Error: The parameter 'scaffoldKey' can't have a value of 'null' [...] 'GlobalKey<ScaffoldState>'

Question

When trying to run the function below, you get this error:

home page

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import '../models/appdata.dart';
import '../partials/customappbar.dart';

class HomePage extends StatelessWidget {

  GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey();
  

  TextStyle styles = TextStyle(
    fontSize: 15,
    fontWeight: FontWeight.bold,
    fontFamily: 'Helvetica Neue'
  );


  @override
  Widget build(BuildContext context){
    return Consumer<AppData>(
      builder: (ctx, appdata, child) => Scaffold(
        key: _scaffoldKey,
        appBar: CustomAppBar(
          scaffoldKey: _scaffoldKey,
          title: 'Pagina Home',
        ),
        drawer: Drawer(),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Container(
                margin: EdgeInsets.only(bottom: 30),
                child: Text("Bem vindo(a) ao:", style: styles,)
              ),
              Image.asset('lib/assets/devstravel_logo.png', width: 200,),
              Container(
                margin: EdgeInsets.only(top: 30),
                child: 
                Text("Seu guia de viagem perfeito", style: styles,
              )
            )
          ],
        )
        ),
      )
    );
  }
}

here customappbar

import 'package:flutter/material.dart';

PreferredSizeWidget  CustomAppBar ({

      GlobalKey<ScaffoldState> scaffoldKey,
      String title = '',
      bool hideSearch = false,
      bool showBack = false
    })

  {

  IconButton drawerIcon = IconButton (
    icon: Icon(Icons.menu, color: Colors.black, size: 30),
    onPressed: (){}        
  );

  IconButton backIcon = IconButton (
    icon: Icon(Icons.arrow_back, color: Colors.black, size: 30),
    onPressed: (){
      scaffoldKey.currentState.openDrawer();
    }        
  );

  IconButton leadingButton = drawerIcon;
  if (showBack){
    leadingButton = backIcon;
  }


  return AppBar(
      backgroundColor: Colors.white,
      elevation: 0,
      centerTitle: false, //aqui eu deixo o texto centralizado 
      title: Text(
        title, 
        style: TextStyle(
          color:Colors.black,
          fontSize: 20,
          fontWeight: FontWeight.bold,
          fontFamily: 'Helvetica Neue'
        ),
      ),
      leading: leadingButton,
      actions: <Widget>[
        !hideSearch ? IconButton(
          icon: Icon(Icons.search, color: Colors.black, size: 30),
          onPressed: (){}        
        ) : Container()
      ],
   );
}

error message:

*: Error: The parameter 'scaffoldKey' can't have a value of 'null' because of its type 'GlobalKey', but the implicit default value is 'null'. lib/…/partials/customappbar.dart:5

• 'GlobalKey' is from 'package:flutter/src/widgets/framework.dart' ('/C:/flutter/packages/flutter/lib/src/widgets/framework.dart'). package:flutter/…/widgets/framework.dart:1
• 'ScaffoldState' is from 'package:flutter/src/material/scaffold.dart' ('/C:/flutter/packages/flutter/lib/src/material/scaffold.dart'). package:flutter/…/material/scaffold.dart:1 Try adding either an explicit non-'null' default value or the 'required' modifier. GlobalKey scaffoldKey,*

What is the problem?? looks like everything is ok.

Answer 1

You are using named constructor which is contains optional parameters. You need to add required or provide default value on parameters.

PreferredSizeWidget CustomAppBar({
  required GlobalKey<ScaffoldState> scaffoldKey, // add required here
  String title = '',
  bool hideSearch = false,
  bool showBack = false,
}) {

And to open drawer using drawerIcon, do

  IconButton drawerIcon = IconButton(
      icon: Icon(Icons.menu, color: Colors.black, size: 30),
      onPressed: () {
        scaffoldKey.currentState?.openDrawer();
      });

More about required-named-parameters.