Using Type Function and pass it into ElevatedButton onPressed, Flutter

Question

I am new to Flutter + Dart. I basically have class in the following. First I have a clas called BottomForm where it have build function that returns ElevatedButton problem when I call Function type variable in onPressed I have an issue saying that: The argument type 'Function' can't be assigned to the parameter type 'void Function()?'.dartargument_type_not_assignable

import 'formbutton.dart';

// Define a corresponding State class.
// This class holds the data related to the Form.
class _MyCustomFormState extends State<MyCustomForm> {
  // Create a text controller and use it to retrieve the current value
  // of the TextField.
  final email = TextEditingController();
  final password = TextEditingController();

  void _logIn() {
    
    print("Logged In.");
  }

  @override
  void dispose() {
    // Clean up the controller when the widget is disposed.
    email.dispose();
    password.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          Padding(
            padding: const EdgeInsets.all(16.0),
            child: TextFormField(
              autocorrect: true,
              controller: email,
            ),
          ),
          ButtonForm(_logIn, "Hello"),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          showDialog(
              context: context,
              builder: (context) {
                return AlertDialog(
                  content: Text(email.text),
                );
              });
        },
        tooltip: "Show me the value",
        child: Icon(Icons.text_fields),
      ),
    );
  }
}

//Define a Custom Widget
class MyCustomForm extends StatefulWidget {
  @override
  _MyCustomFormState createState() => _MyCustomFormState();
}

Than I have a problem in the main class for our Button . When I pass the Function functionApply;

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

class ButtonForm extends StatelessWidget {
  final Function functionApply;

  final String textButton;

  ButtonForm(this.functionApply, this.textButton);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      child: ElevatedButton(
        child: Text(this.textButton),
        onPressed: this.functionApply, // I have a problem here!! 
      ),
    );
  }
}

Answer 1

onPressed is a type of VoidCallback

typedef VoidCallback = void Function()

So instead of using

final Function functionApply;

use

final VoidCallback functionApply;

So your ButtonForm will be

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

class ButtonForm extends StatelessWidget {
  final VoidCallback functionApply;

  final String textButton;

  ButtonForm(this.functionApply, this.textButton);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      child: ElevatedButton(
        child: Text(textButton),
        onPressed: functionApply, // Problem Solved!! 
      ),
    );
  }
}