I want to get the x and y coordinates as long as my finger travels inside the container.

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> {
  final key = GlobalKey();
  // ignore: non_constant_identifier_names
  double X_Position = 0.00;
  // ignore: non_constant_identifier_names
  double Y_Position = 0.00;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: GestureDetector(
        onPanStart: (details) {
          RenderBox box = key.currentContext?.findRenderObject() as RenderBox;
          Offset position = box.localToGlobal(Offset.zero);
          setState(() {
            X_Position = position.dx;
            Y_Position = position.dy;
          });
        },
        onPanUpdate: (details) {
          RenderBox box = key.currentContext?.findRenderObject() as RenderBox;
          Offset position = box.localToGlobal(Offset.zero);
          setState(() {
            X_Position = position.dx;
            Y_Position = position.dy;
          });
        },
        child: Container(
          width: 500,
          height: 500,
          decoration: BoxDecoration(
            border: Border.all(
              color: Colors.black,
              width: 2,
            ),
          ),
        ),
      ),
    );
  }
}

In the above code, I want to get the x and y coordinates according to the movement of my finger inside the container. But I am getting the following error. can you help?

CodePudding user response：

You must give the key to the container as a parameter

Container( key: key, ),

CodePudding user response：

The key isn't attached to any widget:

Attach it like so:

Container(
  key: key,
  width: 500,
  height: 500,
  decoration: BoxDecoration(
     border: Border.all(
       color: Colors.black,
       width: 2,
     ),
   ),
)

EDIT

Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: GestureDetector(
        onPanStart: (details) {
          RenderBox box = key.currentContext?.findRenderObject() as RenderBox;
          Offset position = box.localToGlobal(Offset.zero);
          setState(() {
            X_Position = position.dx;
            Y_Position = position.dy;
          });
        },
        onPanUpdate: (details) {
          RenderBox box = key.currentContext?.findRenderObject() as RenderBox;
          Offset position = box.localToGlobal(Offset.zero);
          setState(() {
            X_Position = position.dx;
            Y_Position = position.dy;
          });
        },
        child: Container(
          width: 500,
          height: 500,
          decoration: BoxDecoration(
            border: Border.all(
              color: Colors.black,
              width: 2,
            ),
          ),
        ),
      ),
    );
  }

This code above will always have the Container at Offset(0,0) that's why the values of X_Position and Y_Position is always 0.

To see changes, put the Container in a Center

Like so:

Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: GestureDetector(
        onPanStart: (details) {
          RenderBox box = key.currentContext?.findRenderObject() as RenderBox;
          Offset position = box.localToGlobal(Offset.zero);
          setState(() {
            X_Position = position.dx;
            Y_Position = position.dy;
          });
        },
        onPanUpdate: (details) {
          RenderBox box = key.currentContext?.findRenderObject() as RenderBox;
          Offset position = box.localToGlobal(Offset.zero);
          setState(() {
            X_Position = position.dx;
            Y_Position = position.dy;
          });
        },
        child: Center(
          child: Container(
            width: 500,
            height: 500,
            decoration: BoxDecoration(
              border: Border.all(
                color: Colors.black,
                width: 2,
              ),
            ),
          ),
        ),
      ),
    );
  }

CodePudding user response：

I prefer just using DragUpdateDetails that comes from onPanUpdate.

body: GestureDetector(
  onTap: () {},
  onPanStart: (details) {
    Offset position = details.localPosition;
    setState(() {
      X_Position = position.dx;
      Y_Position = position.dy;
    });
  },
  onPanUpdate: (DragUpdateDetails details) {
    Offset position = details.localPosition;
    setState(() {
      X_Position = position.dx;
      Y_Position = position.dy;
    });
  },