Add a Fluid view helper and write a functional test for it

[cumuru]

Story

As an integrator, I want a view helper that eases my templates or adds some cool new functionality, because the developer wants to add their test for it ;)

As a developer, I want to have an example for a good view helper test so that I can copy from it for my own extension when I create custom view helpers there.

Acceptance criteria

• New custom view helper created and used somewhere in the Fluid templates
• Functional test that asserts the view helper‘s output
• Test method with #[DataProvider] attribute to show how to test the VH for various input variants

Additional information

This is the code for the functional test I came up with in my own project, maybe it helps. Putting it here so it doesn‘t get lost:

        $template = '{namespace tea=TTN\\Tea\\ViewHelpers}<tea:my.viewHelper attribute="{attribute}">{content}</tea:my.viewHelper>';
        /** @var FluidViewAdapter $view */
        $view = GeneralUtility::makeInstance(ViewFactoryInterface::class)
            ->create(new ViewFactoryData());
        $view->getRenderingContext()->getTemplatePaths()->setTemplateSource($template);
        $view->assignMultiple([
            'attribute' => $attribute,
            'content' => $content,
        ]);
        $result = $view->render();
        $this->assertSame('Expected result', $result);

More code examples (still V11-compatible, so might need some adaption):

• https://github.com/oliverklee-de/oelib/blob/main/Tests/Functional/ViewHelpers/IsFieldEnabledViewHelperTest.php
• https://github.com/oliverklee-de/seminars/blob/main/Tests/Functional/ViewHelpers/SalutationAwareTranslateViewHelperTest.php