text: Extensions
  ==========
  
  Extensions are redistributable software packages specifically designed to be used in Yii applications and provide
  ready-to-use features. For example, the [yiisoft/yii2-debug](tool-debugger.md) extension adds a handy debug toolbar
  at the bottom of every page in your application to help you more easily grasp how the pages are generated. You can
  use extensions to accelerate your development process. You can also package your code as extensions to share with
  other people your great work.
  
  > Info: We use the term "extension" to refer to Yii-specific software packages. For general purpose software packages
    that can be used without Yii, we will refer to them using the term "package" or "library".
  
  
  ## Using Extensions <a name="using-extensions"></a>
  
  To use an extension, you need to install it first. Most extensions are distributed as [Composer](https://getcomposer.org/)
  packages which can be installed by taking the following two simple steps:
  
  1. modify the `composer.json` file of your application and specify which extensions (Composer packages) you want to install.
  2. run `composer install` to install the specified extensions.
  
  Note that you may need to install [Composer](https://getcomposer.org/) if you do not have it.
  
  By default, Composer installs packages registered on [Packagist](https://packagist.org/) - the biggest repository
  for open source Composer packages. You can look for extensions on Packagist. You may also
  [create your own repository](https://getcomposer.org/doc/05-repositories.md#repository) and configure Composer
  to use it. This is useful if you are developing closed open extensions and want to share within your projects.
  
  Extensions installed by Composer are stored in the `BasePath/vendor` directory, where `BasePath` refers to the
  application's [base path](structure-applications.md#basePath).  Because Composer is a dependency manager, when
  it installs a package, it will also install all its dependent packages.
  
  For example, to install the `yiisoft/yii2-imagine` extension, modify your `composer.json` like the following:
  
  ```json
  {
      // ...
  
      "require": {
          // ... other dependencies
  
          "yiisoft/yii2-imagine": "*"
      }
  }
  ```
  
  After the installation, you should see the directory `yiisoft/yii2-imagine` under `BasePath/vendor`. You should
  also see another directory `imagine/imagine` which contains the installed dependent package.
  
  > Info: The `yiisoft/yii2-imagine` is a core extension developed and maintained by the Yii developer team. All
    core extensions are hosted on [Packagist](https://packagist.org/) and named like `yiisoft/yii2-xyz`, where `xyz`
    varies for different extensions.
  
  Now you can use the installed extensions like they are part of your application. The following example shows
  how you can use the `yii\imagine\Image` class provided by the `yiisoft/yii2-imagine` extension:
  
  ```php
  use Yii;
  use yii\imagine\Image;
  
  // generate a thumbnail image
  Image::thumbnail('@webroot/img/test-image.jpg', 120, 120)
      ->save(Yii::getAlias('@runtime/thumb-test-image.jpg'), ['quality' => 50]);
  ```
  
  > Info: Extension classes are autoloaded by the [Yii class autoloader](concept-autoloading.md).
  
  
  ### Installing Extensions Manually <a name="installing-extensions-manually"></a>
  
  In some rare occasions, you may want to install some or all extensions manually, rather than relying on Composer.
  To do so, you should
  
  1. download the extension archive files and unpack them in the `vendor` directory.
  2. install the class autoloaders provided by the extensions, if any.
  3. download and install all dependent extensions as instructed.
  
  If an extension does not have a class autoloader but follows the [PSR-4 standard](http://www.php-fig.org/psr/psr-4/),
  you may use the class autoloader provided by Yii to autoload the extension classes. All you need to do is just to
  declare a [root alias](concept-aliases.md#defining-aliases) for the extension root directory. For example,
  assuming you have installed an extension in the directory `vendor/mycompany/myext`, and the extension classes
  are under the `myext` namespace, then you can include the following code in your application configuration:
  
  ```php
  [
      'aliases' => [
          '@myext' => '@vendor/mycompany/myext',
      ],
  ]
  ```
  
  
  ## Creating Extensions <a name="creating-extensions"></a>
  
  You may consider creating an extension when you feel the need to share with other people your great code.
  An extension can contain any code you like, such as a helper class, a widget, a module, etc.
  
