add concept-autoloading.md -wip

docs/guide-es/concept-autoloading.md

+Autocarga de clases
+===================
+
+Yii depende del [mecanismo de autocarga de clases](http://www.php.net/manual/en/language.oop5.autoload.php) para localizar
+e incluir los archivos de las clases requiridas. Proporciona un cargador de clases de alto rendimiento que cumple con el
+[estandard PSR-4](https://github.com/php-fig/fig-standards/blob/master/proposed/psr-4-autoloader/psr-4-autoloader.md).
+El cargador se instala cuando incluyes el archivo `Yii.php`.
+
+> Nota: Para simplificar la descripción, en esta sección sólo hablaremos de la carga automática de clases. Sin embargo,
+  ten en cuenta que el contenido que describimos aquí también se aplica a la autocarga de interfaces y rasgos (Traits).
+
+
+Usando el Autocargador de Yii <a name="using-yii-autoloader"></a>
+-----------------------------
+
+Para utilizar el el autocargador de clases de Yii, deberías seguir dos reglas básicas cuando desarrolles y nombres tus
+clases:
+
+* Cada clase debe estar bajo un espacio de nombre (namespace). Por ejemplo `foo\bar\MyClass`.
+* Cada clase debe estar guardada en un archivo individual cuya ruta está determinada por el siguiente algoritmo:
+
+```php
+// $className es un nombre completo de clase con las iniciales barras invertidas.
+$classFile = Yii::getAlias('@' . str_replace('\\', '/', $className) . '.php');
+```
+
+For example, if a class name is `foo\bar\MyClass`, the [alias](concept-aliases.md) for the corresponding class file path
+would be `@foo/bar/MyClass.php`. In order for this alias to be able to be resolved into a file path,
+either `@foo` or `@foo/bar` must be a [root alias](concept-aliases.md#defining-aliases).
+
+When you are using the [Basic Application Template](start-basic.md), you may put your classes under the top-level
+namespace `app` so that they can be autoloaded by Yii without the need of defining a new alias. This is because
+`@app` is a [predefined alias](concept-aliases.md#predefined-aliases), and a class name like `app\components\MyClass`
+can be resolved into the class file `AppBasePath/components/MyClass.php`, according to the algorithm we just described.
+
+In the [Advanced Application Template](tutorial-advanced-app.md), each tier has its own root alias. For example,
+the front-end tier has a root alias `@frontend` while the back-end tier `@backend`. As a result, you may
+put the front-end classes under the namespace `frontend` while the back-end classes under `backend`. This will
+allow these classes to be autoloaded by the Yii autoloader.
+
+
+Class Map <a name="class-map"></a>
+---------
+
+The Yii class autoloader supports the *class map* feature which maps class names to the corresponding class file paths.
+When the autoloader is loading a class, it will first check if the class is found in the map. If so, the corresponding
+file path will be included directly without further check. This makes class autoloading super fast. In fact,
+all core Yii classes are being autoloaded this way.
+
+You may add a class to the class map `Yii::$classMap` as follows,
+
+```php
+Yii::$classMap['foo\bar\MyClass'] = 'path/to/MyClass.php';
+```
+
+[Aliases](concept-aliases.md) can be used to specify class file paths. You should set the class map in the
+[bootstrapping](runtime-bootstrapping.md) process so that the map is ready before your classes are used.
+
+
+Using Other Autoloaders <a name="using-other-autoloaders"></a>
+-----------------------
+
+Because Yii embraces Composer as a package dependency manager, it is recommended that you also install
+the Composer autoloader. If you are using some 3rd-party libraries that have their autoloaders, you should
+also install them.
+
+When you are using the Yii autoloader together with other autoloaders, you should include the `Yii.php` file
+*after* all other autoloaders are installed. This will make the Yii autoloader to be the first one responding to
+any class autoloading request. For example, the following code is extracted from
+the [entry script](structure-entry-scripts.md) of the [Basic Application Template](start-basic.md). The first
+line installs the Composer autoloader, while the second line installs the Yii autoloader.
+
+```php
+require(__DIR__ . '/../vendor/autoload.php');
+require(__DIR__ . '/../vendor/yiisoft/yii2/Yii.php');
+```
+
+You may use the Composer autoloader alone without the Yii autoloader. However, by doing so, the performance
+of your class autoloading may be degraded, and you must follow the rules set by Composer in order for your classes
+to be autoloadable.
+
+> Info: If you do not want to use the Yii autoloader, you must create your own version of the `Yii.php` file
+  and include it in your [entry script](structure-entry-scripts.md).
+
+
+Autoloading Extension Classes <a name="autoloading-extension-classes"></a>
+-----------------------------
+
+The Yii autoloader is capable of autoloading [extension](structure-extensions.md) classes. The sole requirement
+is that an extension specifies the `autoload` section correctly in its `composer.json` file. Please refer to the
+[Composer documentation](https://getcomposer.org/doc/04-schema.md#autoload) for more details about specifying `autoload`.
+
+In case you do not use the Yii autoloader, the Composer autoloader can still autoload extension classes for you.