Magento 2: Adding Frontend Asset Files Programmatically

These methods aren’t, as of 2.01, marked as @api safe so they may go away. That said, they’re pretty integral to how the layout works so I feel like they have a good shot of sticking around.

If you want to add a frontend asset to your page layout programmatically and not render the <link/>/<script/> tags yourself, you’ll want to use the asset repository and the asset grouped collection

MagentoFrameworkViewAssetRepository $assetRepository
MagentoFrameworkViewAssetGroupedCollection $assetCollection

Specifically, use the asset repository to create an asset

$identifier = 'css/somefile.css';
$asset      = $assetRepository->createAsset($identifier)    

and then add the asset to the grouped collection

$assetCollection->add($asset);

Unfortunately, you can’t use this technique from a block’s _prepareLayout statement. Magento 2 renders a page in the following method

#File: vendor/magento/framework/View/Result/Page.php 
protected function render(ResponseInterface $response)
{
    $this->pageConfig->publicBuild();
    if ($this->getPageLayout()) {
        $config = $this->getConfig();
        $this->addDefaultBodyClasses();
        $addBlock = $this->getLayout()->getBlock('head.additional'); // todo
        $requireJs = $this->getLayout()->getBlock('require.js');
        $this->assign([
            'requireJs' => $requireJs ? $requireJs->toHtml() : null,
            'headContent' => $this->pageConfigRenderer->renderHeadContent(),
            'headAdditional' => $addBlock ? $addBlock->toHtml() : null,
            'htmlAttributes' => $this->pageConfigRenderer->renderElementAttributes($config::ELEMENT_TYPE_HTML),
            'headAttributes' => $this->pageConfigRenderer->renderElementAttributes($config::ELEMENT_TYPE_HEAD),
            'bodyAttributes' => $this->pageConfigRenderer->renderElementAttributes($config::ELEMENT_TYPE_BODY),
            'loaderIcon' => $this->getViewFileUrl('images/loader-2.gif'),
        ]);

        $output = $this->getLayout()->getOutput();
        $this->assign('layoutContent', $output);
        $output = $this->renderPage();
        $this->translateInline->processResponseBody($output);
        $response->appendBody($output);
    } else {
        parent::render($response);
    }
    return $this;
}       

The frontend assets are rendered by the following method call

$this->pageConfigRenderer->renderHeadContent()    

However, block rendering (and _prepareLayout) doesn’t happen until this method call

$output = $this->getLayout()->getOutput();    

So any thoughts of creating blocks that carry their frontend assets with them is out the window.