composer/doc/faqs/triggers.md

Triggers

What is a trigger?

A trigger is an event that runs a script in a static method, defined by a project. This event is raised before and after each action (install, update).

Where are the event types defined?

It is in the constant property in Composer\Trigger\TriggerEvents class.

How is it defined?

It is defined by adding the triggers key in the extra key to a project's composer.json or package's composer.json.

It is specified as an array of classes with her static method, in associative array define the event's type.

The PSR-0 must be defined, otherwise the trigger will not be triggered.

For any given project:

{
    "extra": {
        "triggers": {
            "post_install": [
                "MyVendor\\MyRootPackage\\MyClass::myStaticMethod"
            ],
            "post_update": [
                "MyVendor\\MyRootPackage\\MyClass::myStaticMethod2"
            ]
        }
    },
    "autoload": {
        "psr-0": {
            "MyVendor\\MyRootPackage": "my/folder/path/that/contains/triggers/from/the/root/project"
        }
    }
}

Trigger Example:

<?php
namespace MyVendor\MyRootPackage;

use Composer\Trigger\TriggerEvent;

class MyClass
{
    public static function myStaticMethod(TriggerEvent $event)
    {
        // code...
    }
    
    public static function myStaticMethod2(TriggerEvent $event)
    {
        // code...
    }
}

Informations:

A declared trigger with non existent file or without registered namespace will be ignored.