Why is my RaisedButton not working in Flutter?

Question

I have been working on a to-do list app in Flutter but in the process, I realized that the add button was not working when it was supposed to add a new task. I don't know why it doesn't work, because I am very new to Flutter. Why is this problem occurring!?

main.dart

  List<String> _toDoItems = [];

  void _addToDoItem(String task) {
    if(task.length > 0) {
      setState(() {  
        _toDoItems.add(task);
      });
    }
  }
  Expanded(
    flex: 8,
    child: TextField(
      autofocus: true,
      onSubmitted: (val) {
        _addToDoItem(val);
      },
    ),
  ),
  Expanded(
    flex: 2,
      child: RaisedButton(
        child: Text('ADD'),
          onPressed: () => _addToDoItem,
      ),
  ),

Answer 1

The RaisedButton widget was deprecated in Flutter 2.0 and replaced with the ElevatedButton widget. It's possible that you're using a newer version of Flutter where RaisedButton is no longer available. Try using ElevatedButton instead.

Answer 2

You're not adding a parameter..

Expanded(
    flex: 2,
      child: RaisedButton(
        child: Text('ADD'),
          onPressed: () => {
           _addToDoItem(something_here),
        },
      ),
  ),

Answer 3

You can copy paste run full code below
You can use TextEditingController and pass controller.text to _addToDoItem
code snippet

TextEditingController _controller = TextEditingController();

Expanded(
      flex: 8,
      child: TextField(
        controller: _controller,
        
Expanded(
      flex: 2,
      child: RaisedButton(
        child: Text('ADD'),
        onPressed: () => _addToDoItem(_controller.text),    

working demo

full code

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  List<String> _toDoItems = [];
  TextEditingController _controller = TextEditingController();

  void _addToDoItem(String task) {
    if (task.length > 0) {
      setState(() {
        _toDoItems.add(task);
      });
    }
  }

  Widget _buildToDoItem(String toDoText) {
    return ListTile(title: Text(toDoText));
  }

  Widget _buildToDoList() {
    return Expanded(
      child: ListView.builder(
        itemBuilder: (context, index) {
          if (index < _toDoItems.length) {
            return _buildToDoItem(_toDoItems[index]);
          }
        },
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: PreferredSize(
            preferredSize: Size.fromHeight(50),
            child: AppBar(
              centerTitle: true,
              backgroundColor: Colors.red,
              title: Text(
                'To Do List',
                style: TextStyle(
                  fontSize: 24,
                  fontWeight: FontWeight.bold,
                ),
              ),
            )),
        backgroundColor: Colors.white,
        body: Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          children: [
            Container(
                margin: EdgeInsets.all(22),
                child: Row(
                  children: [
                    Expanded(
                      flex: 8,
                      child: TextField(
                        controller: _controller,
                        autofocus: true,
                        onSubmitted: (val) {
                          _addToDoItem(val);
                        },
                        decoration: InputDecoration(
                          hintText: 'Add a tak here...',
                          enabledBorder: OutlineInputBorder(
                            borderRadius:
                                BorderRadius.all(Radius.circular(12.0)),
                            borderSide: BorderSide(color: Colors.red, width: 2),
                          ),
                          focusedBorder: OutlineInputBorder(
                            borderRadius:
                                BorderRadius.all(Radius.circular(12.0)),
                            borderSide:
                                BorderSide(color: Colors.red, width: 1.5),
                          ),
                        ),
                      ),
                    ),
                    Expanded(
                      flex: 2,
                      child: RaisedButton(
                        child: Text('ADD'),
                        onPressed: () => _addToDoItem(_controller.text),
                      ),
                    )
                  ],
                )),
            _buildToDoList(),
          ],
        ),
      ),
    );
  }
}