This is the full code sample used in my "Flutter Draggable" example on Instagram.

Flutter - Draggable

import 'package:flutter/material.dart';

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

class ExampleApp extends StatelessWidget {
  final Widget icon = Icon(Icons.sentiment_very_satisfied, size: 100.0);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: [
              Draggable<String>(
                data: "face",
                child: icon,
                childWhenDragging: Opacity(
                  opacity: 0.5,
                  child: icon,
                ),
                feedback: RotationTransition(
                  turns: AlwaysStoppedAnimation(15 / 360),
                  child: icon,
                ),
              ),
              Container(
                width: 100.0,
                height: 100.0,
                color: Color(0xff8fc3ff),
                child: DragTarget<String>(
                  builder: (context, candidateData, rejectedData) {
                    return Container();
                  },
                  onWillAccept: (data) {
                    if (data == "face") {
                      print('Will be accepted.');
                      return true;
                    } else {
                      print('Will be rejected.');
                      return false;
                    }
                  },
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}