Demo the basic action with dialog. (cancelOnTouchOutside, dismiss listener, call GUI function after dismiss)

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Tutorial by Tran Duc Tam',
      home: ProcessPage(),
    );
  }
}

class ProcessPage extends StatefulWidget {
  const ProcessPage();

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

class _ProcessPageState extends State<ProcessPage> {
  final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>();
  DateTime startTime;

  Future performStart(BuildContext context) async {
    startTime = DateTime.now();
    Future.value(start(context)).then((value) {
      Navigator.pop(context, value);
    });
    return await showDialog(
      context: context,
      builder: loadingDialogBuilder,
      barrierDismissible: false,
    );
  }

  Future<bool> start(BuildContext context) async {
    // TODO: Do something
    await Future.delayed(Duration(seconds: 2));
    return true;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: scaffoldKey,
      appBar: AppBar(title: Text('')),
      body: RaisedButton(
          child: Text('LOAD DATA'),
          onPressed: () => performStart(context).then((returnVal) {
                // On dismiss
                if (startTime == null || returnVal == null) {
                  final snackBar = SnackBar(content: Text('Invalid action'));
                  scaffoldKey.currentState.showSnackBar(snackBar);
                } else {
                  final Duration detal = DateTime.now().difference(startTime);
                  final snackBar = SnackBar(
                    content: Text('Process status "$returnVal" after: $detal'),
                    duration: Duration(seconds: 5),
                  );
                  scaffoldKey.currentState.showSnackBar(snackBar);
                }
              })),
    );
  }

  Function(BuildContext) loadingDialogBuilder =
      (BuildContext context) => AlertDialog(
            content: Row(
              crossAxisAlignment: CrossAxisAlignment.center,
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Container(
                  height: 20.0,
                  width: 20.0,
                  child: CircularProgressIndicator(strokeWidth: 2.0),
                ),
                Padding(padding: EdgeInsets.all(20.0), child: Text('LOADING')),
              ],
            ),
          );
}