  It is recommended that you create an extension in terms of a [Composer package](https://getcomposer.org/) so that
  it can be more easily installed and used by other users, liked described in the last subsection.
  
  Below are the basic steps you may follow to create an extension as a Composer package.
  
  1. Create a project for your extension and host it on a VCS repository, such as [github.com](https://github.com).
     The development and maintenance work about the extension should be done on this repository.
  2. Under the root directory of the project, create a file named `composer.json` as required by Composer. Please
     refer to the next subsection for more details.
  3. Register your extension with a Composer repository, such as [Packagist](https://packagist.org/), so that
     other users can find and install your extension using Composer.
  
  
  ### `composer.json` <a name="composer-json"></a>
  
  Each Composer package must have a `composer.json` file in its root directory. The file contains the metadata about
  the package. You may find complete specification about this file in the [Composer Manual](https://getcomposer.org/doc/01-basic-usage.md#composer-json-project-setup).
  The following example shows the `composer.json` file for the `yiisoft/yii2-imagine` extension:
  
  ```json
  {
      // package name
      "name": "yiisoft/yii2-imagine",
  
      // package type
      "type": "yii2-extension",
  
      "description": "The Imagine integration for the Yii framework",
      "keywords": ["yii2", "imagine", "image", "helper"],
      "license": "BSD-3-Clause",
      "support": {
          "issues": "https://github.com/yiisoft/yii2/issues?labels=ext%3Aimagine",
          "forum": "http://www.yiiframework.com/forum/",
          "wiki": "http://www.yiiframework.com/wiki/",
          "irc": "irc://irc.freenode.net/yii",
          "source": "https://github.com/yiisoft/yii2"
      },
      "authors": [
          {
              "name": "Antonio Ramirez",
              "email": "amigo.cobos@gmail.com"
          }
      ],
  
      // package dependencies
      "require": {
          "yiisoft/yii2": "*",
          "imagine/imagine": "v0.5.0"
      },
  
      // class autoloading specs
      "autoload": {
          "psr-4": {
              "yii\\imagine\\": ""
          }
      }
  }
  ```
  
  
  #### Package Name <a name="package-name"></a>
  
  Each Composer package should have a package name which uniquely identifies the package among all others.
  The format of package names is `vendorName/projectName`. For example, in the package name `yiisoft/yii2-imagine`,
  the vendor name and the project name are `yiisoft` and `yii2-imagine`, respectively.
  
  Do NOT use `yiisoft` as vendor name as it is reserved for use by the Yii core code.
  
  We recommend you prefix `yii2-` to the project name for packages representing Yii 2 extensions, for example,
  `myname/yii2-mywidget`. This will allow users to more easily tell whether a package is a Yii 2 extension.
  
  
  #### Package Type <a name="package-type"></a>
  
  It is important that you specify the package type of your extension as `yii2-extension` so that the package can
  be recognized as a Yii extension when being installed.
  
  When a user runs `composer install` to install an extension, the file `vendor/yiisoft/extensions.php`
  will be automatically updated to include the information about the new extension. From this file, Yii applications
  can know which extensions are installed (the information can be accessed via [[yii\base\Application::extensions]].
  
  
  #### Dependencies <a name="dependencies"></a>
  
  Your extension depends on Yii (of course). So you should list it (`yiisoft/yii2`) in the `require` entry in `composer.json`.
  If your extension also depends on other extensions or third-party libraries, you should list them as well.
  Make sure you also list appropriate version constraints (e.g. `1.*`, `@stable`) for each dependent package. Use stable
  dependencies when your extension is released in a stable version.
  
