First shot of a documentation of the new PSR-4 class loader.

components/class_loader/index.rst

@@ -6,6 +6,7 @@ ClassLoader
 
     introduction
     class_loader
+    psr4_class_loader
     map_class_loader
     cache_class_loader
     debug_class_loader

components/class_loader/psr4_class_loader.rst

@@ -0,0 +1,63 @@
+.. index::
+   single: ClassLoader; PSR-4 Class Loader
+
+The PSR-4 Class Loader
+======================
+
+Libraries that follow the `PSR-4`_ standard can be loaded with the ``Psr4ClassLoader``.
+
+.. note::
+
+    If you manage your dependencies via Composer, you get a PSR-4 compatible
+    autoloader out of the box. Use this loader in environments where Composer
+    is not available.
+
+.. tip::
+    All Symfony Components follow PSR-4.
+
+Usage
+-----
+
+The following example demonstrates, how you can use the
+:class:`Symfony\\Component\\ClassLoader\\Psr4ClassLoader` autoloader to use
+Symfony's Yaml component. Let's imagine, you downloaded both components –
+ClassLoader and Yaml – as ZIP packages and unpacked them to a libs directory.
+
+The directory structure will look like this:
+
+.. code-block:: text
+
+    /
+    +- libs
+    |  +- ClassLoader
+    |  |  +- Psr4ClassLoader.php
+    |  |  +- …
+    |  +- Yaml
+    |     +- Yaml.php
+    |     +- …
+    +- config.yml
+    +- test.php
+
+In ``demo.php``, to parse the file ``config.yml``, you can use the following
+code.
+
+.. code-block:: php
+
+    use Symfony\Component\ClassLoader\Psr4ClassLoader;
+    use Symfony\Component\Yaml\Yaml;
+
+    require __DIR__ . '/lib/ClassLoader/Psr4ClassLoader.php';
+
+    $loader = new Psr4ClassLoader();
+    $loader->addPrefix('Symfony\\Component\\Yaml\\', __DIR__ . '/lib/Yaml');
+    $loader->register();
+
+    $data = Yaml::parse(__DIR__ . '/demo.yml');
+
+First of all, we've loaded our class loader manually using ``require`` since we
+don't have an autoload mechanism, yet. With the ``addPrefix()`` call, we told
+the class loader where to look for classes with the namespace prefix
+``Symfony\Component\Yaml\``. After registering the autoloader, the Yaml
+component is ready to use.
+
+.. _PSR-4: http://www.php-fig.org/psr/psr-4/