Jetpack Publicize custom message

jetpack-publicize-custom-message.php

<?php

/**
 * NOTE: the following is a rough example only that would need refinement:
 *   - `_wpas_mess` (wp_postmeta > post_id) will only be set when a custom Publicize message is manually entered.
 *   - It doesn't take into account the max characters for a custom Publicize message.
 */

/**
 * Customizes the Publicize message excerpt. In this example the post author is prefixed.
 *
 * The `publicize_save_meta` hook fires before a post is processed for Publicize.
 * Hook reference: https://developer.jetpack.com/hooks/publicize_save_meta/
 */
function custom_action_publicize_save_meta( $submit_post, $post_id, $service_name, $connection ) {
	$previous_message = get_post_meta( $post_id, '_wpas_mess', true );
	$author = get_user_by(
		'ID',
		get_post_field( 'post_author', $post_id )
	);

	if ( ! $author || ! $author->display_name ) {
		return;
	}

	$updated_message = "By: $author->display_name - " . $previous_message;
	update_post_meta( $post_id, '_wpas_mess', $updated_message );
}

/**
 * We only want to customize the Publicize message when a post is being published,
 * so we can hook into `transition_post_status` and check the post statuses.
 */
function maybe_add_custom_action_publicize_save_meta( $new_status, $old_status, $post ) {
	if ( 'publish' === $new_status && 'publish' !== $old_status && 'post' === $post->post_type ) {
		add_action( 'publicize_save_meta', 'custom_action_publicize_save_meta', 10, 4 );
	}
}
add_action( 'transition_post_status', 'maybe_add_custom_action_publicize_save_meta', 10, 3 );