flutter transparent bg

main.dart

import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart' hide Draggable;

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

class App extends StatefulWidget {
  const App({Key key}) : super(key: key);

  @override
  AppState createState() {
    return AppState();
  }
}

class AppState extends State<App> {
  int counter;

  @override
  void initState() {
    super.initState();
    counter = 0;
  }

  @override
  Widget build(BuildContext context) {
    return Align(
      child: SizedBox(
        child: Draggable(
          child: GestureDetector(
            child: Opacity(
              child: DecoratedBox(
                child: Align(
                  child: Text(
                    'Taps: $counter',
                    style: TextStyle(
                      color: Color(0xFFFFFFFF),
                      fontSize: 24.0,
                      shadows: <BoxShadow>[
                        BoxShadow(
                          blurRadius: 2.0,
                          color: Color(0xFFFFFFFF),
                        ),
                      ],
                    ),
                    textDirection: TextDirection.ltr,
                  ),
                ),
                decoration: BoxDecoration(
                  borderRadius: BorderRadius.all(Radius.circular(100.0)),
                  color: Color(0xFFFF0000),
                  boxShadow: <BoxShadow>[
                    BoxShadow(
                      blurRadius: 20.0,
                      color: Color(0xFFFF0000),
                    ),
                  ],
                ),
              ),
              opacity: 0.5,
            ),
            onTap: () {
              setState(() {
                counter++;
              });
            },
          ),
        ),
        height: 200.0,
        width: 200.0,
      ),
    );
  }
}

class Draggable extends StatelessWidget {
  static const platformChannelDraggable = const MethodChannel('ykmnkmi.dev/draggable');

  final Widget child;

  Draggable({@required this.child});

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      child: child,
      onPanStart: onPanStart,
      onPanUpdate: onPanUpdate,
    );
  }

  void onPanUpdate(DragUpdateDetails details) async {
    await platformChannelDraggable.invokeMethod('onPanUpdate');
  }

  void onPanStart(DragStartDetails details) async {
    await platformChannelDraggable.invokeMethod('onPanStart', {
      'dx': details.globalPosition.dx,
      'dy': details.globalPosition.dy,
    });
  }
}