Initial code

.editorconfig

+# Drupal editor configuration normalization
+# @see http://editorconfig.org/
+
+# This is the top-most .editorconfig file; do not search in parent directories.
+root = true
+
+# All files.
+[*]
+end_of_line = LF
+indent_style = space
+indent_size = 2
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[{composer.json,composer.lock}]
+indent_size = 4

.gitignore

+vendor
+composer.lock

composer.json

+{
+    "name": "lakedrops/composer-dev-environment",
+    "description": "Composer Plugin for development environments",
+    "type": "composer-plugin",
+    "keywords": ["Development"],
+    "homepage": "https://gitlab.paragon-es.de/lakedrops/composer-dev-environment",
+    "license": "GPL-2.0+",
+    "authors": [
+        {
+            "name": "Jürgen Haas",
+            "email": "juergen@paragon-es.de",
+            "homepage": "https://www.paragon-es.de",
+            "role": "Drupal Expert"
+        },
+        {
+            "name": "Richard Papp",
+            "email": "richard.papp@boromino.com",
+            "homepage": "http://www.boromino.com",
+            "role": "Drupal Expert"
+        }
+    ],
+    "support": {
+        "issues": "https://gitlab.paragon-es.de/lakedrops/composer-dev-environment/issues",
+        "source": "https://gitlab.paragon-es.de/lakedrops/composer-dev-environment/tree/master"
+    },
+    "require": {
+        "composer-plugin-api": "^1.0.0",
+        "php": ">=5.4.5"
+    },
+    "autoload": {
+        "psr-4": {
+            "LakeDrops\\ComposerDevEnvironment\\": "src/"
+        }
+    },
+    "extra": {
+        "class": "LakeDrops\\ComposerDevEnvironment\\Plugin"
+    },
+    "require-dev": {
+        "composer/composer": "dev-master",
+        "phpunit/phpunit": "^4.4.0"
+    }
+}

src/Handler.php

+<?php
+
+/**
+ * @file
+ * Contains \LakeDrops\ComposerDevEnvironment\Handler.
+ */
+
+namespace LakeDrops\ComposerDevEnvironment;
+
+use Composer\Package\PackageInterface;
+use Composer\Composer;
+use Composer\IO\IOInterface;
+use Composer\Script\Event as ScriptEvent;
+
+class Handler {
+
+  /**
+   * @var \Composer\Composer
+   */
+  protected $composer;
+
+  /**
+   * @var \Composer\IO\IOInterface
+   */
+  protected $io;
+
+  /**
+   * @var PackageInterface
+   */
+  protected $drupalCorePackage;
+
+  /**
+   * Handler constructor.
+   *
+   * @param Composer $composer
+   * @param IOInterface $io
+   */
+  public function __construct(Composer $composer, IOInterface $io) {
+    $this->composer = $composer;
+    $this->io = $io;
+  }
+
+  /**
+   * Post project create event to execute the scaffolding.
+   *
+   * @param ScriptEvent $event
+   */
+  public function test($event) {
+    print('HALLO, here I am' . PHP_EOL);
+  }
+
+}

src/Plugin.php

+<?php
+
+/**
+ * @file
+ * Contains LakeDrops\ComposerDevEnvironment\Plugin.
+ */
+
+namespace LakeDrops\ComposerDevEnvironment;
+
+use Composer\Composer;
+use Composer\EventDispatcher\EventSubscriberInterface;
+use Composer\IO\IOInterface;
+use Composer\Plugin\PluginInterface;
+use Composer\Script\ScriptEvents;
+
+/**
+ * Composer plugin for handling drupal scaffold.
+ */
+class Plugin implements PluginInterface, EventSubscriberInterface {
+
+  /**
+   * @var \LakeDrops\ComposerDevEnvironment\Handler
+   */
+  protected $handler;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function activate(Composer $composer, IOInterface $io) {
+    $this->handler = new Handler($composer, $io);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    return array(
+      ScriptEvents::PRE_UPDATE_CMD => 'test',
+    );
+  }
+
+  /**
+   * Post create project event callback.
+   *
+   * @param \Composer\Script\Event $event
+   */
+  public function test($event) {
+    $this->handler->test($event);
+  }
+
+}