I have a draggable like such (a is a StatefulWidget):

Draggable(child: a, feedback: a, childWhenDragging: null, data: a)

However, a has a GlobalKey in an effort to preserve state when the widget is dropped. My issue is that when you attempt to drag, a duplicate GlobalKey error is thrown (probably because child and feedback exist on the same frame). Is there any way to achieve this behavior without any errors?

Reproducible example:

import 'package:flutter/material.dart';

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

class SFWidget extends StatefulWidget {
  const SFWidget({Key? key}) : super(key: key);
  @override
  State<SFWidget> createState() => SFWidgetState();
}

class SFWidgetState extends State<SFWidget> {
  String str = "str";
  @override
  Widget build(BuildContext context) {
    return Text(str);
  }

  void edit(String newstr) {
    setState(() {
      str = newstr;
    });
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final key = GlobalKey<SFWidgetState>();
  final widget = SFWidget(key: key);
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Draggable(
        child: widget,
        feedback: widget,
        data: widget,
        childWhenDragging: null,
      ),
    );
  }
}

CodePudding user response：

The problem is that the same widget stays in the starting place if childWhenDragging is null. From the official docs:

If childWhenDragging is null, then the Draggable widget will always display child (and so the drag source representation will not change while a drag is underway).

So, to fix it just default the childWhenDragging to something that does nothing like Container(). Something like below:

      ...

      body: Draggable(
        child: widget,
        feedback: widget,
        data: widget,
        childWhenDragging: Container(), // <- Here
      ),

      ...