Options framework example. Still work in progress and changes weekly.

controller.php

<?php
    public function setup_example_page()
    {
        $page = new Page('sf-example-page');
        $accordion = new Accordion();
        $page->add($accordion);
        do_action('sf-example-page', [$accordion]);
        $save  = new Row();
        $save->add(new NonceField('save-example-settings'));
        $save->add(new SaveButton());
        $save->add(new Row('notifications'));
        $page->add($save);
        return $page;
    }

fields.php

<?php
add_action('sf-example-page', 'examplePage' );
    public function examplePage(IElementGroup $page) {
        $section = new Section('General settings');
        $section->add(UIFactory::inputTextField('setting_key',
            'Setting label',
            'Setting description.'
        ));
        $page->add($section);
    }

settings-page.php

<div class="wrap so-fo">
    <h2>Example page</h2>
    <div id="sf-examples-wrap">
        <form action="<?php echo admin_url('admin.php?controller=settings&action=save')?>" method="post">
            <?php $page->render() ?>
        </form>
    </div>
</div>