draggable_example.dart

import 'package:flutter/material.dart';
import 'package:flutter/painting.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: 'Draggable Example',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Draggable Example'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {

    bool accepted = false;

    final simpleBox = (String text, Color  color) => Padding(
      padding: EdgeInsets.only(left: 10.0, right: 10.0),
        child: Container(
          width: 90.0,
          height: 60.0,
          decoration: BoxDecoration(
            borderRadius: BorderRadius.circular(10.0),
            color: color
          ),
          child: Center(
            child:  Text(text, style: TextStyle(fontSize: 15.0, color: Colors.white, fontWeight: FontWeight.bold),)
        ),
      ),
    );

    final dashedBox = () => Padding(
      padding: EdgeInsets.only(left: 10.0, right: 10.0),
        child: Container(
          width: 90.0,
          height: 60.0,
          decoration: BoxDecoration(
            borderRadius: BorderRadius.circular(10.0),
            border: Border.all(),
            color: Colors.transparent
          ),
      ),
    );

    return Scaffold(      
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: <Widget>[
            Text("Draggable/DragTarget example", style: TextStyle(fontSize: 19.0),),
            SizedBox(height: 50.0,),

            // This is de main widget, It is the widget 
            // that provides the ability to drag towards 
            // a target in our case a 'DragTarget'
            Draggable(
              // Here are the data to send to 'Drag Target'
              // It can be of any type
              data: "Hello!",
              
              // You can set a default widget
              child: simpleBox('Dragme!', Colors.blue),

              // But also a widget that stays in the container
              childWhenDragging: simpleBox("Bye!", Colors.grey),

              // And a widget that will appear when you start to drag
              feedback: simpleBox('Yupi!', Colors.blue[300])
            ),


            SizedBox(height: 20.0,),

            // This is the target that receives the data 
            DragTarget<String>(
              // Called to build the contents of this widget
              builder: (context, candidates, rejects) {
                return accepted ? simpleBox('Thanks!', Colors.green) : dashedBox();
              },

              // Called to determine whether this widget is 
              // interested in receiving a given piece of data 
              // being dragged over this drag target.
              onWillAccept: (value){
                return true;
              },
              
              // Called when an acceptable piece of data was dropped over this drag target.
              onAccept: (value) {
                accepted = true;
              },

              // Called when a given piece of data being
              // dragged over this target leaves the target.
              onLeave: (value) {
                accepted = true;
              },              
            )
          ],
        )
      ),
    );
  }
}