Show alert dialog on app main screen load automatically in flutter

Question

I want to show alert dialog based on a condition. Not based on user interaction such as button press event.

If a flag is set in app state data alert dialog is shown otherwise its not.

Below is the sample alert dialog which I want to show

  void _showDialog() {
    // flutter defined function
    showDialog(
      context: context,
      builder: (BuildContext context) {
        // return object of type Dialog
        return AlertDialog(
          title: new Text("Alert Dialog title"),
          content: new Text("Alert Dialog body"),
          actions: <Widget>[
            // usually buttons at the bottom of the dialog
            new FlatButton(
              child: new Text("Close"),
              onPressed: () {
                Navigator.of(context).pop();
              },
            ),
          ],
        );
      },
    );
  }

I tried to call that method inside main screen widget's build method but it gives me error -

 The context used to push or pop routes from the Navigator must be that of a widget that is a descendant of a Navigator widget.
E/flutter ( 3667): #0      Navigator.of.<anonymous closure> (package:flutter/src/widgets/navigator.dart:1179:9)
E/flutter ( 3667): #1      Navigator.of (package:flutter/src/widgets/navigator.dart:1186:6)
E/flutter ( 3667): #2      showDialog (package:flutter/src/material/dialog.dart:642:20)

Problem is I don't know from where I should call that _showDialog method?

Answer 1

for me i using this so each time the user click the page then user will show this disclaimer

void showDisclaimerPopup() {
    showDialog(
      context: context,
      builder: (BuildContext context) => AlertDialog(
        title: const Text(
          'Q and A ?',
        ),
        content: SizedBox(
          width: double.maxFinite,
          child: ListView.builder(
            shrinkWrap: true,
            itemCount: disclaimerPoints.length,
            itemBuilder: (BuildContext context, int index) {
              return Padding(
                padding: const EdgeInsets.symmetric(vertical: 4.0),
                child: Row(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: [
                    const Text(
                      '• ',
                      style: TextStyle(fontSize: 18),
                    ),
                    Expanded(
                      child: Text(
                        disclaimerPoints[index],
                        style: const TextStyle(fontSize: 16),
                      ),
                    ),
                  ],
                ),
              );
            },
          ),
        ),
        shape: const RoundedRectangleBorder(
          borderRadius: BorderRadius.all(Radius.circular(25.0)),
        ),
        actions: [
          Center(
            child: ElevatedButton(
              style: ElevatedButton.styleFrom(
                shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(25),
                ),
              ),
              child: Text(
                'okay',
                style: txtSubheadBlack,
              ),
              onPressed: () {
                Navigator.pop(context);
              },
            ),
          ),
        ],
      ),
    );   }

  @override   void initState() {
    super.initState();
    Future.delayed(Duration.zero, () => showDisclaimerPopup());   }

Answer 2

I would place it in initState of a State (of a StatefulWidget).

Placing it in the build method of a Stateless widget is tempting, but that will trigger your alert multiple times.

In this example below, it displays an alert when the device is not connected to the Wifi, showing a [try again] button if it's not.

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

void main() => runApp(MaterialApp(title: "Wifi Check", home: MyPage()));

class MyPage extends StatefulWidget {
    @override
    _MyPageState createState() => _MyPageState();
}

class _MyPageState extends State<MyPage> {
    bool _tryAgain = false;

    @override
    void initState() {
      super.initState();
      _checkWifi();
    }

    _checkWifi() async {
      // the method below returns a Future
      var connectivityResult = await (new Connectivity().checkConnectivity());
      bool connectedToWifi = (connectivityResult == ConnectivityResult.wifi);
      if (!connectedToWifi) {
        _showAlert(context);
      }
      if (_tryAgain != !connectedToWifi) {
        setState(() => _tryAgain = !connectedToWifi);
      }
    }

