FLUTTER => using inputFormatters for things other than inputFormatting

main.dart

import 'dart:async';

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

void main() => runApp(
    MaterialApp(home: new MyApp())
);

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() {
    return new MyAppState();
  }
}

class MyAppState extends State<MyApp> {
  
  final FocusNode emailFocusNode = new FocusNode();
  final KeyboardListener keyboardListen = new KeyboardListener();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Form(
          child: TextFormField(
            inputFormatters: <TextInputFormatter> [keyboardListen],
            focusNode: emailFocusNode,
            decoration: new InputDecoration(
              labelText: "Email",
              hintText: 'you@swol.com',
            ),
            //onFieldSubmitted: (value) => focusEmail(),
          ),
        ),
      ),
    );
  }
}

class KeyboardListener extends TextInputFormatter {

  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue,
      TextEditingValue newValue,
      ){
    print("find key pressed from a bit of processing with these two values\n " + oldValue.text + " vs " + newValue.text);
    return newValue;
  }
}