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 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

To use an extension, you need to install it first. Most extensions are distributed as Composer 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 php composer.phar install to install the specified extensions.

You may need to install Composer if you do not have it. Composer is a dependency manager. This means when installing a package, it will install all its dependent packages automatically.

By default, Composer installs packages registered on Packagist - the biggest repository for open source Composer packages. You can look for extensions on Packagist. You may also create your own 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.

For example, to install the yiisoft/yii2-imagine extension, modify your composer.json like the following:

{
    // ...

    "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 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:

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.

Creating Extensions

An extension can contain any code you like, such as a helper class, a widget, a module, etc.

You may consider creating an extension when you feel the need to redistribute some of your great code so that they can be easily reused by other people or in your other projects.

It is recommended that you create an extension in terms of a Composer package so that it can be more easily installed and used by other users, liked described in the last subsection.

Basic Steps

Below are the basic steps you may follow to create an extension.

1. Create a project for your extension and host it on a VCS repository, such as github.com. 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 so that other users can find and install your extension. If you are creating an open source extension, you can register it with Packagist; If you are creating a private extension for internal use, you may register it with your own repository.

composer.json

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. You may also refer to the following example which is the composer.json file for the yiisoft/yii2-imagine extension:

{
    // 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\\": ""
        }
    }
}

Info: It is important that you specify the package type as yii2-extension so that the package can be recognized as a Yii extension when being installed.

Package Names

Each extension, when released as a Composer package, should have a package name which uniquely identifies itself among all other packages. 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 this is reserved for use by the Yii core code.

Also, to easily tell from package name whether a package is a Yii extension, we recommend you prefix yii2- to the project name.

Namespaces

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 or PSR-0 standard.

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.

Class Autoloading

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:

{
    // ....

    "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 an alias for each listed root namespace. The alias will refer to the directory corresponding to the root namespace. For example, the above autoload declaration will correspond to an alias named @yii/imagine.

Bootstrapping Classes

Sometimes, you may want your extension to execute some code during the bootstrapping process 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,

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,

{
    // ...

    "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()]] during the bootstrapping process for every request.

Installing Extensions Manually

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, 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 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:

[
    'aliases' => [
        '@myext' => '@vendor/mycompany/myext',
    ],
]

Core Extensions

Best Practices