Added support to insert an event handler at the beginning of class-level event handler queue

framework/CHANGELOG.md

@@ -38,6 +38,7 @@ Yii Framework 2 Change Log
 - Enh: Added support for using sub-queries when building a DB query with `IN` condition (qiangxue)
 - Enh: Supported adding a new response formatter without the need to reconfigure existing formatters (qiangxue)
 - Enh: Added `yii\web\UrlManager::addRules()` to simplify adding new URL rules (qiangxue)
+- Enh: Added support to insert an event handler at the beginning of class-level event handler queue (qiangxue)
 - Chg #3036: Upgraded Twitter Bootstrap to 3.1.x (qiangxue)
 - Chg #3175: InvalidCallException, InvalidParamException, UnknownMethodException are now extended from SPL BadMethodCallException (samdark)
 - Chg: Replaced `clearAll()` and `clearAllAssignments()` in `yii\rbac\ManagerInterface` with `removeAll()`, `removeAllRoles()`, `removeAllPermissions()`, `removeAllRules()` and `removeAllAssignments()` (qiangxue)

framework/base/Event.php

@@ -74,11 +74,19 @@ class Event extends Object
      * @param callable $handler the event handler.
      * @param mixed $data the data to be passed to the event handler when the event is triggered.
      * When the event handler is invoked, this data can be accessed via [[Event::data]].
+     * @param boolean $append whether to append new event handler to the end of the existing
+     * handler list. If false, the new handler will be inserted at the beginning of the existing
+     * handler list.
      * @see off()
      */
-    public static function on($class, $name, $handler, $data = null)
+    public static function on($class, $name, $handler, $data = null, $append = true)
     {
-        self::$_events[$name][ltrim($class, '\\')][] = [$handler, $data];
+        $class = ltrim($class, '\\');
+        if ($append || empty(self::$_events[$name][$class])) {
+            self::$_events[$name][$class][] = [$handler, $data];
+        } else {
+            array_unshift(self::$_events[$name][$class], [$handler, $data]);
+        }
     }
 
     /**