Restore split button for TYPO3 v9 forms

ContentButtonBarHook.php

<?php

namespace Vendor\Extension\Hook;

use TYPO3\CMS\Backend\Template\Components\ButtonBar;
use TYPO3\CMS\Backend\Template\Components\Buttons\InputButton;
use TYPO3\CMS\Backend\Template\Components\Buttons\LinkButton;
use TYPO3\CMS\Core\Http\ServerRequest;
use TYPO3\CMS\Core\Imaging\Icon;
use TYPO3\CMS\Core\Imaging\IconFactory;
use TYPO3\CMS\Core\Utility\GeneralUtility;

/**
 * This class contains a hook to add a "Save & Close" action to the toolbar.
 */
class ContentButtonBarHook
{
    /** @var ServerRequest */
    protected $request;

    /**
     * Creates an instance of this class.
     *
     * @param ServerRequest $request
     */
    public function __construct(?ServerRequest $request = null)
    {
        $this->request = $request ?? $GLOBALS['TYPO3_REQUEST'];
    }

    /**
     * Changes the button bar
     *
     * @param  array $parameters
     * @param ButtonBar $buttonBar
     * @return array
     */
    public function getButtons(array $parameters, ButtonBar $buttonBar): array
    {
        if ($this->getTableName() === 'tt_content') {
            $this->updateButtons($parameters['buttons'], $buttonBar);
        }

        return $parameters['buttons'];
    }

    /**
     * Updates buttons.
     *
     * @param array $buttonGroups
     * @param ButtonBar $buttonBar
     */
    protected function updateButtons(array &$buttonGroups, ButtonBar $buttonBar)
    {
        foreach ($buttonGroups as $sideName => $buttonGroupSide) {
            foreach ($buttonGroupSide as $posiition => $buttonGroup) {
                foreach ($buttonGroup as $key => $button) {
                    $type = $button->getType();
                    switch ($type) {
// Uncomment below if you want to have "icon-only" close & delete buttons like in v8
//                        case LinkButton::class:
//                            switch ($button->getClasses()) {
//                                case 't3js-editform-close':
//                                case 't3js-editform-delete-record':
//                                    $button->setShowLabelText(false);
//                                    break;
//                            }
//                            break;
                        case InputButton::class:
                            if ($button->getName() === '_savedok') {
                                $iconFactory = GeneralUtility::makeInstance(IconFactory::class);
                                $splitSaveButton = $buttonBar->makeSplitButton();
                                $splitSaveButton->addItem($button, true);
                                $splitSaveButton->addItem(
                                    $buttonBar->makeInputButton()
                                        ->setName('_savedoknew')
                                        ->setValue('1')
                                        ->setForm($button->getForm())
                                        ->setTitle($GLOBALS['LANG']->sL('LLL:EXT:core/Resources/Private/Language/locallang_core.xlf:rm.saveNewDoc'))
                                        ->setIcon(
                                            $iconFactory->getIcon(
                                                'actions-document-save-new',
                                                Icon::SIZE_SMALL
                                            )
                                        )
                                );
                                $splitSaveButton->addItem(
                                    $buttonBar->makeInputButton()
                                        ->setName('_savedokview')
                                        ->setValue('1')
                                        ->setForm($button->getForm())
                                        ->setTitle($GLOBALS['LANG']->sL('LLL:EXT:core/Resources/Private/Language/locallang_common.xlf:saveAndViewPage'))
                                        ->setIcon(
                                            $iconFactory->getIcon(
                                                'actions-document-save-close',
                                                Icon::SIZE_SMALL
                                            )
                                        )
                                );
                                $splitSaveButton->addItem(
                                    $buttonBar->makeInputButton()
                                        ->setName('_saveandclosedok')
                                        ->setValue('1')
                                        ->setForm($button->getForm())
                                        ->setTitle($GLOBALS['LANG']->sL('LLL:EXT:core/Resources/Private/Language/locallang_common.xlf:saveAndClose'))
                                        ->setIcon(
                                            $iconFactory->getIcon(
                                                'actions-document-save-close',
                                                Icon::SIZE_SMALL
                                            )
                                        )
                                );
                                $buttonGroups[$sideName][$posiition][$key] = $splitSaveButton;
                            }
                            break;
                    }
                }
            }
        }
    }

    /**
     * Fetches table name from the request.
     *
     * @return string|null
     */
    protected function getTableName(): ?string
    {
        $table = array_key_first($this->request->getQueryParams()['edit'] ?? []);
        if (!is_string($table) || empty($table)) {
            return null;
        }

        return $table;
    }
}

ext_localconf.php

<?php

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['Backend\Template\Components\ButtonBar']['getButtonsHook'][1597666436]
        = \Vendor\Extension\Hook\ContentButtonBarHook::class . '->getButtons';