This is the perfect example of "Just because you can do something doesn't mean you should". There was quite a lot of hacking to get this to work. I'm hoping that Active Admin 4 will remove the need for most of this hackery.

Loading HTMX

First we need to load in the htmx js file

# /config/initializers/active_admin.rb
ActiveAdmin.setup do |config|
  config.register_javascript '/vendor/htmx/1.9.9.js'
end

Then we need to add hx-boost to the body tag

# /config/initializers/active_admin.rb
module ActiveAdmin
  module Views
    module Pages
      class Base
        alias_method :original_build_page, :build_page
        def build_page
          within body(class: body_classes, "hx-boost" => "true") do
            original_build_page # Ensures that the rest of the page is built as usual
          end
        end
      end
    end
  end
end

Fixing issues

All htmx requests are sent with an Accept header of */*. Active Admin doesn't allow this, so we need to override the restrict_format_access! method.

First create a controller concern

# app/controllers/concerns/admin/common_admin_functions.rb
module Admin
  module HtmxEnable
    extend ActiveSupport::Concern

    included do
      def restrict_format_access!
        unless request.format.html? || request.format.to_s == '*/*'
          presenter = active_admin_config.get_page_presenter(:index)
          download_formats = (presenter || {}).fetch(:download_links, active_admin_config.namespace.download_links)

          unless build_download_formats(download_formats).include?(request.format.symbol)
            raise ActiveAdmin::AccessDenied.new(current_active_admin_user, :index)
          end
        end
      end
    end
  end
end

Once the concern is created, we need to tell Active Admin to use it.

# /config/initializers/active_admin.rb
Rails.application.config.to_prepare do
  ActiveAdmin::ResourceController.class_eval do
    include Admin::HtmxEnable
  end
end

Loading Javascript

After htmx has loaded in new content, we need to apply behaviours to it again. But active admin doesn't have a single function that wraps all of it's javascript. So instead we have to reload the entire active_admin.js file 😆.

// app/assets/javascript/active_admin.js

document.addEventListener('DOMContentLoaded', function() {
  document.body.addEventListener('htmx:afterSwap', function(event) {
    const script = document.createElement('script');
    script.id = 'application-js';
    script.src = '/assets/active_admin.js?' + new Date().getTime(); // Cache-busting
    document.head.appendChild(script);
  });
});

Issues

• Hx-boost uses inner-swap, which means that the body classes don't change. Active admin form styling depends on the body class. So if you load up an edit page, click on a side menu link to an index page, the body still has the edit class and the index page is missing CSS.

• Doesn't play well with Arctic Admin's javascript - because Arctic Admin uses functions to identify dom elements, the same behaviour gets added multiple times - this breaks the filter panel on the right, causing repeated animations.