Filter post before *editing*

Question

I know there are a lot of hooks in WP for altering a posts content (and other fields) before the post is written to the DB upon saving; but what I am interested in doing is running my custom filter before the post is loaded for editing. So basically when my custom post type is loaded by /wp-admin/post.php?post=##&action=edit, I would like to run my filter on it before the content is displayed in the WYSIWYG editor. Is that possible or no?

Answer 1

You can use the the_editor_content filter:

function wpa_editor_content( $content ) {
    global $post;
    return "this post's id is $post->ID " . $content;
}
add_filter( 'the_editor_content', 'wpa_editor_content' );

Answer 2

This is quite an old question but I came across it while searching for some answers of my own and thought I'd add an additional answer.

WordPress has a filter called the_content() which allows you to filter the content returned to almost any function, including the editor.

Check out the documentation here: https://developer.wordpress.org/reference/hooks/the_content/

Code example from the docs, adapted to determine if we're in the admin:

add_filter( 'the_content', 'filter_the_content_in_the_main_loop', 1 );
 
function filter_the_content_in_the_main_loop( $content ) {
 
    // Check if we're in the wp-admin
    if ( is_admin ) {
        return $content . esc_html__( 'I’m filtering the content in the admin', 'wporg');
    }
 
    return $content;
}