Htmx reusing existing URLs, avoid duplicating url handling

Problem/Motivation

When setting the Htmx value necessary we almost always need some URL to send the request to (using the get/post/put/delete methods). We also have the need to set the _wrapper_format query parameter on htmx requests.

It can be a pain to have to manage the urls, in some case we don't know what they are exactly until later in the process.

There are several approaches we can take:

Setting the request URL

Some of it is made easier by #3546105: Automatically manage _wrapper_format for HTMX requests and clean up ajax_page_state in urls. We're assuming this gets in.

1. on the backend create a duplicate URL object and use it in get/post/…
   • This can be tricky to find what is the url of the link/form being processed
   • Extra work on the backend to manage the URLs

   $url = Url::fromRoute('some.route');
   (new Htmx())
     ->get($url)
     ->applyTo($element);
   
   // hx-get="/some/url?_wrapper_format=drupal_htmx"
   

2. create a new syntax to use as get/post/… argument: attr(href)?_wrapper_format=drupal_htmx
   • non standards but implementation at the htmx:configRequest step is the way to go

   (new Htmx())
     ->get('attr(href)')
     ->applyTo($element);
   
   // hx-get="attr(href)?_wrapper_format=drupal_htmx"
   

3. use hx-boost
   • Recommanded way to go about reusing an existing url, but it push url needs to be set every time, so similar situation as 1
   • Need to clean up the push-url value to remove _wrapper_format and ajax_page_state informations from the URL
   • Need to be careful about more things

   (new Htmx())
     ->boost(TRUE)
     ->applyTo($element);
   

4. Change the behavior of an empty hx-get/post/… to behave like hx-boost for getting the url to send the request to
   • Not a standard behavior of htmx
   • Nothing extra needed on Drupal side

   (new Htmx())
     ->get() // behaves like attr(href) or hx-boost to et the URL
     ->applyTo($element);
   
   // hx-get="?_wrapper_format=drupal_htmx"
   

Adding the wrapper format when using hx-boost

When using an url from the backend it's not much of a problem. This becomes a problem when using hx-boost where we reuse the "normal" url. Without help we would need to do this for a working set of attributes:

$url = Url::fromRoute('some.route');
(new Htmx())
  ->boost(TRUE)
  // necessary to make sure we don't use the url with 
  // _wrapper_format and ajax_page_state query string
  ->pushUrl($url)
  ->vals(['_wrapper_format' => 'drupal_htmx'])
  // if this is a form with hx-params, _wrapper_format needs to be allowed
  ->params([...$params, '_wrapper_format'])
  ->applyTo($element);

Since we need to explicitly set a url, this is pointless.

1. automatically add the _wrapper_format parameter to all htmx request on the frontend
   • Problem we can't get a full page easily, would need a custom way to bypass the behavior
2. use hx-vals to add the _wrapper_format to all htmx requests (setting hx-vals on body for example)
   • problem when allowlisting with hx-params for forms, need to add _wrapper_format to the list otherwise it's ommited
   • Need to change MainContentSubscriber backend so that _wrapper_format in $_POST data is taken into account
3. Create and use a new data attribute that gets picked up by drupal and adds the wrapper format and ajax page state when necessary.
   • This can be tricky to find what is the url of the link/form being processed
   • Extra work on the backend to manage the URLs

   (new Htmx())
     ->boost(full_page: TRUE)
     ->applyTo($element);
   
   // "data-hx-drupal-full-page" attribute used in htmx-assets.js 
   

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet