For each trigger a followed by step can be defined.

classes/step_manager.php

@@ -56,11 +56,15 @@ class step_manager extends subplugin_manager {
      * @param int $subpluginid id of the subplugin
      * @return trigger_subplugin
      */
-    private function get_subplugin_by_id($subpluginid) {
+    public function get_subplugin_by_id($subpluginid) {
         global $DB;
         $record = $DB->get_record('tool_cleanupcourses_step', array('id' => $subpluginid));
+        if ($record) {
             $subplugin = subplugin::from_record($record);
             return $subplugin;
+        } else {
+            return null;
+        }
     }
 
     /**
@@ -101,4 +105,13 @@ class step_manager extends subplugin_manager {
         $transaction->allow_commit();
     }
 
+    /**
+     * Gets the list of currently enabled trigger subplugins.
+     * @return array of enabled trigger subplugins.
+     */
+    public function get_steps() {
+        global $DB;
+        return $DB->get_records('tool_cleanupcourses_step');
+    }
+
 }

classes/trigger_manager.php

@@ -109,6 +109,34 @@ class trigger_manager extends subplugin_manager {
         $transaction->allow_commit();
     }
 
+    /**
+     * Changes the followedby of a trigger.
+     * @param int $subpluginid id of the trigger
+     * @param int $followedby id of the step
+     */
+    public function change_followedby($subpluginid, $followedby) {
+        global $DB;
+        $transaction = $DB->start_delegated_transaction();
+
+        $subplugin = $this->get_subplugin_by_id($subpluginid);
+        if (!$subplugin) {
+            return; //TODO: Throw error.
+        }
+        $step_manager = new step_manager();
+        $step = $step_manager->get_subplugin_by_id($followedby);
+
+        // If step is not defined clear followedby.
+        if ($step) {
+            $subplugin->followedby = $step->id;
+        } else {
+            $subplugin->followedby = null;
+        }
+
+        $this->insert_or_update($subplugin);
+
+        $transaction->allow_commit();
+    }
+
     /**
      * Removes a subplugin from the sortindex and adjusts all other indizes.
      * @param trigger_subplugin $toberemoved
@@ -132,8 +160,12 @@ class trigger_manager extends subplugin_manager {
     private function get_subplugin_by_id($subpluginid) {
         global $DB;
         $record = $DB->get_record('tool_cleanupcourses_trigger', array('id' => $subpluginid));
+        if ($record) {
             $subplugin = trigger_subplugin::from_record($record);
             return $subplugin;
+        } else {
+            return null;
+        }
     }
 
     /**
@@ -217,6 +249,9 @@ class trigger_manager extends subplugin_manager {
         if ($action === ACTION_DOWN_SUBPLUGIN) {
             $this->change_sortindex($subplugin, false);
         }
+        if ($action === ACTION_FOLLOWEDBY_SUBPLUGIN) {
+            $this->change_followedby($subplugin, optional_param('followedby', null, PARAM_INT));
+        }
     }
 
 }

classes/trigger_table.php

@@ -33,18 +33,19 @@ class trigger_table extends \table_sql {
     public function __construct($uniqueid) {
         parent::__construct($uniqueid);
         global $PAGE;
-        $this->set_sql("id, name, enabled, sortindex", '{tool_cleanupcourses_trigger}', "TRUE");
+        $this->set_sql("id, name, enabled, sortindex, followedby", '{tool_cleanupcourses_trigger}', "TRUE");
         $this->define_baseurl($PAGE->url);
         $this->pageable(false);
         $this->init();
     }
 
     public function init() {
-        $this->define_columns(['name', 'enabled', 'sortindex']);
+        $this->define_columns(['name', 'enabled', 'sortindex', 'followedby']);
         $this->define_headers([
             get_string('subplugin_name', 'tool_cleanupcourses'),
             get_string('subplugin_enabled', 'tool_cleanupcourses'),
-            get_string('subplugin_sortindex', 'tool_cleanupcourses')
+            get_string('subplugin_sortindex', 'tool_cleanupcourses'),
+            get_string('subplugin_followedby', 'tool_cleanupcourses'),
             ]);
         $this->sortable(false, 'sortindex');
         $this->setup();
@@ -131,4 +132,30 @@ class trigger_table extends \table_sql {
                 null , array('title' => $alt)) . ' ';
     }
 
+    /**
+     * Render followedby column.
+     * @param $row
+     * @return string action button for enabling/disabling of the subplugin
+     */
+    public function col_followedby($row) {
+        global $PAGE, $OUTPUT;
+
+        $manager = new step_manager();
+        $steps = $manager->get_steps();
+        $options = array();
+        foreach ($steps as $id => $step) {
+            $options[$id] = get_string('pluginname', 'cleanupcoursesstep_' . $step->name);
+        }
+
+        // Determine, which step is selected.
+        $selected = '';
+        if ($row->followedby !== null) {
+            $selected = (int) $row->followedby;
+        }
+
+        return $OUTPUT->single_select(new \moodle_url($PAGE->url,
+            array('action' => ACTION_FOLLOWEDBY_SUBPLUGIN, 'subplugin' => $row->id, 'sesskey' => sesskey())),
+            'followedby', $options, $selected);
+    }
+
 }
\ No newline at end of file

lang/en/tool_cleanupcourses.php

@@ -35,6 +35,7 @@ $string['subplugin_name'] = 'Subplugin Name';
 $string['subplugin_type'] = 'Subplugin Type';
 $string['subplugin_enabled'] = 'Enabled';
 $string['subplugin_sortindex'] = 'Up/Down';
+$string['subplugin_followedby'] = 'Followed by';
 
 $string['trigger'] = 'Trigger';
 $string['step'] = 'Process step';

lib.php

@@ -26,3 +26,4 @@ define('ACTION_ENABLE_SUBPLUGIN', 'enable');
 define('ACTION_DISABLE_SUBPLUGIN', 'disable');
 define('ACTION_UP_SUBPLUGIN', 'up');
 define('ACTION_DOWN_SUBPLUGIN', 'down');
+define('ACTION_FOLLOWEDBY_SUBPLUGIN', 'followedby');
\ No newline at end of file