Detect `TextField` focus/unfocus (focus and blur in HTML terminology)

textfield_focus.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  // Data will be available here
  final _inputController = TextEditingController();
  // Focus will be manipulated thru this
  final _inputFocus = FocusNode();

  // This is what we display and mutate when there is update in focus
  bool focused = false;

  @override
  void initState() {
    super.initState();

    _inputFocus.addListener(() {
      // We'll be notified here when there is a change in focus
      print('focus change: ${_inputFocus.hasFocus}');
      setState(() {
        focused = _inputFocus.hasFocus;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        // This allows losing the focus when the user taps on anything else
        FocusScopeNode currentFocus = FocusScope.of(context);
        if (!currentFocus.hasPrimaryFocus) {
          currentFocus.unfocus();
        }
      },
      child: Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text(
                'Focus:',
              ),
              Text(
                '$focused',
                style: Theme.of(context).textTheme.headline4,
              ),
              Container(height: 16.0),
              Padding(
                padding: const EdgeInsets.all(16.0),
                child: TextField(
                  controller: _inputController,
                  focusNode: _inputFocus,
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}