First version

.gitignore

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+/composer.lock
+/vendor/

composer.json

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+{
+    "name": "jvasseur/mail-sender",
+    "description": "Send mails with Swiftmailer and Twig",
+    "type": "library",
+    "license": "MIT",
+    "authors": [
+        {
+            "name": "Jérôme Vasseur",
+            "email": "jerome.vasseur@jhome.fr"
+        }
+    ],
+    "autoload": {
+        "psr-4": {
+            "Jvasseur\\MailSender\\": "src/"
+        }
+    },
+    "require": {
+        "php": ">=5.4"
+    }
+}

src/MailSender.php

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+<?php
+
+namespace Jvasseur\MailSender;
+
+class MailSender
+{
+    /**
+     * @var \Swift_Mailer
+     */
+    private $mailer;
+
+    /**
+     * @var \Twig_Environement
+     */
+    private $twig;
+
+    public function __construct(\Swift_Mailer $mailer, \Twig_Environement $twig)
+    {
+        $this->mailer = $mailer;
+        $this->twig = $twig;
+    }
+
+    /**
+     * @param string $name
+     * @param array  $context
+     */
+    public function send($name, array $context = [])
+    {
+        $template = $this->twig->loadTemplate($name);
+
+        $blocks = [];
+        foreach (['from', 'to', 'subject', 'body_txt', 'body_html'] as $blockName) {
+            $rendered = $this->renderBlock($template, $blockName, $context);
+
+            if ($rendered) {
+                $blocks[$blockName] = $rendered;
+            }
+        }
+
+        $blocks = array_merge($context, $blocks);
+
+        $mail = new \Swift_Message();
+        $mail->setSubject($blocks['subject']);
+        $mail->setFrom($blocks['from']);
+
+        if (isset($blocks['to'])) {
+            $mail->setTo($blocks['to']);
+        }
+
+        if (isset($blocks['body_txt'] && $blocks['body_html']) {
+            $mail->setBody($block['body_txt']);
+            $mail->addPart($block['body_html'], 'text/html');
+        } elseif (isset($blocks['body_txt']) {
+            $mail->setBody($block['body_txt']);
+        } elseif (isset($blocks['body_html'])) {
+            $mail->setBody($block['body_html'], 'text/html');
+        }
+
+        $this->mailer->send($mail);
+    }
+
+    /**
+     * Renders a Twig block.
+     *
+     * see {@link https://github.com/twigphp/Twig/issues/676#issuecomment-15842093}
+     *
+     * @param \Twig_Template $template
+     * @param string         $block
+     * @param array          $context
+     *
+     * @return string
+     *
+     * @throws \Exception
+     */
+    private function renderBlock(\Twig_Template $template, $block, array $context)
+    {
+        $context = $template->getEnvironment()->mergeGlobals($context);
+
+        $level = ob_get_level();
+        ob_start();
+        try {
+            $rendered = $template->renderBlock($block, $context);
+            ob_end_clean();
+
+            return $rendered;
+        } catch (\Exception $e) {
+            while (ob_get_level() > $level) {
+                ob_end_clean();
+            }
+
+            throw $e;
+        }
+    }
+}