flush_rewrite_rules() not working on updating Settings API

Question

I'm providing a checkbox to my plugin Settings API to enable/disable certain Rewrite rules to my plugins CPT. I want to flush the rewrite rules when the specific setting is changed. So I did the following:

function my_flush_rules_while_rewrite_changed( $old_values, $new_values ) {
    if( $old_values['rewrite_url'] != $new_values['rewrite_url'] ) {
        flush_rewrite_rules();
        //echo "Rewrite rules flushed";
        //exit();
    }
}

add_action( 'update_option_my_settings', 'my_flush_rules_while_rewrite_changed', 10, 2 );

It's not flushing the rewrite rules. Though I tested without the conditions, and tested that the echod text is displayed if used with exit(). So, I'm sure, the function is being called, but sad part is, it's NOT WORKING.

I thought it's the Rewrite Rules plugin's problem, but the actual rewriting isn't working without saving Permalink changes. So it's actually a problem with my code - it's not working.

But why?

Answer 1

flush_rewrite_rules is firing, but it's too late; the previous rewrite setting has already determined your register_post_type args at the start of the request, so you're just flushing the "old" rules.

You could try re-registering the post type just before flushing, but I'm not sure of the wider implications or even if this will work.

If it were me, I'd use the logic you've already got to stash a flag in the settings, then handle on the subsequent request:

function my_flush_rules_while_rewrite_changed( $new_values, $old_values ) {
    if( empty( $new_values['rewrite_url'] ) && ! empty( $old_values['rewrite_url'] ) || ! empty( $new_values['rewrite_url'] ) && empty( $old_values['rewrite_url'] ) ) {
        $new_values['flush_rewrite_rules'] = true;
    }

    return $new_values;
}

add_filter( 'pre_update_option_my_custom_settings', 'my_flush_rules_while_rewrite_changed', 11, 2 );

And then as an example:

add_action( 'init', function () {
    $settings = get_option( 'my_custom_settings' );
    if ( ! empty( $settings['flush_rewrite_rules'] ) ) {
        flush_rewrite_rules();
        unset( $settings['flush_rewrite_rules'] );
        update_option( 'my_custom_settings', $settings );
    }
});