Move container classes from preprocess to templates

template_preprocess_container adds one static class. No comments to change in either the preprocess or the twig template.

Thanks @Sutharsan! I think we need to create a variable in preprocess to use in the Twig template to set this class conditionally. Otherwise the form-wrapper class will be added unnecessarily.

Hmm, did I really overlook that :/
Not 100% sure about the name 'has_parent'. But it matches with FAPI #array_parents

+++ b/core/modules/system/templates/container.html.twig
@@ -9,10 +9,16 @@
+{% if has_parent %}
+  <div{{ attributes.addClass('form-wrapper') }}>{{ children }}</div>
+{% else %}
+  <div{{ attributes }}>{{ children }}</div>
+{% endif %}

The method we've preferred for setting conditional classes is a ternary operator in a set classes block at the top. So something like this:

has_parent ? 'form-wrapper'

You can look at the meta for an example of how this is done.

The advantage of this approach is that it isolates the variation to just the class name. Otherwise if somebody wants to change the HTML, they have to change it in two places.

Thanks for the patches!

#7 comment included.

However, .... this will fail some tests because <div{{ attributes.addClass() }}>results in <div class="">. This is because addClass assumes existing classes.

I created #2330731: Attribute::addClass() adds empty class to fix the empty class problem.

Great, this looks pretty good to go after that gets committed. The only thing is according to our guidelines in #2322163: [meta] Consensus Banana Phase 1, move CSS classes from preprocess to twig templates. since this is using a ternary it should be in a set tag, but we can discuss that as well.

For anyone following along at home, we are now blocked on #2336355: Refactor Attribute rendering so that class="" is not printed.

Good to go now.

Committed/pushed to 8.0.x, thanks!

How can i get views name (or any other views attributes) in container.html.twig
to set individual class for its wrapper? i'm using bootstrap theme.
Should i initially set a var in a theme_preprocess_page(), and then check its value in the template file?

No: i should use hook_preprocess_container()

/**
 * Implements hook_preprocess_container()
 */
function modulename_preprocess_container(&$vars) {
    // we add element_name to global element wrapper
    // use element_name in container.html.twig
    if (isset($vars['element']['#name'])) {
        $vars['element_name'] = $vars['element']['#name'];
    }
}