'This expression has a type of 'void' so its value can't be used.' setState() error in Flutter

Try this:

class ListOfIngs extends State<ListOfIngsWidget> {  final int countIngs;  final myController = new TextEditingController();  ListOfIngs(    this.countIngs,  );  Widget build(BuildContext contex) {    return new Container(        child: ListView.builder(            itemCount: countIngs,            itemBuilder: (context,index){              return Container(                child: TextField(                  controller: myController,                  decoration: InputDecoration(                    border: OutlineInputBorder(),                    labelText: 'Ingredient $index',                  ),                ),              );            },          ),       );  }}

A few important things first:

• Every TextField needs its own TextEditingController, so in your case you need a list of TextEditingControllers.
• With TextField() you create an object of type TextField. In the brackets you can't just write a for loop, because parameters to create the object are expected here.
• Ingredient $countIngs' always gives you only the length. What you want to have is your variable i, which increases in every loop pass.

import 'package:flutter/material.dart';class ListOfIngsWidget extends StatefulWidget {  final int countIngs;  const ListOfIngsWidget(this.countIngs, {Key key}) : super(key: key);  @override  _ListOfIngsState createState() => _ListOfIngsState();}class _ListOfIngsState extends State<ListOfIngsWidget> {  List<TextEditingController> _controllerList = [];  List<TextField> _textFieldList = [];    @override  void initState() {    for (int i=0; i<widget.countIngs; i++) {      TextEditingController controller = TextEditingController();      TextField textField = TextField(        controller: controller,        decoration: InputDecoration(          border: OutlineInputBorder(),          labelText: 'Ingredient $i',        ),      );      _textFieldList.add(textField);      _controllerList.add(controller);    }    super.initState();  }  @override  Widget build(BuildContext context) {    return new Container(      child: Flexible(        child: ListView(children: _textFieldList),      ),    );  }}

You can now call the widget for example with ListOfIngsWidget(5).

The problem is with the ListView. It does not have a children: [] atribute. You need to build the widget list before

List<Widget> list = [];for(int i = 0; i < countIngs; i ++) {  list.add(TextField(    controller: myController,    decoration: InputDecoration(      border: OutlineInputBorder(),      labelText: 'Ingredient $i',    ),  ));}

Then build the ListView

...  ListView(    children: list  )...

Note: If you are building multiple TextFields, make sure you create a different controller for each of them.

You can also use ListView.builder(), which is better for many items. You can make use of functions inside of it.