A widget that will detect inactivity of the user.

inactivity_detector.dart

import 'package:async/async.dart';
import 'package:flutter/material.dart';

final navigatorKey = GlobalKey<NavigatorState>();

class InactivityObserver extends StatefulWidget {
  const InactivityObserver({
    super.key,
    required this.inactivityTimeout,
    required this.child,
  });

  final Duration inactivityTimeout;
  final Widget child;

  @override
  State<InactivityObserver> createState() => _InactivityObserverState();
}

class _InactivityObserverState extends State<InactivityObserver> {
  late final timer = RestartableTimer(
    widget.inactivityTimeout,
    onInactivityTimeout,
  );

  @override
  void initState() {
    super.initState();
    // This is needed to initialize the timer after the first frame.
    WidgetsBinding.instance.addPostFrameCallback((_) {
      timer.reset();
    });
  }

  @override
  void dispose() {
    timer.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerDown: (details) => timer.reset(),
      // TODO: add other events that should reset the timer.
      child: widget.child,
    );
  }

  void onInactivityTimeout() {
    final localContext = navigatorKey.currentContext;

    // If there is no navigator, we can't show a dialog and should reset the
    // timer.
    if (localContext == null) return timer.reset();

    const InactivityDialog().showMe(localContext).whenComplete(() {
      timer.reset();
    });
  }
}

class InactivityDialog extends StatelessWidget with ShowableDialogMixin {
  const InactivityDialog({super.key});

  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: const Text('Inactivity detected'),
      content: const Text(
        'You have been inactive for a while. Do you want to continue?',
      ),
      actions: [
        TextButton(
          onPressed: () => Navigator.pop(context),
          child: const Text("I'm still here!"),
        ),
      ],
    );
  }
}

mixin ShowableDialogMixin on Widget {
  Future<T?> showMe<T>(BuildContext context) {
    return showDialog<T>(
      context: context,
      builder: (context) => this,
    );
  }
}