Show dialog on widget

We have to display the dialog once done with building the widget. You can use Future.delayed function like below(I tested, it is working).

class XxxxxWidget extends StatelessWidget {

@override
Widget build(BuildContext context) {
// [NG]We want to show dialog on Container widget.

 Future.delayed(Duration.zero, () => showMyDialog(context)); // import 'dart:async';
 return Container(
  child: FlatButton(.... //same as question

Explaination:

As Dart is based on single-threaded event loop, when we create an async tasks, it will put those events in the end of the event queue and continue it's current execution. Please refer below example for more details,

void main() {
  print("first");
  Future(() => print("second"));
  print("third");
  Future(() => print("forth"));

}

Output will be

first
third
second
forth

It is very similar to

DispatchQueue.main.async {
 print("Async1") //printJob
}

Once done with building the widget display the dialog. Check out my answer for similar problem.

some of this is ambiguous, but you can show an alert dialog in a widgets init method like this

Future showDialog() async {

 WidgetsBinding.instance.addPostFrameCallback((_) async {
  await showDialog<String>(
    context: context,
    builder: (BuildContext context) => AlertDialog(
          title: const Text('Title'),
          content: const Text('Content')
          actions: <Widget>[
            FlatButton(
              child: const Text('OK'),
              onPressed: () {
                Navigator.of(context).pop();
              },
            ),
          ],
        ),
  );
});
}

this will wait for the first frame to be drawn before showing the dialog so you can call it from init

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

   showDialog();
}