Flutter – How to run code after some delay in Flutter

Kotlin

    Handler(Looper.getMainLooper()).postDelayed({
      //Do something after 100ms
    }, 100)

Java

    final Handler handler = new Handler(Looper.getMainLooper());
    handler.postDelayed(new Runnable() {
      @Override
      public void run() {
        //Do something after 100ms
      }
    }, 100);

The member variable context can be accessed during initState but can't be used for everything. This is from the flutter for initState documentation:

You cannot use [BuildContext.inheritFromWidgetOfExactType] from this method. However, [didChangeDependencies] will be called immediately following this method, and [BuildContext.inheritFromWidgetOfExactType] can be used there.

You could move your initialization logic to didChangeDependencies, however that might not be exactly what you want as didChangeDependencies can be called multiple times in the lifecycle of the widget.

If you instead make an asynchronous call which delegates your call until after the widget has been initialized, you can then use context as you intend.

A simple way to do that is to use a future.

Future.delayed(Duration.zero,() {
  ... showDialog(context, ....)
}

Another way, which may be more 'correct', is to use flutter's scheduler to add a post-frame callback:

SchedulerBinding.instance.addPostFrameCallback((_) {
  ... showDialog(context, ....)
});

And finally, here's a little trick I like to do to use asynchronous calls in the initState function:

() async {
  await Future.delayed(Duration.zero);
  ... showDialog(context, ...)      
}();

Here's a fully fleshed out example using the simple Future.delayed:

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: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  bool _checkConfiguration() => true;

  void initState() {
    super.initState();
    if (_checkConfiguration()) {
      Future.delayed(Duration.zero,() {
        showDialog(context: context, builder: (context) => AlertDialog(
          content: Column(
            children: <Widget>[
              Text('@todo')
            ],
          ),
          actions: <Widget>[
            FlatButton(onPressed: (){
              Navigator.pop(context);
            }, child: Text('OK')),
          ],
        ));
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
    );
  }
}

With more context from the OP provided in comments, I can give a slightly better solution to their specific problem. Depending on the app, you may actually want to make a decision based on which page to show depending on whether it's the first time the app is opened i.e. set home to something different. And dialogs aren't necessarily the best UI element on mobile; it may be better to show a full page with the settings they need to add and a next button.