public function install_addon( $addon_or_id, $action = 'install' ) {

		// Setup the addon.
		$addon = is_a( $addon_or_id, 'LLMS_Add_On' ) ? $addon_or_id : llms_get_add_on( $addon_or_id );
		if ( ! $addon ) {
			return new WP_Error( 'invalid_addon', __( 'Invalid add-on ID.', 'lifterlms' ) );
		}

		if ( ! in_array( $action, array( 'install', 'update' ), true ) ) {
			return new WP_Error( 'invalid_action', __( 'Invalid action.', 'lifterlms' ) );
		}

		if ( ! $addon->is_installable() ) {
			return new WP_Error( 'not_installable', __( 'Add-on cannot be installable.', 'lifterlms' ) );
		}

		// Make sure it's not already installed.
		if ( 'install' === $action && $addon->is_installed() ) {
			// Translators: %s = Add-on name.
			return new WP_Error( 'installed', sprintf( __( '%s is already installed', 'lifterlms' ), $addon->get( 'title' ) ) );
		}

		// Get download info via llms.com api.
		$dl_info = $addon->get_download_info();
		if ( is_wp_error( $dl_info ) ) {
			return $dl_info;
		}
		if ( ! isset( $dl_info['data']['url'] ) ) {
			return new WP_Error( 'no_url', __( 'An error occured while attempting to retrieve add-on download information. Please try again.', 'lifterlms' ) );
		}

		require_once ABSPATH . 'wp-admin/includes/file.php';
		include_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
		WP_Filesystem();

		$skin = new Automatic_Upgrader_Skin();

		if ( 'plugin' === $addon->get_type() ) {

			$upgrader = new Plugin_Upgrader( $skin );

		} elseif ( 'theme' === $addon->get_type() ) {

			$upgrader = new Theme_Upgrader( $skin );

		} else {

			return new WP_Error( 'inconceivable', __( 'The requested action is not possible.', 'lifterlms' ) );

		}

		if ( 'install' === $action ) {
			remove_filter( 'upgrader_package_options', array( $this, 'upgrader_package_options' ) );
			$result = $upgrader->install( $dl_info['data']['url'] );
			add_filter( 'upgrader_package_options', array( $this, 'upgrader_package_options' ) );
		} elseif ( 'update' === $action ) {
			$result = $upgrader->upgrade( $addon->get( 'update_file' ) );
		}

		if ( is_wp_error( $result ) ) {
			return $result;
		} elseif ( is_wp_error( $skin->result ) ) {
			return $skin->result;
		} elseif ( is_null( $result ) ) {
			return new WP_Error( 'filesystem', __( 'Unable to connect to the filesystem. Please confirm your credentials.', 'lifterlms' ) );
		}

		return true;

	}