Working with View Models

We like Magento view models a lot! In comparison to custom block classes, they are more reusable and composable. You can add an arbitrary number of view models to any template block via layout XML.

But after introducing a bunch of useful view models in Hyvä we got tired of all that seemingly unnecessary XML and thought of a simpler solution:

The View Model Registry

In a Hyvä theme, in every template, a variable $viewModels is automatically available, introduced by hyva-themes/magento2-theme-module, similar to the existing $block and $escaper variables. You can use it to fetch any view model (i.e. any class that implements ArgumentInterface).

Example:

/** @var \Hyva\Theme\Model\ViewModelRegistry $viewModels */
$currentProduct = $viewModels->require(\Hyva\Theme\ViewModel\CurrentProduct::class);

Tip

It is no longer needed to declare view models in XML!

You can read more about the usage of ViewModels versus Blocks here: Firegento - Better Blocks: Magento 2 PHP View Models.

View Model Cache Tags

View models are often used to provide data to be rendered in a template.

In case you need the cache tags from some entities to be included in the HTTP response for the FPC cache record, you can implement the Magento\Framework\DataObject\IdentityInterface and return the required cache tags.
They will be included in the HTTP response X-Magento-Tags header automatically.

For more information have a look at the in depth documentation.

View Model cache tags in ESI cached blocks

If a block has a ttl="..." attribute in layout XML, it will be cached by Varnish and included as an ESI section.
If the view model also implements the IdentityInterface to provide cache tags to the full page cache, this leads to the situation where the cache tags are added to both the full page cache record and also the ESI cache record.

However, the desired behavior is that the cache tags are only added to the ESI cache record.

To allow the view model registry to detect if the view model is rendered in a template that as part of an ESI section, the $block variable needs to be passed to the $viewModelRegistry->require() method as a second argument.

/** @var \Hyva\Theme\Model\ViewModelRegistry $viewModels */
$currentProduct = $viewModels->require(\Hyva\Theme\ViewModel\CurrentProduct::class, $block);

Only pass $block when required

The $block argument only needs to be specified for view models with cache tags that are used in templates rendered in an ESI section.

In the default Hyvä theme, this is only true for the top menu block templates Magento_Theme/templates/html/header/menu/desktop.phtml and Magento_Theme/templates/html/header/menu/mobile.phtml.

Do not all the $block argument everywhere just in case.