How do I access post meta data when publishing a new post in Gutenberg?

I am updating one of my plugins for Gutenberg. I need to check for the presence of a custom post meta field when a post is updated or published, so I’ve hooked into publish_post:

function process_meta( $ID, $post ) {
    if (get_post_meta($ID, 'my_post_meta_field', true)) {
        // do something
    }
}
add_action('publish_post', 'process_meta', 10, 2);

The problem is that post meta fields have not yet been saved to the database when this hook (or any others that I’m aware of) fires, so unless a post draft has already been saved, my_post_meta_field always appears empty at this point in my code.

In the old version of my plugin, I would simply check the contents of the $_POST[] array. However, Gutenberg now saves content directly via the WordPress API, so there is no POST data to intercept.

How can I work around this problem?

The solution is to use a different hook which fires after post meta data has been saved by Gutenberg to the WP API. After reading this related discussion on Github and inspecting this code by n7studios, I discovered that rest_after_insert_post is a hook to use.

So, the way to reliably check for the existence of a WordPress Post Meta field when using Gutenberg:

function read_my_meta_field( $post, $request ) {
    $my_meta_field = get_post_meta($post->ID, 'my_post_meta_field', true);

    // Process $my_meta_field as desired...
}
add_action('rest_after_insert_post', 'read_my_meta_field', 10, 2);

Note that you can handle meta fields from other post types by simply changing post in the first parameter to the add_action() function, so instead of using 'rest_after_insert_post' you would use 'rest_after_insert_whatever'.

Finally, here is a more complete answer to my own question, which checks the meta field only on post publication.

function run_on_post_publication( $ID, $post ) {
    // Only register this hook if we really are publishing a new post
    add_action('rest_after_insert_post', 'read_my_postmeta_field', 10, 2);
}
add_action('publish_post', 'run_on_post_publication', 10, 2);

function read_my_postmeta_field( $post, $request ) {
    $my_meta_field = get_post_meta( $post->ID, 'my_post_meta_field', true);

    // Process $my_meta_field as desired...
}

For reference, if you want more control over when, exactly, your code runs, check out the ‘transition_post_status’ hook.