Fix zendframework/zend-navigation#23

Running tests of zend-navigation against all v2 components (no v3 components)
revealed a circular dependency condition in the navigation helpers related to
`getEventManager()`.

In v2, `getEventManager()` has lazy-loaded an EM instance, and an initializer
was checking the returned instance to see if its `SharedEventManager` instance
was present and/or the same as the one in the container; if not, it was
re-injecting the EM instance from the container.  Unfortunately, this fails now,
as the call to `setEventManager()` now attaches the default listeners, and
requires that a shared manager is present.

This patch changes the behavior to the following:

- `getEventManager()` now *never* lazy-loads an instance. This ensures that the
  initializer doesn't lead to lazy-population of the shared manager.
- `setEventManager()` was updated to check that we have a shared manager before
  attempting to call `setDefaultListeners()`.
- Internally, if an EM instance is needed, it now lazy-creates one, *with a
  shared manager*, and calls `setEventManager()` with the new EM instance.
- The EM initializer in the helper plugin amanger was updated to first check
  that we have an `EventManager` instance before attempting to inject one.

src/Helper/Navigation/AbstractHelper.php

@@ -342,7 +342,8 @@ public function accept(AbstractPage $page, $recursive = true)
      */
     protected function isAllowed($params)
     {
-        $results = $this->getEventManager()->trigger(__FUNCTION__, $this, $params);
+        $events = $this->getEventManager() ?: $this->createEventManager();
+        $results = $events->trigger(__FUNCTION__, $this, $params);
         return $results->last();
     }
 
@@ -513,22 +514,23 @@ public function setEventManager(EventManagerInterface $events)
 
         $this->events = $events;
 
-        $this->setDefaultListeners();
+        if ($events->getSharedManager()) {
+            $this->setDefaultListeners();
+        }
 
         return $this;
     }
 
     /**
-     * Get the event manager.
+     * Get the event manager, if present.
+     *
+     * Internally, the helper will lazy-load an EM instance the first time it
+     * requires one, but ideally it should be injected during instantiation.
      *
-     * @return  EventManagerInterface
+     * @return  null|EventManagerInterface
      */
     public function getEventManager()
     {
-        if (null === $this->events) {
-            $this->setEventManager($this->createEventManager());
-        }
-
         return $this->events;
     }
 
@@ -956,7 +958,13 @@ protected function setDefaultListeners()
             return;
         }
 
-        $this->getEventManager()->getSharedManager()->attach(
+        $events = $this->getEventManager() ?: $this->createEventManager();
+
+        if (! $events->getSharedManager()) {
+            return;
+        }
+
+        $events->getSharedManager()->attach(
             'Zend\View\Helper\Navigation\AbstractHelper',
             'isAllowed',
             ['Zend\View\Helper\Navigation\Listener\AclListener', 'accept']
@@ -975,9 +983,13 @@ private function createEventManager()
     {
         $r = new ReflectionClass(EventManager::class);
         if ($r->hasMethod('setSharedManager')) {
-            return new EventManager();
+            $events = new EventManager();
+            $events->setSharedManager(new SharedEventManager());
+        } else {
+            $events = new EventManager(new SharedEventManager());
         }
 
-        return new EventManager(new SharedEventManager());
+        $this->setEventManager($events);
+        return $events;
     }
 }

src/HelperPluginManager.php

@@ -375,6 +375,11 @@ public function injectEventManager($first, $second)
             return;
         }
 
+        if (! $container->has('EventManager')) {
+            // If the container doesn't have an EM service, do nothing.
+            return;
+        }
+
         $events = $helper->getEventManager();
         if (! $events || ! $events->getSharedManager() instanceof SharedEventManagerInterface) {
             $helper->setEventManager($container->get('EventManager'));

test/Helper/Navigation/AbstractHelperTest.php

@@ -76,4 +76,9 @@ public function testHasRoleChecksMemberVariable()
         $this->_helper->setRole($role);
         $this->assertEquals(true, $this->_helper->hasRole());
     }
+
+    public function testEventManagerIsNullByDefault()
+    {
+        $this->assertNull($this->_helper->getEventManager());
+    }
 }

test/Helper/Navigation/AbstractTest.php

@@ -23,14 +23,9 @@
 
 /**
  * Base class for navigation view helper tests
- *
- * @group      Zend_View
- * @group      Zend_View_Helper
  */
 abstract class AbstractTest extends \PHPUnit_Framework_TestCase
 {
-    const REGISTRY_KEY = 'Zend_Navigation';
-
     /**
      * @var
      */