Craft 3 Entry Field Type Validation

ValidateService.php

<?php
/**
 * Craft 3 Entry Validation
 * 
 * This goes in the main plugin class.
 * 
 * Event::on(
 *    Entry::class,
 *    Entry::EVENT_BEFORE_SAVE,
 *    function (ModelEvent $event) {
 *        if (!$event->isNew) { // prevent new entry/draft error
 *            PluginName::$plugin->validateService->validateEntry($event);
 *        }
 *    }
 * );
 */

namespace ns\plugin\services;

use Craft;
use craft\base\Component;
use craft\elements\Entry;

class ValidateService extends Component
{
    public function validateEntry($event)
    {
        $sender = $event->sender;
        $handle = $sender->section->handle;

        if ($handle == 'sessionName') {
            foreach ($sender->fieldName->all() as $entryField) {
                $entry = Entry::findOne($fieldName->id);

                if (empty($entry->textField)) { // text
                    $sender->addError('fieldName', Craft::t('pluginName', $entry->title . ' must have a value for field1.'));
                    return $event->isValid = false;
                }

                if (count($entry->assetField) == 0) { // asset
                    $sender->addError('fieldName', Craft::t('pluginName', $entry->title . ' must have at least one asset in field2.'));
                    return $event->isValid = false;
                }
            }
        }
    }
}