    @override
    Widget build(BuildContext context) {
      var body = Container(
        alignment: Alignment.center,
        child: _tryAgain
          ? RaisedButton(
              child: Text("Try again"),
              onPressed: () {
                _checkWifi();
            })
          : Text("This device is connected to Wifi"),
      );

      return Scaffold(
        appBar: AppBar(title: Text("Wifi check")),
        body: body
      );
    }

    void _showAlert(BuildContext context) {
      showDialog(
          context: context,
          builder: (context) => AlertDialog(
            title: Text("Wifi"),
            content: Text("Wifi not detected. Please activate it."),
          )
      );
    }
}

Answer 3

Simply override initState and call your _showDialog method inside Future or Timer:

@override
void initState() {
  super.initState();

  // Use either of them. 
  Future(_showDialog);
  Timer.run(_showDialog); // Requires import: 'dart:async'
}

Answer 4

If you are using blocs then use a BlocListener as suggested by @mirkancal in this answer: Flutter: bloc, how to show an alert dialog

Answer 5

This is how I achieved this in a simple way:

1. Add https://pub.dev/packages/shared_preferences

2. Above the build method of your main screen (or any desired widget):

   Future checkFirstRun(BuildContext context) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    bool isFirstRun = prefs.getBool('isFirstRun') ?? true;
   
    if (isFirstRun) {
      // Whatever you want to do, E.g. Navigator.push()
      prefs.setBool('isFirstRun', false);
    } else {
      return null;
    }
   }
   

3. Then on your widget's initState:

   @override
   void initState() {
     super.initState();
     WidgetsBinding.instance.addPostFrameCallback((_) => checkFirstRun(context));
   }
   

This ensures the function is run after the widget is built.

Answer 6

I solved it using a package developed by Flutter Community. here https://pub.dev/packages/after_layout

Add this to your pubspec.yaml

after_layout: ^1.0.7+2

And try below example

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

class DialogDemo extends StatefulWidget {
  @override
  _DialogDemoState createState() => _DialogDemoState();
}

class _DialogDemoState extends State<DialogDemo>
    with AfterLayoutMixin<DialogDemo> {
  @override
  void initState() {
    super.initState();
  }

  @override
  void afterFirstLayout(BuildContext context) {
    _neverSatisfied();
  }

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Container(
        decoration: BoxDecoration(color: Colors.red),
      ),
    );
  }

  Future<void> _neverSatisfied() async {
    return showDialog<void>(
      context: context,
      barrierDismissible: false, // user must tap button!
      builder: (BuildContext context) {
        return AlertDialog(
          title: Text('Rewind and remember'),
          content: SingleChildScrollView(
            child: ListBody(
              children: <Widget>[
                Text('You will never be satisfied.'),
                Text('You\’re like me. I’m never satisfied.'),
              ],
            ),
          ),
          actions: <Widget>[
            FlatButton(
              child: Text('Regret'),
              onPressed: () {
                Navigator.of(context).pop();
              },
            ),
          ],
        );
      },
    );
  }
}

Answer 7

You have to wrap the content inside another Widget (preferably Stateless).

Example:

Change From:

  import 'package:flutter/material.dart';

  void main() {
    runApp(new MyApp());
  }

  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
          title: 'Trial',
          home: Scaffold(
              appBar: AppBar(title: Text('List scroll')),
              body: Container(
                child: Text("Hello world"),
              )));
    }
  }

to this:

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

  void main() {
    runApp(new MyApp());
  }

  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
          title: 'Trial',
          home: Scaffold(
              appBar: AppBar(title: Text('List scroll')), body: new MyHome()));
    }
  }

  class MyHome extends StatelessWidget { // Wrapper Widget
    @override
    Widget build(BuildContext context) {
      Future.delayed(Duration.zero, () => showAlert(context));
      return Container(
        child: Text("Hello world"),
      );
    }

    void showAlert(BuildContext context) {
      showDialog(
          context: context,
          builder: (context) => AlertDialog(
                content: Text("hi"),
              ));
    }
  }

Note: Refer here for wrapping show alert inside Future.delayed(Duration.zero,..)