Which filter fires upon setting a featured image

Question

When I am creating/editing a post and set the featured image, I’d like to display a message to the screen if the file dimensions or size are below a certain threshold. But I want the featured image to still be set.

The message is simply a reminder to myself if I have set a featured image that doesn’t meet certain criteria. I need to do this BEFORE saving the post, ideally right after I set the featured image.

I couldn’t determine which add_filter I would use in my child functions.php to display a message at this point.

I found this example ( https://developer.wordpress.org/reference/hooks/admin_post_thumbnail_html/#user-contributed-notes ) but couldn't get it to work.

Do you have a recommendation?

UPDATE: It turns out that admin_post_thumbnail_html filter no longer works in WordPress > 5.0 : https://core.trac.wordpress.org/ticket/45688 And doing something comparable since 5.0 seems convoluted/requires javascript: https://github.com/danielbachhuber/gutenberg-migration-guide/blob/master/filter-admin-post-thumbnail-html.md

Answer 1

A nearly identical use case for your question is found in the WordPress documentation for the filter admin_post_thumbnail_html.

A filter hooked to this will receive the HTML for the admin screen as $content. Modify and return this markup as you wish.

function filter_featured_image_admin_text( $content, $post_id, $thumbnail_id ){
    $help_text = '<p>' . __( 'Please use an image that is 1170 pixels wide x 658 pixels tall.', 'my_domain' ) . '</p>';
return $help_text . $content;
}

add_filter( 'admin_post_thumbnail_html', 'filter_featured_image_admin_text', 10, 3 );

Full details here: https://developer.wordpress.org/reference/hooks/admin_post_thumbnail_html/