Positioned widget with GestureDetector, Dragging a widget will go off the screen

Question

Im trying to Drag a container with GestureDetector, but when i dragg to the edges of the screen the container is going off the screen.

I wanted it to stop when it reaches the edges of the screen.

Here is the example code

import 'package:flutter/material.dart';

class DrawDraggableRectangle extends StatefulWidget {
  const DrawDraggableRectangle({super.key});

  @override
  State<DrawDraggableRectangle> createState() => _DrawDraggableRectangleState();
}

class _DrawDraggableRectangleState extends State<DrawDraggableRectangle> {
  double zoneHeight = 122;
  double zoneWidth = 112;
  double topY = 21;
  double leftX = 16;
  late double initX;
  late double initY;

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        Positioned(
          top: topY,
          left: leftX,
          child: GestureDetector(
            onPanStart: (DragStartDetails details) {
              setState(() {
                initX = details.globalPosition.dx;
                initY = details.globalPosition.dy;
              });
            },
            onPanUpdate: (DragUpdateDetails details) {
              final dx = details.globalPosition.dx - initX;
              final dy = details.globalPosition.dy - initY;
              initX = details.globalPosition.dx;
              initY = details.globalPosition.dy;
              setState(() {
                topY = topY + dy;
                leftX = leftX + dx;
              });
            },
            child: Container(
              height: zoneHeight,
              width: zoneWidth,
              //color: Colors.red.withOpacity(0.5),
              decoration: BoxDecoration(
                color: Colors.transparent,
                border: Border.all(
                    color: Colors.pink, // Set border color
                    width:
                        3.0), // Set border width// Make rounded corner of border
              ),
            ),
          ),
        ),
      ],
    );
  }
}

Answer 1

You can try to clamp() the values assigned to y and x to the boundaries of your screen. In the example below I used double.infinity for the upper value as we don't know exact size of the screen at this time.

            onPanUpdate: (DragUpdateDetails details) {
              final dx = details.globalPosition.dx - initX;
              final dy = details.globalPosition.dy - initY;
              initX = details.globalPosition.dx;
              initY = details.globalPosition.dy;
              setState(() {
                topY = (topY + dy).clamp(0.0, double.infinity);
                leftX = (leftX + dx).clamp(0.0, double.infinity);
              });
            },

See this in practice using DartPad

If you'd like to make sure that the Positioned stays within arbitrary bounds you may try to use LayoutBuilder around Stack to get the constraints. Then you would be able to clamp the right and bottom edge in a similar way.