Flutter back button is hidden on navigation - flutter

Question

Why MaterialApp hides back button from AppBar for my new screen. I have many screens in my app, where I need to use ThemeData. To do that, I use MaterialApp in another screen, but I don't see any back arrow on the AppBar().
return MaterialApp(
theme: ThemeData(
fontFamily: 'IranSansLight',
textTheme: TextTheme(
headline: TextStyle(fontSize: 72.0, fontWeight: FontWeight.bold, fontFamily: 'IranSansLight'),
title: TextStyle(fontSize: 36.0, fontStyle: FontStyle.italic, fontFamily: 'IranSansLight'),
body1: Theme.of(context).textTheme.caption.copyWith(fontFamily: 'IranSansLight', color: Colors.black)),
),
home: ScopedModel(
model: CounterModel(),
child: Directionality(
textDirection: TextDirection.rtl,
child: Scaffold(
appBar: AppBar(automaticallyImplyLeading:true,title: Text('aaaa'),),
body: Container(
)),
),
),
);
}

Answer 1

You are using MaterialApp for all individual screens, this is not the correct way, all you need is just one MaterialApp for the entire app.
void main() {
runApp(
MaterialApp(home: HomePage()), // MaterialApp widget should be used only here
);
}
And whenever you need to use Theme in your 2nd screen, use something like:
#override
Widget build(BuildContext context) {
return Theme(
data: Theme.of(context).copyWith(
// override whatever you need like
textTheme: TextTheme(...),
),
child: Scaffold(),
);
}