How to show a popup on app start in Flutter

You can use Shared Preferences

When launching the app, get the value from shared pref (Ex: isFirstLoaded). If isFirstLoaded == true then show the dialog.

When the dialog is dismissed, set isFirstLoaded = false and save to shared preferences.

Below is an example (please note that the example doesn't handle the dismiss event when tapping on Back key).

Add shared_preferences to your pubspec.yaml

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

void main() {
  runApp(MyStatelessApp());
}

class MyStatelessApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Stateless Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: StatelessWidgetDemo(),
    );
  }
}

class StatelessWidgetDemo extends StatelessWidget {
  final keyIsFirstLoaded = 'is_first_loaded';

  @override
  Widget build(BuildContext context) {
    Future.delayed(Duration.zero, () => showDialogIfFirstLoaded(context));
    return SafeArea(
        child: Scaffold(
            appBar: AppBar(
              title: Text('Flutter Stateless Demo'),
            ),
            body: Center(
              child: Text('Hello'),
            )));
  }

  showDialogIfFirstLoaded(BuildContext context) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    bool isFirstLoaded = prefs.getBool(keyIsFirstLoaded);
    if (isFirstLoaded == null) {
      showDialog(
        context: context,
        builder: (BuildContext context) {
          // return object of type Dialog
          return AlertDialog(
            title: new Text("Title"),
            content: new Text("This is one time dialog"),
            actions: <Widget>[
              // usually buttons at the bottom of the dialog
              new FlatButton(
                child: new Text("Dismiss"),
                onPressed: () {
                  // Close the dialog
                  Navigator.of(context).pop();
                  prefs.setBool(keyIsFirstLoaded, false);
                },
              ),
            ],
          );
        },
      );
    }
  }

There are many options to make a dialog or something else.

You can use Alert Dialog, Dialog Awesome, and more.

But if you want to make your custom dialog or something like that then you can use Overlay Widget.

Check the link: https://api.flutter.dev/flutter/widgets/OverlayEntry-class.html

also tutorial: https://www.youtube.com/watch?v=A3co3Tskjtg

call your dailog inside initState method (from main or statup page):

  @override
  void initState() {
  WidgetsBinding.instance.addPostFrameCallback(_showOpenDialog);
  super.initState();
  }

  // show Open Dialog method
  _showOpenDialog(_) {
    showDialog(
        context: context,
        builder: (context) {
          return YourCustomDialog();
        });
  }

class MyWidget extends StatelessWidget {
  bool firstBuild = true;

  // show Open Dialog method
  _showOpenDialog(context) {
    showDialog(
        context: context,
        builder: (context) {
          return Dialog();
        });
  }

  @override
  Widget build(BuildContext context) {
    WidgetsBinding.instance.addPostFrameCallback((_) {
      if (firstBuild) {
        firstBuild = false;
        _showOpenDialog(context);
      }
    });
    return Text('Hello, World!');
  }
}