How to set logged in username inside a top navigationBar? - flutter

Question

How can I get the username from Firebase in this code?
I have followed a guide on youtube for the design.
I have a top navigation bar that has a CustomText with the username.
But in this there is no class (stless/stfull). So how can I get my username and put in in my CustomText?
When I have a normal Statefull class I manage to do it, but when there is noe class I don't know how.
Here is the code:
AppBar(
leading: !ResponsiveWidget.isSmallScreen(context) ? Row(
children: [
Padding(
padding: const EdgeInsets.only(left: 16),
child: Image.asset("assets/icons/logo.png", width: 28,),
),
],
) : IconButton(icon: Icon(Icons.menu), onPressed: (){
key.currentState.openDrawer();
}),
title: Container(
child: Row(
children: [
Visibility(
visible: !ResponsiveWidget.isSmallScreen(context),
child: CustomText(text: "Dash", color: lightGrey, size: 20, weight: FontWeight.bold,)),
Expanded(child: Container()),
IconButton(icon: Icon(Icons.settings, color: dark,), onPressed: (){}),
Stack(
children: [
IconButton(icon: Icon(Icons.notifications, color: dark.withOpacity(.7),), onPressed: (){}),
Positioned(
top: 7,
right: 7,
child: Container(
width: 12,
height: 12,
padding: EdgeInsets.all(4),
decoration: BoxDecoration(
color: active,
borderRadius: BorderRadius.circular(30),
border: Border.all(color: light, width: 2)
),
),
)
],
),
Container(
width: 1,
height: 22,
color: lightGrey,
),
SizedBox(width: 24,),
CustomText(text: 'username', color: lightGrey,),
SizedBox(width: 16,),
Container(
decoration: BoxDecoration(
color: active.withOpacity(.5),
borderRadius: BorderRadius.circular(30)
),
child: Container(
decoration: BoxDecoration(
color: Colors.white,
borderRadius: BorderRadius.circular(30)
),
padding: EdgeInsets.all(2),
margin: EdgeInsets.all(2),
child: CircleAvatar(
backgroundColor: light,
child: Icon(Icons.person_outline, color: dark,),
),
),
)
],
),
),
iconTheme: IconThemeData(color: dark),
elevation: 0,
backgroundColor: Colors.transparent,
);