Added priority to event listener. Higher priority means more importance.

tests/phpunit/opencart/system/engine/EventTest.php

@@ -0,0 +1,38 @@
+<?php
+
+class EventTest extends OpenCartTest {
+	public function testEventOrderedExecution() {
+		$eventMock = $this->getMockBuilder('Event')
+			->setMethods(array('createAction'))
+			->disableOriginalConstructor()
+			->getMock();
+
+		$actionMock = $this->getMockBuilder('Action')
+			->disableOriginalConstructor()
+			->getMock();
+
+		$actionMock->expects($this->exactly(3))
+			->method('execute');
+
+		$eventMock->expects($this->at(0))
+			->method('createAction')
+			->with($this->equalTo('SomeExtraAction'), $this->equalTo(array()))
+			->will($this->returnValue($actionMock));
+
+		$eventMock->expects($this->at(1))
+			->method('createAction')
+			->with($this->equalTo('SomeAction'), $this->equalTo(array()))
+			->will($this->returnValue($actionMock));
+
+		$eventMock->expects($this->at(2))
+			->method('createAction')
+			->with($this->equalTo('SomeAnotherAction'), $this->equalTo(array()))
+			->will($this->returnValue($actionMock));
+
+		$eventMock->register('some.event', 'SomeAction', 10);
+		$eventMock->register('some.event', 'SomeAnotherAction', 1);
+		$eventMock->register('some.event', 'SomeExtraAction', 100);
+
+		$eventMock->trigger('some.event');
+	}
+}

upload/system/engine/event.php

@@ -1,25 +1,48 @@
 <?php
 class Event {
 	private $data = array();
+	private $registry;
 
 	public function __construct($registry) {
 		$this->registry = $registry;
 	}
 
-	public function register($key, $action) {
-		$this->data[$key][] = $action;
+	public function register($key, $action, $priority = 0) {
+		$this->data[$key][] = [
+			'action' => $action,
+			'priority' => (int)$priority,
+		];
 	}
 
 	public function unregister($key, $action) {
-		unset($this->data[$key]);
+		if (isset($this->data[$key])) {
+			foreach ($this->data[$key] as $index => $event) {
+				if ($event['action'] == $action) {
+					unset($this->data[$key][$index]);
+				}
+			}
+		}
 	}
 
 	public function trigger($key, &$arg = array()) {
 		if (isset($this->data[$key])) {
+			usort($this->data[$key], array("Event", "cmpByPriority"));
 			foreach ($this->data[$key] as $event) {
-				$action = new Action($event, $arg);
+				$action = $this->createAction($event['action'], $arg);
 				$action->execute($this->registry);
 			}
 		}
 	}
+
+	protected static function cmpByPriority($a, $b) {
+		if ($a['priority'] == $b['priority']) {
+			return 0;
+		}
+
+		return ($a['priority'] > $b['priority']) ? -1 : 1;
+	}
+
+	protected function createAction($action, &$arg) {
+		return new Action($action, $arg);
+	}
 }