  Most JavaScript/CSS packages are managed using [Bower](http://bower.io/) and/or [NPM](https://www.npmjs.org/),
  instead of Composer. Yii uses the [Composer asset plugin](https://github.com/francoispluchino/composer-asset-plugin)
  to enable managing these kinds of packages through Composer. If your extension depends on a Bower package, you can
  simply list the dependency in `composer.json` like the following:
  
  ```json
  {
      // package dependencies
      "require": {
          "bower-asset/jquery": ">=1.11.*"
      }
  }
  ```
  
  The above code states that the extension depends on the `jquery` Bower package. In general, you can use
  `bower-asset/PackageName` to refer to a Bower package in `composer.json`, and use `npm-asset/PackageName`
  to refer to a NPM package. When Composer installs a Bower or NPM package, by default the package content will be
  installed under the `@vendor/bower/PackageName` and `@vendor/npm/Packages` directories, respectively.
  These two directories can also be referred to using the shorter aliases `@bower/PackageName` and `@npm/PackageName`.
  
  For more details about asset management, please refer to the [Assets](structure-assets.md#bower-npm-assets) section.
  
  
  #### Class Autoloading <a name="class-autoloading"></a>
  
  In order for your classes to be autoloaded by the Yii class autoloader or the Composer class autoloader,
  you should specify the `autoload` entry in the `composer.json` file, like shown below:
  
  ```json
  {
      // ....
  
      "autoload": {
          "psr-4": {
              "yii\\imagine\\": ""
          }
      }
  }
  ```
  
  You may list one or multiple root namespaces and their corresponding file paths.
  
  When the extension is installed in an application, Yii will create for each listed root namespace
  an [alias](concept-aliases.md#extension-aliases) that refers to the directory corresponding to the namespace.
  For example, the above `autoload` declaration will correspond to an alias named `@yii/imagine`.
  
  
  ### Recommended Practices <a name="recommended-practices"></a>
  
  Because extensions are meant to be used by other people, you often need to take extra development effort. Below
  we introduce some common and recommended practices in creating high quality extensions.
  
  
  #### Namespaces <a name="namespaces"></a>
  
  To avoid name collisions and make the classes in your extension autoloadable, you should use namespaces and
  name the classes in your extension by following the [PSR-4 standard](http://www.php-fig.org/psr/psr-4/) or
  [PSR-0 standard](http://www.php-fig.org/psr/psr-0/).
  
  You class namespaces should start with `vendorName\extensionName`, where `extensionName` is similar to the project name
  in the package name except that it should not contain the `yii2-` prefix. For example, for the `yiisoft/yii2-imagine`
  extension, we use `yii\imagine` as the namespace its classes.
  
  Do not use `yii`, `yii2` or `yiisoft` as vendor name. These names are reserved for use by the Yii core code.
  
  
  #### Bootstrapping Classes <a name="bootstrapping-classes"></a>
  
  Sometimes, you may want your extension to execute some code during the [bootstrapping process](runtime-bootstrapping.md)
  stage of an application. For example, your extension may want to respond to the application's `beginRequest` event
  to adjust some environment settings. While you can instruct users of the extension to explicitly attach your event
  handler in the extension to the `beginRequest` event, a better way is to do this automatically.
  
  To achieve this goal, you can create a so-called *bootstrapping class* by implementing [[yii\base\BootstrapInterface]].
  For example,
  
  ```php
  namespace myname\mywidget;
  
  use yii\base\BootstrapInterface;
  use yii\base\Application;
  
  class MyBootstrapClass implements BootstrapInterface
  {
      public function bootstrap($app)
      {
          $app->on(Application::EVENT_BEFORE_REQUEST, function () {
               // do something here
          });
      }
  }
  ```
  
  You then list this class in the `composer.json` file of your extension like follows,
  
  ```json
  {
      // ...
  
      "extra": {
          "bootstrap": "myname\\mywidget\\MyBootstrapClass"
      }
  }
  ```
  
  When the extension is installed in an application, Yii will automatically instantiate the bootstrapping class
  and call its [[yii\base\BootstrapInterface::bootstrap()|bootstrap()]] method during the bootstrapping process for
  every request.
  
  
  #### Working with Databases <a name="working-with-databases"></a>
  
  Your extension may need to access databases. Do not assume that the applications that use your extension will always
  use `Yii::$db` as the DB connection. Instead, you should declare a `db` property for the classes that require DB access.
  The property will allow users of your extension to customize which DB connection they would like your extension to use.
  As an example, you may refer to the [[yii\caching\DbCache]] class and see how it declares and uses the `db` property.
  
