visibility_aware_state.dart

import 'package:flutter/material.dart';

/*This must be added to your MaterialApp`s navigatorObservers, like:
* navigatorObservers: [routeObserver]
*/
final routeObserver = RouteObserver<PageRoute>(); 

abstract class VisibilityAwareState<T extends StatefulWidget>
    extends State<T> with WidgetsBindingObserver, RouteAware {
  bool _isOnScreen = true;
  bool _isAppResumed = true;

  @override
  void initState() {
    WidgetsBinding.instance.addObserver(this);
    super.initState();
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    routeObserver.unsubscribe(this);
    super.dispose();
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    routeObserver.subscribe(this, ModalRoute.of(context));
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    if (state == AppLifecycleState.paused) {
      _isAppResumed = false;
    }
    if (state == AppLifecycleState.resumed) {
      _isAppResumed = true;
    }

    onVisibilityChanged(_isOnScreen && _isAppResumed);
  }

  @override
  void didPush() {
    _isOnScreen = true;
    onVisibilityChanged(_isOnScreen && _isAppResumed);
  }

  @override
  void didPushNext() {
    _isOnScreen = false;
    onVisibilityChanged(_isOnScreen && _isAppResumed);
  }

  @override
  void didPop() {
    _isOnScreen = false;
    onVisibilityChanged(_isOnScreen && _isAppResumed);
  }

  @override
  void didPopNext() {
    _isOnScreen = true;
    onVisibilityChanged(_isOnScreen && _isAppResumed);
  }

  void onVisibilityChanged(bool isVisible);
}