ActionController

use Psr\Http\Message\ResponseInterface;
use T3docs\BlogExample\Domain\Model\Blog;
use TYPO3\CMS\Extbase\Annotation\IgnoreValidation;
use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;

class BlogController extends ActionController
{
    /**
     * Displays a form for creating a new blog
     *
     * @IgnoreValidation("newBlog")
     */
    public function newAction(?Blog $newBlog = null): ResponseInterface
    {
        $this->view->assignMultiple([
            'newBlog' => $newBlog,
            'administrators' => $this->administratorRepository->findAll(),
        ]);
        return $this->htmlResponse();
    }
}

use Psr\Http\Message\ResponseInterface;
use T3docs\BlogExample\Domain\Model\Blog;
use T3docs\BlogExample\Exception\NoBlogAdminAccessException;
use TYPO3\CMS\Extbase\Annotation\Validate;

class BlogController extends AbstractController
{
    /**
     * Updates an existing blog
     *
     * $blog is a not yet persisted clone of the original blog containing
     * the modifications
     *
     * @Validate(param="blog", validator="T3docs\BlogExample\Domain\Validator\BlogValidator")
     * @throws NoBlogAdminAccessException
     */
    public function updateAction(Blog $blog): ResponseInterface
    {
        $this->checkBlogAdminAccess();
        $this->blogRepository->update($blog);
        $this->addFlashMessage('updated');
        return $this->redirect('index');
    }
}

class BackendController extends ActionController
{
    protected function initializeAction()
    {
        $this->pageUid = (int)($this->request->getQueryParams()['id'] ?? 0);
        parent::initializeAction();
    }
}

use Psr\Http\Message\ResponseInterface;
use T3docs\BlogExample\Domain\Model\Blog;
use TYPO3\CMS\Core\Pagination\SimplePagination;
use TYPO3\CMS\Extbase\Http\ForwardResponse;
use TYPO3\CMS\Extbase\Pagination\QueryResultPaginator;

class PostController extends \T3docs\BlogExample\Controller\AbstractController
{
    /**
     * Displays a list of posts. If $tag is set only posts matching this tag are shown
     */
    public function indexAction(
        ?Blog $blog = null,
        string $tag = '',
        int $currentPage = 1
    ): ResponseInterface {
        if ($blog == null) {
            return (new ForwardResponse('index'))
                ->withControllerName('Blog')
                ->withExtensionName('blog_example')
                ->withArguments(['currentPage' => $currentPage]);
        }
        $this->blogPageTitleProvider->setTitle($blog->getTitle());
        if (empty($tag)) {
            $posts = $this->postRepository->findByBlog($blog);
        } else {
            $tag = urldecode($tag);
            $posts = $this->postRepository->findByTagAndBlog($tag, $blog);
            $this->view->assign('tag', $tag);
        }
        $paginator = new QueryResultPaginator(
            $posts,
            $currentPage,
            (int)($this->settings['itemsPerPage'] ?? 3),
        );
        $pagination = new SimplePagination($paginator);
        $this->view->assignMultiple([
            'paginator' => $paginator,
            'pagination', $pagination,
            'pages' => range(1, $pagination->getLastPageNumber()),
            'blog' => $blog,
            'posts' => $posts,
        ]);
        return $this->htmlResponse();
    }
}

<?php
defined('TYPO3') or die();

use TYPO3\CMS\Extbase\Utility\ExtensionUtility;
use FriendsOfTYPO3\BlogExample\Controller\PostController;
use FriendsOfTYPO3\BlogExample\Controller\CommentController;

ExtensionUtility::configurePlugin(
   'BlogExample',
   'PostSingle',
   [PostController::class => 'show', CommentController::class => 'create'],
   [CommentController::class => 'create']
);

<?php

defined('TYPO3') or die();

use T3docs\BlogExample\Controller\BlogController;
use T3docs\BlogExample\Controller\CommentController;
use T3docs\BlogExample\Controller\PostController;
use TYPO3\CMS\Extbase\Utility\ExtensionUtility;

ExtensionUtility::configurePlugin(
    'BlogExample',
    'PostSingle',
    [
        PostController::class => 'show',
        CommentController::class => 'create',
        BlogController::class => 'index',
    ],
    [
        // Non-cached actions
        CommentController::class => 'create',
    ],
);

// ...

<?php

use Psr\Http\Message\ResponseInterface;
use TYPO3\CMS\Core\Http\PropagateResponseException;
use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;

final class MyController extends ActionController
{
    public function downloadAction(): ResponseInterface
    {
        // ... do something (set $filename, $filePath, ...)

        $response = $this->responseFactory->createResponse()
            // Must not be cached by a shared cache, such as a proxy server
            ->withHeader('Cache-Control', 'private')
            // Should be downloaded with the given filename
            ->withHeader('Content-Disposition', sprintf('attachment; filename="%s"', $filename))
            ->withHeader('Content-Length', (string)filesize($filePath))
            // It is a PDF file we provide as a download
            ->withHeader('Content-Type', 'application/pdf')
            ->withBody($this->streamFactory->createStreamFromFile($filePath));

        throw new PropagateResponseException($response, 200);
    }
}

<?php

use Psr\Http\Message\ResponseInterface;
use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;

final class MyController extends ActionController
{
    public function redirectAction(): ResponseInterface
    {
        // ... do something (set $value, ...)

        $uri = $this->uriBuilder->uriFor('show', ['parameter' => $value]);

        // $uri could also be https://example.com/any/uri
        // $this->resourceFactory is injected as part of the `ActionController` inheritance
        return $this->responseFactory->createResponse(307)
            ->withHeader('Location', $uri);
    }
}