Easier requirements for SilverStripe 4

Page.php

<?php

use SilverStripe\CMS\Controllers\ContentController;
use SilverStripe\CMS\Model\SiteTree;

/**
 * Class Page
 */
class Page extends SiteTree
{
    private static $db = [];

    private static $has_one = [];
}

/**
 * Class PageController
 */
class PageController extends ContentController
{
    private static $allowed_actions = [];

    protected function init()
    {
        parent::init();

        $css = [
            '/themes/default/bower_components/bootstrap/dist/css/bootstrap.min.css',
        ];

        $js = [
            '/themes/default/bower_components/jquery/dist/jquery.min.js',
            '/themes/default/bower_components/bootstrap/dist/js/bootstrap.min.js',
        ];

        $afterCss = [];
        $afterJs = [];

        foreach ($this->getPageRequirements() as $key => $requirements) {
            foreach ($requirements as $requirement) {
                if (is_array($requirement)) {
                    list($requirement, $position) = $requirement;

                    if ($position == 'after') {
                        $lib = 'after' . ucfirst($key);
                        ${$lib}[] = $requirement;
                        continue;
                    }
                }

                ${$key}[] = $requirement;
            }
        }

        $css[] = '/themes/default/css/main.min.css';
        $js[] = '/themes/default/js/main.min.js';
        
        $css = array_merge($css, $afterCss);
        $js = array_merge($js, $afterJs);

        \SilverStripe\View\Requirements::combine_files('common.min.css', $css);
        \SilverStripe\View\Requirements::combine_files('common.min.js', $js);
    }

    public function getPageRequirements()
    {
        return [
            'css' => [],
            'js'  => []
        ];
    }
}

// Then you can override getPageRequirements in all Page subclasses to inject requirements before or after
// your main.min.js or main.min.css
//
// public function getPageRequirements()
// {
//     return [
//         'css' => [
//              '/themes/default/js/form-validator.min.js',
//              ['/themes/default/js/pages/contact-us.min.js', 'after']
//          ],
//         'js'  => []
//     ];
// }