How to add custom meta box when you have a custom page template file

Question

I've been trying to add a custom meta box in the Edit Page panel for a page using a specific template, in this case front-page.php. I have attempted to use both get_page_template_slug( $post_id ) and get_post_meta( $post_id, '_wp_page_template', true ). But it seems to me that both of these methods merely look for metadata in the database.

Which is a problem, because that's not how my custom template is registered. I've built a custom theme, and I have a page set as the "Front Page", and then a front-page.php file that Wordpress is automatically assigning to said front page. As far as I can tell, there's nothing in the database that says the front page post uses the front-page.php template, which is why the above methods don't work.

So, my question is this: if I have a custom theme with custom page templates that are assigned solely by their filenames, how would I detect those templates so I can add custom meta boxes to their respective Edit Panels?

Answer 1

The front page configuration is a site option. You can know if the site is configured to display a page with get_option( 'show_on_front' ) and get the ID of the page with get_option( 'page_on_front' ).

You could do something like this:

add_action( 'add_meta_boxes', 'cyb_add_metaboxes', 10, 2 ); 
function cyb_add_metaboxes( $post_type, $post ) {

   if( $post->ID == get_option( 'page_on_front' ) ) {

      add_meta_box('front-page-metabox', 'Front page metabox', 'cyb_front_page_metabox', 'page' );

   }

}

// Metabox callback
function cyb_front_page_metabox() {
}

But with this method, the page needs to be created and assigned to front page before the metabox is rendered. I don't know what kind of data you want to store in the meta fields for the front page, but I think you should explore the customize API, specially if you want to use the metabox for site's front page configuration or site's front page presentational options.