qa: provide tests for ListenerProviderAggregate

Creates ListenerProviderAggregateTest, which tests the behavior of
ListenerProviderAggregate. Discovered two issues while doing so, which
are now resolved.

Author: weierophinney

src/ListenerProvider/ListenerProviderAggregate.php

@@ -13,12 +13,15 @@
 
 class ListenerProviderAggregate implements ListenerProviderInterface
 {
+    /**
+     * @var ListenerProviderInterface[]
+     */
     private $providers = [];
 
     public function getListenersForEvent(object $event) : iterable
     {
-        foreach ($providers as $provider) {
-            yield from $provider;
+        foreach ($this->providers as $provider) {
+            yield from $provider->getListenersForEvent($event);
         }
     }
 

test/ListenerProvider/ListenerProviderAggregateTest.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * @see       https://github.com/phly/phly-event-dispatcher for the canonical source repository
+ * @copyright Copyright (c) 2018-2019 Matthew Weier O'Phinney (https:/mwop.net)
+ * @license   https://github.com/phly/phly-event-dispatcher/blob/master/LICENSE.md New BSD License
+ */
+
+declare(strict_types=1);
+
+namespace PhlyTest\EventDispatcher\ListenerProvider;
+
+use Phly\EventDispatcher\ListenerProvider\ListenerProviderAggregate;
+use PHPUnit\Framework\TestCase;
+use Psr\EventDispatcher\ListenerProviderInterface;
+use PhlyTest\EventDispatcher\TestAsset\TestEvent;
+
+class ListenerProviderAggregateTest extends TestCase
+{
+    public function testAggregateYieldsFromAttachedProviders()
+    {
+        $event = new TestEvent();
+        $listener = function (TestEvent $event) {
+        };
+
+        $provider = $this->prophesize(ListenerProviderInterface::class);
+        $provider
+            ->getListenersForEvent($event)
+            ->willReturn([$listener]);
+
+        $aggregate = new ListenerProviderAggregate();
+        $aggregate->attach($provider->reveal());
+
+        $listeners = iterator_to_array($aggregate->getListenersForEvent($event));
+
+        $this->assertSame([$listener], $listeners);
+    }
+}