feature #2311 Add a simple Twig_RuntimeLoaderInterface implementation (chalasr)

This PR was merged into the 1.x branch.

Discussion
----------

Add a simple Twig_RuntimeLoaderInterface implementation

Next to https://github.com/symfony/symfony/pull/21023

This is related to the BC break reported in symfony/symfony#21008 which has been introduced in symfony/symfony#20093 when decoupling extensions from definitions.

What I propose here is to ease the upgrade to symfony 3.2+ by adding a simple `Twig_RuntimeLoaderInterface` implementation here, useful only when using the twig-bridge outside of the symfony fullstack framework (with the Form component for instance).

Upgrading would be as simple as:

```diff
$twig = new Twig_Environment(...);
$rendererEngine = new TwigRendererEngine(array('form_div_layout.html.twig'), $twig);
- $twig->addExtension(new FormExtension(new TwigRenderer($rendererEngine, $csrfTokenManager)));
+ $twig->addExtension(new FormExtension());
+ $twig->addRuntimeLoader(new Twig_RuntimeLoader(array(TwigRenderer::class => new TwigRenderer($rendererEngine, $csrfTokenManager)));
```

Instead of having to write this runtime loader yourself.
Please see symfony/symfony#21008 for details and a concrete example of how this could help.

Commits
-------

91c8d59 Add a Twig_FactoryRuntimeLoader

lib/Twig/FactoryRuntimeLoader.php

@@ -0,0 +1,40 @@
+<?php
+
+/*
+ * This file is part of Twig.
+ *
+ * (c) Fabien Potencier
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+/**
+ * Lazy loads the runtime implementations for a Twig element.
+ *
+ * @author Robin Chalas <robin.chalas@gmail.com>
+ */
+class Twig_FactoryRuntimeLoader implements Twig_RuntimeLoaderInterface
+{
+    private $map;
+
+    /**
+     * @param array $map An array of format [classname => factory callable]
+     */
+    public function __construct(array $map = array())
+    {
+        $this->map = $map;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function load($class)
+    {
+        if (isset($this->map[$class])) {
+            $runtimeFactory = $this->map[$class];
+
+            return $runtimeFactory();
+        }
+    }
+}

test/Twig/Tests/RuntimeFactoryLoaderTest.php

@@ -0,0 +1,32 @@
+<?php
+
+/*
+ * This file is part of Twig.
+ *
+ * (c) Fabien Potencier
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+class Twig_Tests_FactoryRuntimeLoaderTest extends PHPUnit_Framework_TestCase
+{
+    public function testLoad()
+    {
+        $loader = new Twig_FactoryRuntimeLoader(array('stdClass' => 'getRuntime'));
+
+        $this->assertInstanceOf('stdClass', $loader->load('stdClass'));
+    }
+
+    public function testLoadReturnsNullForUnmappedRuntime()
+    {
+        $loader = new Twig_FactoryRuntimeLoader();
+
+        $this->assertNull($loader->load('stdClass'));
+    }
+}
+
+function getRuntime()
+{
+    return new stdClass();
+}