Add helper class for single-event subscription

src/Symfony/Component/EventDispatcher/CHANGELOG.md

@@ -1,6 +1,11 @@
 CHANGELOG
 =========
 
+4.4.0
+-----
+
+ * added `EventListenerTrait` helper class for single-event subscription.
+
 4.3.0
 -----
 

src/Symfony/Component/EventDispatcher/EventListenerTrait.php

@@ -0,0 +1,61 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\EventDispatcher;
+
+use Symfony\Contracts\EventDispatcher\Event as ContractsEvent;
+
+/**
+ * Helper class for single-event subscription.
+ *
+ * @author Yonel Ceruto <yonelceruto@gmail.com>
+ */
+trait EventListenerTrait
+{
+    /**
+     * {@inheritdoc}
+     */
+    final public static function getSubscribedEvents(): array
+    {
+        return [static::getEventName() => [static::getMethodName(), static::getPriority()]];
+    }
+
+    public static function getEventName(): string
+    {
+        $method = new \ReflectionMethod(static::class, static::getMethodName());
+
+        if (0 === $method->getNumberOfParameters()) {
+            throw new \LogicException(sprintf('We cannot infer the Event name from "%s::%s()" method without parameters, you must to define one or implement the "%s::getEventName()" method to return it explicitly.', static::class, $method->getName(), static::class));
+        }
+
+        $parameter = $method->getParameters()[0];
+
+        if (null === $type = $parameter->getType()) {
+            throw new \LogicException(sprintf('We cannot infer the Event name from "%s::%s()" method without type-hint the "$%s" parameter, you must to type-hint it or implement the "%s::getEventName()" method to return it explicitly.', ContractsEvent::class, $method->getName(), $parameter->getName(), static::class));
+        }
+
+        if ($type->isBuiltin()) {
+            throw new \LogicException(sprintf('The "$%s" parameter defined in "%s::%s()" method must be a class of "%s", "%s" given.', $parameter->getName(), static::class, $method->getName(), ContractsEvent::class, $type->getName()));
+        }
+
+        return $type->getName();
+    }
+
+    public static function getMethodName(): string
+    {
+        return '__invoke';
+    }
+
+    public static function getPriority(): int
+    {
+        return 0;
+    }
+}

src/Symfony/Component/EventDispatcher/Tests/EventDispatcherTest.php

@@ -14,6 +14,7 @@
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\EventDispatcher;
+use Symfony\Component\EventDispatcher\EventListenerTrait;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Contracts\EventDispatcher\Event as ContractsEvent;
 
@@ -437,6 +438,16 @@ public function testLegacySignatureWithNewEventObject()
         $this->expectExceptionMessage('Argument 1 passed to "Symfony\Component\EventDispatcher\EventDispatcherInterface::dispatch()" must be an object, string given.');
         $this->dispatcher->dispatch('foo', new ContractsEvent());
     }
+
+    public function testSingleEventSubsriber()
+    {
+        $subscriber = new TestSingleEventListener();
+        $this->dispatcher->addSubscriber($subscriber);
+        $this->assertTrue($this->dispatcher->hasListeners(MyEvent::class));
+
+        $this->dispatcher->dispatch(new MyEvent());
+        $this->assertTrue($subscriber->invoked);
+    }
 }
 
 class CallableClass
@@ -480,6 +491,22 @@ public function foo($e, $name, $dispatcher)
     }
 }
 
+class MyEvent extends ContractsEvent
+{
+}
+
+class TestSingleEventListener implements EventSubscriberInterface
+{
+    use EventListenerTrait;
+
+    public $invoked = false;
+
+    public function __invoke(MyEvent $event): void
+    {
+        $this->invoked = true;
+    }
+}
+
 class TestEventSubscriber implements EventSubscriberInterface
 {
     public static function getSubscribedEvents(): array