Demo loading dialog and controll the back button pressed.

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
  @override
  Widget build(BuildContext context) {
    final title = 'Ignore Pointer';

    return MaterialApp(
      title: title,
      home: WillPopScope(
        onWillPop: () async {
          // Android側onBackPress()メソッド見たい。
          Navigator.pop(context);
          return true;
        },
        child: Scaffold(
          key: _scaffoldKey,
          appBar: AppBar(
            title: Text(title),
          ),
          body: Stack(
            children: [
              Center(
                child: RaisedButton(
                  child: Text('Click me and wait snackbar'),
                  color: Colors.red,
                  textColor: Colors.yellow,
                  padding: EdgeInsets.fromLTRB(10, 10, 10, 10),
                  splashColor: Colors.grey,
                  onPressed: () {
                    _scaffoldKey.currentState.showSnackBar(
                      SnackBar(
                        content: Text('Hi, I"m coming'),
                        duration: Duration(seconds: 1),
                      ),
                    );
                  },
                ),
              ),
              AbsorbPointer(
                absorbing: true, // ⭐︎既定な値はtrue => 変換して見てください。
                child: Container(
                  width: double.infinity,
                  height: double.infinity,
                  child: Center(
                    child: Container(
                      width: 100,
                      height: 100,
                      child: CircularProgressIndicator(),
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}