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Written documentation on Custom Installers

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# Custom installers
## Synopsis
At times it may be necessary for a package to require additional actions during installation, such as installing packages outside of the default `vendor` library.
In these cases you could consider creating a Custom Installer to handle your specific logic.
## Calling a Custom Installer
Suppose that your project already has a Custom Installer for specific modules then invoking that installer is a matter of defining the correct [type][1] in your package file.
> _See the next chapter for an instruction how to create Custom Installers._
Every Custom Installer defines which [type][1] string it will recognize. Once recognized it will completely override the default installer and only apply its own logic.
An example use-case would be:
> phpDocumentor features Templates that need to be installed outside of the default /vendor folder structure. As such they have chosen to adopt the `phpdocumentor-template` [type][1] and create a Custom Installer to send these templates to the correct folder.
An example composer.json of such a template would be:
{
"name": "phpdocumentor/template.responsive",
"type": "phpdocumentor-template",
}
With the package definition shown in the example will any project that includes this package try to find an appropriate installer and use that to execute this [type][1].
> **IMPORTANT**: the host project will not recognize the [type][1] if the installer's package is not included. Thus a composer.json consuming a template package will always need to _require_ the template's installer as well.
## Creating an Installer
A Custom Installer is a defined as a class that implements the [\Composer\Installer\InstallerInterface][3] and is contained in a Composer package that has the [type][1] `composer-installer`.
A basic Installer would thus compose of two files:
1. the package file: composer.json
2. The Installer class, i.e.: \Composer\Installer\MyInstaller.php
> **NOTE**: _The namespace does not need to be \Composer\Installer, it may be anything that you would want._
### composer.json
The package file is the same as any other package file but with the following requirements:
1. the [type][1] attribute must be `composer-installer`
2. the [extra][2] attribute must contain an element `class` defining the class name of the installer (including namespace)
Example:
{
"name": "phpdocumentor/template-installer",
"type": "composer-installer",
"license": "MIT",
"autoload": {
"psr-0": {"phpDocumentor\\Composer": "src/"}
},
"extra": {"class": "\\phpDocumentor\\Composer\\TemplateInstaller"}
}
### The Custom Installer class
The class that executes the custom installation should implement the [\Composer\Installer\InstallerInterface][3] (or extend another installer that implements that interface).
The class may be placed in any location and have any name, as long as it matches the `extra.class` element in the package definition.
It will also define the [type][1] string as it will be recognized by packages that will use this installer in the `supports()` method.
> **NOTE**: _choose your [type][1] name carefully, it is recommended to follow the format: `vendor-type`_. For example: `phpdocumentor-template`
The InstallerInterface class defines the following methods (please see the source for the exact signature):
* **supports()**, here you test whether the passed [type][1] matches the name that you declared for this installer (see the example).
* **isInstalled()**, determines whether a supported package is installed or not.
* **install()**, here you can determine the actions that need to be executed upon installation.
* **update()**, here you define the behavior that is required when Composer is invoked with the update argument.
* **uninstall()**, here you can determine the actions that need to be executed when the package needs to be removed.
* **getInstallPath()**, this method should return the location where the package is to be installed, _relative from the location of composer.json._
Example:
namespace phpDocumentor\Composer;
use Composer\Package\PackageInterface;
class TemplateInstaller extends \Composer\Installer\LibraryInstaller
{
/**
* {@inheritDoc}
*/
public function getInstallPath(PackageInterface $package)
{
if (substr($package->getPrettyName(), 0, 23) != 'phpdocumentor/template.') {
throw new \InvalidArgumentException(
'Unable to install template, phpdocumentor templates should always start their package name with "phpdocumentor/template."'
);
}
return 'data/templates/'.substr($package->getPrettyName(), 23);
}
/**
* {@inheritDoc}
*/
public function supports($packageType)
{
return (bool)('phpdocumentor-template' === $packageType);
}
}
The example demonstrates that it is quite simple to extend the [\Composer\Installer\LibraryInstaller][4] class to strip a prefix (`phpdocumentor/template.`) and use the remaining part to assemble a completely different installation path.
> _Instead of installing to `/vendor` will any package installed using this Installer be put in the `/data/templates/<stripped name>` folder._
[1]: http://getcomposer.org/doc/04-schema.md#type
[2]: http://getcomposer.org/doc/04-schema.md#extra
[3]: https://github.com/composer/composer/blob/master/src/Composer/Installer/InstallerInterface.php
[4]: https://github.com/composer/composer/blob/master/src/Composer/Installer/LibraryInstaller.php