public function process_rsvp_step( array $args ) {
		$result = [
			'success' => null,
			'errors'  => [],
		];

		// Process the attendee.
		if ( 'success' === $args['step'] ) {
			$first_attendee = $this->parse_attendee_details();

			/**
			 * These are the inputs we should be seeing:
			 *
			 * tribe_tickets[{$ticket_id}][ticket_id] (previously product_id[])
			 * tribe_tickets[{$ticket_id}][quantity] (previously quantity_{$ticket_id})
			 * tribe_tickets[{$ticket_id}][attendees][0][order_status] (previously attendee[order_status])
			 * tribe_tickets[{$ticket_id}][attendees][0][full_name] (previously attendee[full_name])
			 * tribe_tickets[{$ticket_id}][attendees][0][email] (previously attendee[email])
			 * tribe_tickets[{$ticket_id}][attendees][0][meta][{$field_slug}] (previously tribe-tickets-meta[{$ticket_id}][0][{$field_slug}])
			 * tribe_tickets[{$ticket_id}][attendees][1][full_name] (new for IAC)
			 * tribe_tickets[{$ticket_id}][attendees][1][email] (new for IAC)
			 * tribe_tickets[{$ticket_id}][attendees][1][meta][{$field_slug}] (previously tribe-tickets-meta[{$ticket_id}][1][{$field_slug}])
			 */
			$attendee_ids = $this->generate_tickets( $args['post_id'], false );

			if ( false === $attendee_ids ) {
				$result['success']  = false;
				$result['errors'][] = __( 'Your RSVP was unsuccessful, please try again.', 'event-tickets' );

				return $result;
			}

			$attendee_ids = implode( ',', $attendee_ids );

			$nonce_action = 'tribe-tickets-rsvp-opt-in-' . md5( $attendee_ids );

			$result['success']     = true;
			$result['opt_in_args'] = [
				'is_going'     => ! empty( $first_attendee['order_status'] ) ? 'yes' === $first_attendee['order_status'] : false,
				'checked'      => false,
				'attendee_ids' => $attendee_ids,
				'opt_in_nonce' => wp_create_nonce( $nonce_action ),
			];
		} elseif ( 'opt-in' === $args['step'] ) {
			/**
			 * These are the inputs we should be seeing:
			 *
			 * opt_in
			 * attendee_ids
			 * opt_in_nonce
			 */
			// @todo Handle opt-in setting for each attendee in order.
			$optout = ! tribe_is_truthy( tribe_get_request_var( 'opt_in', true ) );

			$attendee_ids = Tribe__Utils__Array::list_to_array( tribe_get_request_var( 'attendee_ids', [] ) );
			$attendee_ids = array_map( 'absint', $attendee_ids );

			$attendee_ids_flat = implode( ',', $attendee_ids );

			$nonce_value  = tribe_get_request_var( 'opt_in_nonce', '' );
			$nonce_action = 'tribe-tickets-rsvp-opt-in-' . md5( $attendee_ids_flat );

			if ( false === wp_verify_nonce( $nonce_value, $nonce_action ) ) {
				$result['success']  = false;
				$result['errors'][] = __( 'Unable to verify your opt-in request, please try again.', 'event-tickets' );

				return $result;
			}

			foreach ( $attendee_ids as $attendee_id ) {
				// @todo This class is not setting $this->attendee_optout_key.
				update_post_meta( $attendee_id, self::ATTENDEE_OPTOUT_KEY, (int) $optout );
			}

			$result['success']     = true;
			$result['opt_in_args'] = [
				'is_going'     => true,
				'checked'      => ! $optout,
				'attendee_ids' => $attendee_ids_flat,
				'opt_in_nonce' => $nonce_value,
			];
		}

		return $result;
	}