  If your extension needs to create specific DB tables or make changes to DB schema, you should
  
  - provide [migrations](db-migrations.md) to manipulate DB schema, rather than using plain SQL files;
  - try to make the migrations applicable to different DBMS;
  - avoid using [Active Record](db-active-record.md) in the migrations.
  
  
  #### Using Assets <a name="using-assets"></a>
  
  If your extension is a widget or a module, chances are that it may require some [assets](structure-assets.md) to work.
  For example, a module may display some pages which contain images, JavaScript, and CSS. Because the files of an
  extension are all under the same directory which is not Web accessible when installed in an application, you have
  two choices to make the asset files directly accessible via Web:
  
  - ask users of the extension to manually copy the asset files to a specific Web-accessible folder;
  - declare an [asset bundle](structure-assets.md) and rely on the asset publishing mechanism to automatically
    copy the files listed in the asset bundle to a Web-accessible folder.
  
  We recommend you use the second approach so that your extension can be more easily used by other people.
  Please refer to the [Assets](structure-assets.md) section for more details about how to work with assets in general.
  
  
  #### Internationalization and Localization <a name="i18n-l10n"></a>
  
  Your extension may be used by applications supporting different languages! Therefore, if your extension displays
  content to end users, you should try to [internationalize and localize](tutorial-i18n.md) it. In particular,
  
  - If the extension displays messages intended for end users, the messages should be wrapped into `Yii::t()`
    so that they can be translated. Messages meant for developers (such as internal exception messages) do not need
    to be translated.
  - If the extension displays numbers, dates, etc., they should be formatted using [[yii\i18n\Formatter]] with
    appropriate formatting rules.
  
  For more details, please refer to the [Internationalization](tutorial-i18n.md) section.
  
  
  #### Testing <a name="testing"></a>
  
  You want your extension to run flawlessly without bringing problems to other people. To reach this goal, you should
  test your extension before releasing it to public.
  
  It is recommended that you create various test cases to cover your extension code rather than relying on manual tests.
  Each time before you release a new version of your extension, you may simply run these test cases to make sure
  everything is in good shape. Yii provides testing support, which can help you to more easily write unit tests,
  acceptance tests and functionality tests. For more details, please refer to the [Testing](test-overview.md) section.
  
  
  #### Versioning <a name="versioning"></a>
  
  You should give each release of your extension a version number (e.g. `1.0.1`). We recommend you follow the
  [semantic versioning](http://semver.org) practice when determining what version numbers should be used.
  
  
  #### Releasing <a name="releasing"></a>
  
  To let other people know your extension, you need to release it to public.
  
  If it is the first time you release an extension, you should register it on a Composer repository, such as
  [Packagist](https://packagist.org/). After that, all you need to do is simply creating a release tag (e.g. `v1.0.1`)
  on the VCS repository of your extension and notify the Composer repository about the new release. People will
  then be able to find the new release, and install or update the extension through the Composer repository.
  
  In the releases of your extension, besides code files you should also consider including the followings to
  help other people learn about and use your extension:
  
  * A readme file in the package root directory: it describes what your extension does and how to install and use it.
    We recommend you write it in [Markdown](http://daringfireball.net/projects/markdown/) format and name the file
    as `readme.md`.
  * A changelog file in the package root directory: it lists what changes are made in each release. The file
    may be written in Markdown format and named as `changelog.md`.
  * An upgrade file in the package root directory: it gives the instructions on how to upgrade from older releases
    of the extension. The file may be written in Markdown format and named as `upgrade.md`.
  * Tutorials, demos, screenshots, etc.: these are needed if your extension provides many features that cannot be
    fully covered in the readme file.
  * API documentation: your code should be well documented to allow other people more easily read and understand it.
    You may refer to the [Object class file](https://github.com/yiisoft/yii2/blob/master/framework/base/Object.php)
    to learn how to document your code.
  
