function bp_notifications_get_all_notifications_for_user( $user_id = 0 ) {

	// Default to displayed user if no ID is passed.
	if ( empty( $user_id ) ) {
		$user_id = ( bp_displayed_user_id() ) ? bp_displayed_user_id() : bp_loggedin_user_id();
	}

	// Get notifications out of the cache, or query if necessary.
	$notifications = wp_cache_get( 'all_for_user_' . $user_id, 'bp_notifications' );
	if ( false === $notifications ) {
		$notifications = BP_Notifications_Notification::get( array(
			'user_id' => $user_id
		) );
		wp_cache_set( 'all_for_user_' . $user_id, $notifications, 'bp_notifications' );
	}

	/**
	 * Filters all notifications for a user.
	 *
	 * @since 2.1.0
	 *
	 * @param array $notifications Array of notifications for user.
	 * @param int   $user_id       ID of the user being fetched.
	 */
	return apply_filters( 'bp_notifications_get_all_notifications_for_user', $notifications, $user_id );
}