[hugopeek]

This revives the old 'composer' flag from @Jako . If set to true, the vendor folder will be moved temporarily and restored after the transport package is created.

[Jako]

In that case, a composer.phar file has to be added in the core/components/<namespace> folder and the following script can be added the gpm.json in the build -> scriptsAfter section:

"build": {
    "scriptsAfter": [
        "script.composer.php"
    ]
}

script.composer.php

<?php
/**
 * Resolve Composer dependencies
 *
 * @package gpm
 * @subpackage build
 *
 * @var array $options
 * @var xPDOTransport $transport
 */

use Composer\Console\HtmlOutputFormatter;
use Composer\Factory;
use Composer\Installer;
use Composer\IO\BufferIO;
use Symfony\Component\Console\Output\StreamOutput;
use xPDO\Transport\xPDOTransport;

$success = true;
if ($transport->xpdo) {
    switch ($options[xPDOTransport::PACKAGE_ACTION]) {
        case xPDOTransport::ACTION_INSTALL:
        case xPDOTransport::ACTION_UPGRADE:
            $transport->xpdo->log(xPDO::LOG_LEVEL_INFO, 'Installing/updating dependencies, this may take some time...');

            $path = MODX_CORE_PATH . "components/{$transport->name}/";

            // Set Composer environment variables
            putenv("COMPOSER={$path}composer.json");
            putenv("COMPOSER_HOME={$path}.composer");
            putenv("COMPOSER_VENDOR_DIR={$path}vendor/");

            // Change the path to the package namespace folder, to prevent autoloading other namespaces
            chdir($path);

            require "phar://{$path}composer.phar/vendor/autoload.php";

            // Run Composer without proc_open/exec
            $io = new BufferIO('', StreamOutput::VERBOSITY_NORMAL, new HtmlOutputFormatter());
            $composer = Factory::create($io);
            $install = Installer::create($io, $composer);
            $install
                ->setPreferDist(true)
                ->setDevMode(false)
                ->setOptimizeAutoloader(true)
                ->setUpdate(false)
                ->setPreferStable(true);

            try {
                $install->run();
            } catch (Exception $e) {
                $transport->xpdo->log(xPDO::LOG_LEVEL_ERROR, get_class($e) . ' installing dependencies: ' . $e->getMessage());
                echo get_class($e) . ': ' . $e->getMessage() . "\n";
                $success = false;
            }

            $output = $io->getOutput();
            $output = nl2br(trim($output));
            $transport->xpdo->log(xPDO::LOG_LEVEL_INFO, $output);
            break;
        case xPDOTransport::ACTION_UNINSTALL:
            break;
    }
}
return $success;