor;
+use MailPoet\Cron\Workers\Scheduler as SchedulerWorker;
+use MailPoet\Cron\Workers\SendingQueue\SendingQueue as SendingQueueWorker;
+use MailPoet\Models\Setting;
+
+if(!defined('ABSPATH')) exit;
+
+class TaskScheduler {
+  const METHOD_WORDPRESS = 'WordPress';
+  const METHOD_MAILPOET = 'MailPoet';
+
+  function __construct() {
+    $this->method = self::getCurrentMethod();
+  }
+
+  function init() {
+    // configure task scheduler only outside of cli environment
+    if(php_sapi_name() === 'cli') return;
+    switch($this->method) {
+      case self::METHOD_MAILPOET:
+        return $this->configureMailpoetScheduler();
+      break;
+      case self::METHOD_WORDPRESS:
+        return $this->configureWordpressScheduler();
+      break;
+      default:
+        throw new \Exception(__("Task scheduler is not configured"));
+      break;
+    };
+  }
+
+  function configureMailpoetScheduler() {
+    try {
+      $supervisor = new Supervisor();
+      $supervisor->checkDaemon();
+    } catch(\Exception $e) {
+      // exceptions should not prevent the rest of the site loading
+    }
+  }
+
+  function configureWordpressScheduler() {
+    $scheduled_queues = SchedulerWorker::getScheduledQueues();
+    $running_queues = SendingQueueWorker::getRunningQueues();
+    // run cron only when there are scheduled queues ready to be processed
+    // or are already being processed
+    if($scheduled_queues || $running_queues) {
+      return $this->configureMailpoetScheduler();
+    }
+    return;
+  }
+
+  static function getAvailableMethods() {
+    return array(
+      'mailpoet' => self::METHOD_MAILPOET,
+      'wordpress' => self::METHOD_WORDPRESS
+    );
+  }
+
+  static function getCurrentMethod() {
+    return Setting::getValue('task_scheduler.method');
+  }
+}