main.dart

class SecondScreen extends StatefulWidget {
  @override
  _SecondScreenState createState() => _SecondScreenState();
}

class _SecondScreenState extends State<SecondScreen> {
  final TextEditingController _controller = TextEditingController();

  String savedText = "";

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: () async {
        if (_controller.text == savedText) {
          return true;
        }
        final result = await showDialog(
          context: context,
          child: AlertDialog(
            title: Text("Are you sure?"),
            content: Text("All unsaved changes would be lost"),
            actions: <Widget>[
              FlatButton(
                child: Text('No'),
                onPressed: () {
                  Navigator.pop(context, false);
                },
              ),
              FlatButton(
                child: Text('Yes', style: TextStyle(color: Colors.red)),
                onPressed: () {
                  Navigator.pop(context, true);
                },
              ),
            ],
          ),
        );
        return result;
      },
      child: Scaffold(
        resizeToAvoidBottomInset: false,
        appBar: AppBar(),
        body: Container(
          margin: EdgeInsets.symmetric(horizontal: 16),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text("Saved text: $savedText"),
              SizedBox(height: 16),
              TextField(controller: _controller),
              SizedBox(height: 16),
              RaisedButton(
                child: Text("Save"),
                onPressed: () {
                  setState(() {
                    savedText = _controller.text;
                  });
                },
              ),
            ],
          ),
        ),
      ),
    );
  }
}

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https://gist.github.com/GursheeshSingh/b94df59b1434dcebce9178eac38102c3?permalink_comment_id=5012686#gistcomment-5012686