Flutter : flutter : Unsupported operation: Cannot add to an unmodifiable list

Question

I want to program a shopping list app it throws me from line 41 the debug error

"UnsupportedError (Unsupported operation: Cannot add to an unmodifiable list)".

Source of the Add Button

ElevatedButton(
                  onPressed: () {
                    shoppingList.add(ShoppingItem(
                        name: _textEditingController.text, done: false));
                    Navigator.pop(context);
                  },
                  child: const Text('Add')
                  ),

the whole source code of the AddShoppingItemScreen

import 'package:flutter/material.dart';
import 'package:shoppinglist/screens/shopping_item.dart';
import 'config.dart';

class AddShoppingItemScreen extends StatefulWidget {
  const AddShoppingItemScreen({Key? key}) : super(key: key);

  @override
  State<AddShoppingItemScreen> createState() => _AddShoppingItemScreenState();
}

class _AddShoppingItemScreenState extends State<AddShoppingItemScreen> {
  final TextEditingController _textEditingController =
      TextEditingController(text: '');

  @override
  Widget build(BuildContext context) {
    final shoppingList = context
        .dependOnInheritedWidgetOfExactType<Configuration>()!
        .shoppingList;

    return Scaffold(
        appBar: AppBar(
          title: const Text('Add Item'),
        ),
        body: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Column(
            children: [
              TextFormField(
                controller: _textEditingController,
                autofocus: true,
                decoration: InputDecoration(
                  border: OutlineInputBorder(
                    borderRadius: BorderRadius.circular(10),
                  ),
                ),
              ),
              ElevatedButton(
                  onPressed: () {
                    shoppingList.add(ShoppingItem(
                        name: _textEditingController.text, done: false));
                    Navigator.pop(context);
                  },
                  child: const Text('Add')
                  ),
            ],
          ),
        ));
  }
}

Where is Configuration declared? Specifically: what is the type of Configuration.shoppingList? — Roger Lipscombe
– Roger Lipscombe, Commented Jul 18, 2022 at 19:34
Remove the const modifier from your shoppingList list declaration. — Alex
– Alex, Commented Jul 18, 2022 at 19:57

Wali Seddiqi · Accepted Answer · 2022-09-01 11:05:47Z

1

you can use something like this. creating an "Add" method inside the DataModel

void main(){

  NumberList list=NumberList([45,64,7]);
  
  list.numbers.forEach((e)=>print(e));///45,64,7
  list= list.add(5);//adds 5 in here it adds another Int and push into list variable itself
  list.numbers.forEach((e)=>print(e));///45,64,7,5
}
class NumberList {
  final List<int> _numbers;
 
  NumberList(this._numbers);
  NumberList add(int number) {
    return NumberList(_numbers.toList()..add(number));
  }
}

edited Sep 1, 2022 at 11:05

answered Jul 18, 2022 at 20:41

Wali Seddiqi

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