import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      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> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeMetrics() {
    final bottomPadding = WidgetsBinding.instance.window.viewInsets.bottom;
    final isKeyboardVisible = bottomPadding > 0;
    print(
      'isKeyboardVisible: $isKeyboardVisible, bottomPadding: $bottomPadding',
    );
    if (!isKeyboardVisible && focusNode.hasFocus) {
       focusNode.unfocus();
    }
  }

  final focusNode = FocusNode();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              GestureDetector(
                behavior: HitTestBehavior.translucent,
                onTap: () => focusNode.unfocus(),
                child: Container(
                  height: 200,
                  color: Colors.red,
                  child:
                      Center(child: Text('Click here to unfocus text field')),
                ),
              ),
              TextField(focusNode: focusNode),
            ],
          ),
        ));
  }
}