class External_Service_Module extends POS_Module {
    public $id = 'external_service';
    public $name = 'External Service';

    public function get_sync_hook_name() {
        return 'pos_sync_' . $this->id;
    }

    public function register_sync( $interval = 'hourly' ) {
        $hook_name = $this->get_sync_hook_name();
        add_action( $hook_name, array( $this, 'sync' ) );
        if ( ! wp_next_scheduled( $hook_name ) ) {
            wp_schedule_event( time(), $interval, $hook_name );
        }
    }

    public function sync() {
        $this->log( 'EMPTY SYNC' );
    }
}

class Evernote_Module extends External_Service_Module {
    public $id = 'evernote';
    public $name = 'Evernote';
    public $description = 'Syncs with evernote service';
    public $advanced_client = null;

    public function __construct( \POS_Module $notes_module ) {

        $this->register_sync( 'hourly' );
    }

    /**
     * Sync with Evernote. This is triggered by the cron job.
     *
     * @see register_sync
     */
    public function sync() {
        $this->log( 'Syncing Evernote triggering ' );
        $usn = get_option( $this->get_setting_option_name( 'usn' ), 0 );

        $sync_chunk = $this->advanced_client->getNoteStore()->getFilteredSyncChunk( $usn, 100, $sync_filter );
        if ( ! $sync_chunk ) {
            $this->log( 'Evernote: Sync failed', E_USER_WARNING );
            return;
        }
        if ( ! empty( $sync_chunk->chunkHighUSN ) && $sync_chunk->chunkHighUSN > $usn ) {
            // We want to unschedule any regular sync events until we have initial sync complete.
            wp_unschedule_hook( $this->get_sync_hook_name() );
            // We will schedule ONE TIME sync event for the next page.
            update_option( $this->get_setting_option_name( 'usn' ), $sync_chunk->chunkHighUSN );
            wp_schedule_single_event( time() + 60, $this->get_sync_hook_name() );
            $this->log( "Scheduling next page chunk with cursor {$sync_chunk->chunkHighUSN}" );
        } else {
            $this->log( 'Evernote: Full sync completed' );
        }
        // ACTUALLY PROCESS ITEMS HERE
    }

}

<?php

class Readwise extends External_Service_Module {
    public $id = 'readwise';
    public $name = 'Readwise';
    public $description = 'Syncs with readwise service';

    public function __construct( $notes_module ) {
        $this->register_sync( 'hourly' );
    }

    public function sync() {
        $this->log( '[DEBUG] Syncing readwise triggering ' );

        $query_args = array();
        $page_cursor = get_option( $this->get_setting_option_name( 'page_cursor' ), null );
        if ( $page_cursor ) {
            $query_args['pageCursor'] = $page_cursor;
        } else {
            $last_sync = get_option( $this->get_setting_option_name( 'last_sync' ) );
            if ( $last_sync ) {
                $query_args['updatedAfter'] = $last_sync;
            }
        }

        $request = wp_remote_get(
            'https://readwise.io/api/v2/export/?' . http_build_query( $query_args ),
            array(
                'headers' => array(
                    'Authorization' => 'Token ' . $token,
                ),
            )
        );
        if ( is_wp_error( $request ) ) {
            $this->log( '[ERROR] Fetching readwise ' . $request->get_error_message(), E_USER_WARNING );
            return false; // Bail early
        }

        $body = wp_remote_retrieve_body( $request );
        $data = json_decode( $body );
        $this->log( "[DEBUG] Readwise Syncing {$data->count} highlights" );

        //phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
        if ( ! empty( $data->nextPageCursor ) ) {
            // We want to unschedule any regular sync events until we have initial sync complete.
            wp_unschedule_hook( $this->get_sync_hook_name() );
            // We will schedule ONE TIME sync event for the next page.
            //phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
            update_option( $this->get_setting_option_name( 'page_cursor' ), $data->nextPageCursor );
            wp_schedule_single_event( time() + 60, $this->get_sync_hook_name() );
            $this->log( "Scheduling next page sync with cursor {$data->nextPageCursor}" );
        } else {
            $this->log( '[DEBUG] Full sync completed' );
            update_option( $this->get_setting_option_name( 'last_sync' ), gmdate( 'c' ) );
            delete_option( $this->get_setting_option_name( 'page_cursor' ) );
        }

        foreach ( $data->results as $book ) {
            $this->sync_book( $book );
        }
    }

}

/**
* This is hooked into the save_post action of the notes module.
* Every time a post is updated, this will check if it is in the synced notebooks and sync it to evernote.
* It will then receive the returned content and update the post, so some content may be lost if it is not handled by evernote
*
* @param int $post_id
* @param \WP_Post $post
* @param bool $update
*/
public function sync_note_to_evernote( int $post_id, \WP_Post $post, bool $update ) {
    $guid = get_post_meta( $post->ID, 'evernote_guid', true );

    if ( $guid ) {
        $note = $this->advanced_client->getNoteStore()->getNote( $guid, false, false, false, false );
        if ( $note ) {
            $note->title = $post->post_title;
            $note->content = self::html2enml( $post->post_content );
            $result = $this->advanced_client->getNoteStore()->updateNote( $note );
        }
        return;
    }

    // edam note
    $note = new \EDAM\Types\Note();
    $note->title = $post->post_title;
    $note->content = self::html2enml( $post->post_content );

    $result = $this->advanced_client->getNoteStore()->createNote( $note );
    if ( $result ) {
        $this->update_note_from_evernote( $result, $post );
    }

}

/**
* This is called when a note is updated from evernote.
* It will update the post with the new data.
* It is triggered by both directions of the sync:
* - When a note is updated in evernote, it will be updated in WordPress
* - When a note is updated in WordPress, it will be updated in evernote and then the return will be passed here.
*
* @param \EDAM\Types\Note $note
* @param \WP_Post $post
* @param bool $sync_resources - If true, it will also upload note resources. We want this in most cases, EXCEPT when we are sending the data from WordPress and know the response will not have new resources for us.
*/
public function update_note_from_evernote( \EDAM\Types\Note $note, \WP_Post $post, $sync_resources = false ): int {
    remove_action( 'save_post_' . $this->notes_module->id, array( $this, 'sync_note_to_evernote' ), 10 );
    // updates updates 
    add_action( 'save_post_' . $this->notes_module->id, array( $this, 'sync_note_to_evernote' ), 10, 3 );
    return $post->ID;
}