Detect back button press while dialog is open in flutter

Because my reputation not enough to comment on the accepted answer, I want to give other alternative for onPressed: () {}. You can use onPressed: () => null. No warning will pop up.

 @override
      Widget build(BuildContext context) {
        return WillPopScope(
          onWillPop: _onBackPressed,
          child: Scaffold(
            
          ),
        );
      }
    
      Future<bool> _onBackPressed() async {
        return await showDialog(
            context: context, builder: (context) => ExitAppDialogBox());
      }

Back button won't close the dialog.

showDialog(
  context: context,
  barrierDismissible: false,
  builder: (BuildContext context) {
    return WillPopScope(
      onWillPop: () async => false,
      child: AlertDialog(
        title: Text('Title'),
        content: Text('This is Demo'),
        actions: <Widget>[
          FlatButton(
            onPressed: () => Navigator.pop(context),
            child: Text('Go Back'),
          ),
        ],
      ),
    );
  },
);

Three ways to stop dialog getting closed by Android Back Button

Option one:

           onWillPop: () {
                          return Future.value(false);
                        },

Option Two:

    onWillPop: () async {
                          return false;
                        },

Option Three:

 onWillPop: () {}, // This will give surpress warning, try to avoid this one.