update.php

<?php

class ThemeUpdateHandler {
	/**
	 * Checks if the current version of the base theme is old, triggers
	 * and update message if it is.
	 *
	 * If a new version is available, we save the new version number to the database,
	 * so we can pull it back later and run version specific upgrades
	 *
	 * @param $transient:object
	 */
	public function check($transient) {
		if(!empty($transient->checked[get_config('paths')['parent-theme']])) {
			$ch = curl_init();

			curl_setopt($ch, CURLOPT_URL, get_config('update')['json']);

			curl_setopt($ch, CURLOPT_TIMEOUT, 3);
			curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

			$result = curl_exec($ch);

			if(curl_exec($ch) === false) {
				debug_log('CURL failed on theme update json file, error code:' . curl_errno($ch));
			} elseif(!empty($result)) {
				$data = json_decode($result, true);

				if(version_compare($transient->checked[get_config('paths')['parent-theme']], $data['new_version'], '<')) {
					$versions = json_encode(array('new' => $data['new_version'], 'old' => $transient->checked[get_config('paths')['parent-theme']]));

					if(!get_option('pm_upgrade_versions')) {
						add_option('pm_upgrade_versions', $versions); 
					} else {
						update_option('pm_upgrade_versions', $versions);
					}

					$transient->response[get_config('paths')['parent-theme']] = $data;
				}
			}

			curl_close($ch);
		}

		return $transient;
	}

	public function modify_source($source, $remote_source, $hook_extra) {
		$exploded = explode('/', $source);
		$exploded[count($exploded) - 2] = get_config('paths')['parent-theme'];
		$newSource = implode('/', $exploded);

		if(!rename($source, $newSource)) { debug_log('Can\'t rename source'); return; }

		$source = $newSource;

		return $source;
	}

	public function before_install($options) {
		$options['hook_extra']['double_check'] = get_config('paths')['parent-theme'];

		return $options;
	}

	/**
	 * Wordpress seems to update the theme but not with the same name,
	 * instead it creates a folder pm-{hash}, so we copy that directory
	 * to the correct folder, then remove the old folder
	 *
	 * @param $response:int
	 * @param $hook_extra:array
	 * @param $result:array
	 *
	 * @return null
	 */
	public function after_install($response, $hook_extra, $result) {
		//double check we are actually updating the base theme
		if($hook_extra['theme'] === $hook_extra['double_check']) {
			$filesystem = new WP_Filesystem_Direct(array());

			$filesystem->delete($result['source'], true);
		}
	}
}

$handler = new ThemeUpdateHandler;

add_filter('upgrader_source_selection', array($handler, 'modify_source'), 5, 3);
add_filter('upgrader_package_options', array($handler, 'before_install'), 5, 3);
add_filter('upgrader_post_install', array($handler, 'after_install'), 5, 3);
add_filter('pre_set_site_transient_update_themes', array($handler, 'check'));