How to listen focus change in flutter?

In addtion you can use Focus widget.

Focus(
  child: TextFormField(...),
  onFocusChange: (hasFocus) {
    if(hasFocus) {
      // do stuff
    }
  },
)

Using FocusNode

Here is a fully correct answer. addListener shall be in initState(), not build(), because that would result in adding a listener every time a widget is built and you most likely don't want that.

import 'package:flutter/material.dart';

class SomeWidget extends StatefulWidget {
  @override
  _SomeWidgetState createState() => _SomeWidgetState();
}

class _SomeWidgetState extends State<SomeWidget> {
  final _focusNode = FocusNode();
    
  @override
  void initState() {
    super.initState();
    _focusNode.addListener(() {
      print("Has focus: ${_focusNode.hasFocus}");
    });
  }
    
  @override
  Widget build(BuildContext context) {
    return TextField(focusNode: _focusNode);
  }
    
  @override
  void dispose() {
    _focusNode.dispose();
    super.dispose();
  }
}

Using flutter_hooks package

If you're by any chance using the flutter_hooks package, there is a dedicated useFocusNode hook.

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

class SomeWidget extends HookWidget {
  @override
  Widget build(BuildContext context) {
    final focusNode = useFocusNode();
    useEffect(() {
      focusNode.addListener(() {
        print("Has focus: ${focusNode.hasFocus}");
      });
      return; // You need this return if you have missing_return lint
    }, [focusNode]);

    return TextField(focusNode: focusNode);
  }
}