Swift_Message creation is now lazy when using callback

Swift_Message object is not created until right before the message is
sent, if it is sent at all. This prevents the expensive Swift_Mailer
autoloader from being invoked.

Swift_Mailer is now also required to run test suite

composer.json

@@ -23,7 +23,8 @@
         "ruflin/elastica": "0.90.*",
         "doctrine/couchdb": "~1.0@dev",
         "aws/aws-sdk-php": "~2.4, >2.4.8",
-        "videlalvaro/php-amqplib": "~2.4"
+        "videlalvaro/php-amqplib": "~2.4",
+        "swiftmailer/swiftmailer": "~5.3"
     },
     "suggest": {
         "graylog2/gelf-php": "Allow sending log messages to a GrayLog2 server",

src/Monolog/Handler/SwiftMailerHandler.php

@@ -32,13 +32,8 @@ class SwiftMailerHandler extends MailHandler
     public function __construct(\Swift_Mailer $mailer, $message, $level = Logger::ERROR, $bubble = true)
     {
         parent::__construct($level, $bubble);
-        $this->mailer  = $mailer;
-        if (!$message instanceof \Swift_Message && is_callable($message)) {
-            $message = call_user_func($message);
-        }
-        if (!$message instanceof \Swift_Message) {
-            throw new \InvalidArgumentException('You must provide either a Swift_Message instance or a callable returning it');
-        }
+
+        $this->mailer = $mailer;
         $this->message = $message;
     }
 
@@ -47,10 +42,31 @@ class SwiftMailerHandler extends MailHandler
      */
     protected function send($content, array $records)
     {
-        $message = clone $this->message;
+        $this->mailer->send($this->buildMessage($content));
+    }
+
+    /**
+     * Creates instance of Swift_Message to be sent
+     *
+     * @param string $content
+     * @return \Swift_Message
+     */
+    protected function buildMessage($content)
+    {
+        $message = null;
+        if ($this->message instanceof \Swift_Message) {
+            $message = clone $this->message;
+        } else if (is_callable($this->message)) {
+            $message = call_user_func($this->message);
+        }
+
+        if (!$message instanceof \Swift_Message) {
+            throw new \InvalidArgumentException('Could not resolve message as instance of Swift_Message or a callable returning it');
+        }
+
         $message->setBody($content);
         $message->setDate(time());
 
-        $this->mailer->send($message);
+        return $message;
     }
 }

tests/Monolog/Handler/SwiftMailerHandlerTest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace Monolog\Handler;
+
+use Monolog\Logger;
+use Monolog\TestCase;
+
+class SwiftMailerHandlerTest extends TestCase
+{
+    /** @var \Swift_Mailer|\PHPUnit_Framework_MockObject_MockObject */
+    private $mailer;
+
+    public function setUp()
+    {
+        $this->mailer = $this
+            ->getMockBuilder('Swift_Mailer')
+            ->disableOriginalConstructor()
+            ->getMock();
+    }
+
+    public function testMessageCreationIsLazyWhenUsingCallback()
+    {
+        $this->mailer->expects($this->never())
+            ->method('send');
+
+        $callback = function () {
+            throw new \RuntimeException('Swift_Message creation callback should not have been called in this test');
+        };
+        $handler = new SwiftMailerHandler($this->mailer, $callback);
+
+        $records = [
+            $this->getRecord(Logger::DEBUG),
+            $this->getRecord(Logger::INFO),
+        ];
+        $handler->handleBatch($records);
+    }
+}