  > Info: Your code comments can be written in Markdown format. The `yiisoft/yii2-apidoc` extension provides a tool
    for you to generate pretty API documentation based on your code comments.
  
  > Info: While not a requirement, we suggest your extension adhere to certain coding styles. You may refer to
    the [core framework code style](https://github.com/yiisoft/yii2/wiki/Core-framework-code-style).
  
  
  ## Core Extensions <a name="core-extensions"></a>
  
  Yii provides the following core extensions that are developed and maintained by the Yii developer team. They are all
  registered on [Packagist](https://packagist.org/) and can be easily installed as described in the
  [Using Extensions](#using-extensions) subsection.
  
  - [yiisoft/yii2-apidoc](https://github.com/yiisoft/yii2-apidoc):
    provides an extensible and high-performance API documentation generator. It is also used to generate the core
    framework API documentation.
  - [yiisoft/yii2-authclient](https://github.com/yiisoft/yii2-authclient):
    provides a set of commonly used auth clients, such as Facebook OAuth2 client, GitHub OAuth2 client.
  - [yiisoft/yii2-bootstrap](https://github.com/yiisoft/yii2-bootstrap):
    provides a set of widgets that encapsulate the [Bootstrap](http://getbootstrap.com/) components and plugins.
  - [yiisoft/yii2-codeception](https://github.com/yiisoft/yii2-codeception):
    provides testing support based on [Codeception](http://codeception.com/).
  - [yiisoft/yii2-debug](https://github.com/yiisoft/yii2-debug):
    provides debugging support for Yii applications. When this extension is used, a debugger toolbar will appear
    at the bottom of every page. The extension also provides a set of standalone pages to display more detailed
    debug information.
  - [yiisoft/yii2-elasticsearch](https://github.com/yiisoft/yii2-elasticsearch):
    provides the support for using [Elasticsearch](http://www.elasticsearch.org/). It includes basic querying/search
    support and also implements the [Active Record](db-active-record.md) pattern that allows you to store active records
    in Elasticsearch.
  - [yiisoft/yii2-faker](https://github.com/yiisoft/yii2-faker):
    provides the support for using [Faker](https://github.com/fzaninotto/Faker) to generate fake data for you.
  - [yiisoft/yii2-gii](https://github.com/yiisoft/yii2-gii):
    provides a Web-based code generator that is highly extensible and can be used to quickly generate models,
    forms, modules, CRUD, etc.
  - [yiisoft/yii2-imagine](https://github.com/yiisoft/yii2-imagine):
    provides commonly used image manipulation functions based on [Imagine](http://imagine.readthedocs.org/).
  - [yiisoft/yii2-jui](https://github.com/yiisoft/yii2-jui):
    provides a set of widgets that encapsulate the [JQuery UI](http://jqueryui.com/) interactions and widgets.
  - [yiisoft/yii2-mongodb](https://github.com/yiisoft/yii2-mongodb):
    provides the support for using [MongoDB](http://www.mongodb.org/). It includes features such as basic query,
    Active Record, migrations, caching, code generation, etc.
  - [yiisoft/yii2-redis](https://github.com/yiisoft/yii2-redis):
    provides the support for using [redis](http://redis.io/). It includes features such as basic query,
    Active Record, caching, etc.
  - [yiisoft/yii2-smarty](https://github.com/yiisoft/yii2-smarty):
    provides a template engine based on [Smarty](http://www.smarty.net/).
  - [yiisoft/yii2-sphinx](https://github.com/yiisoft/yii2-sphinx):
    provides the support for using [Sphinx](http://sphinxsearch.com). It includes features such as basic query,
    Active Record, code generation, etc.
  - [yiisoft/yii2-swiftmailer](https://github.com/yiisoft/yii2-swiftmailer):
    provides email sending features based on [swiftmailer](http://swiftmailer.org/).
  - [yiisoft/yii2-twig](https://github.com/yiisoft/yii2-twig):
    provides a template engine based on [Twig](http://twig.sensiolabs.org/).
  