Custom code for Message stack and Swiftmailer to create messages about new/changed nodes and comments and email them to subscribers.

The following modules are used:

• Message
• Message Notify
• Message Subscribe
• Message UI
• Mailsystem
• Swiftmailer
• Diff

The logic used for subscribing users to content and sending them emails includes the following. See message_integration.module and the message templates for more details.

• All active users will automatically be sent all new content.
• When a node is first published, the code will render the message view mode of the node in an HTML email and send it to the subscriber list.
• When a new revision is created in a published node, the code will use Diff module to just display what changed and send the diff in an HTML email to the subscriber list.
• When new comments are added, the code will notify the users subscribed to the node. The email will display the compact view mode of the content and the default view mode of the new comment an HTML email.

This code is too opinionated to use as a normal contributed module. But you you can fork it, adjust it to your own needs, and use it elsewhere.

To use this on your own site, fork this code, then add the requirements and patches in composer.json to your primary composer.json.

    "repositories": {
        "drupal": {
            "type": "composer",
            "url": "https://packages.drupal.org/8"
        },
        "forked-repo/message_integration": {
            "type": "vcs",
            "url": "https://github.com/forked-repo/message_integration.git",
            "no-api": true
        }
    },

• Make sure all the required code (with patches) is available by running composer require forked-repo/message_integration.
• Enable this module, which will install all the other required modules: drush en message_integration
• Lots of tweaky little steps can't be done until after everything is installed. Do them by running drush message_integration:configure after enabling the modules.
• Enable the message view mode to control the display that will be rendered in messages.
• Subscribe to some content.
• Create/edit content and add comments to it.
• Run cron to trigger queued messages and emails.
• Navigate to admin/content/messages to see the generated messages.
• Navigate to admin/structure/message to edit and change message templates.

Review the code in message_implementation.module to see what hooks are being used to generate messages. You can alter them as needed. You'll also want to edit the list of libraries used to generate HTML email to add your own theme libraries.