Flutter Panable/Moveable/Draggable Widget

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      home: Stack(
        children: [
          PanableWidget(),
        ],
      ),
    ),
  );
}

/// A pan-able/move-able/draggable widget that doesn't pan/move/drag beyond the dimensions of the screen.
class PanableWidget extends StatefulWidget {
  @override
  _PanableWidgetState createState() => _PanableWidgetState();
}

class _PanableWidgetState extends State<PanableWidget> {
  double containerHeight = 100;
  double containerWidth = 100;

  double xPosition = 0;
  double yPosition = 0;
  Color color;

  @override
  void initState() {
    super.initState();
    color = Colors.blue;
  }

  @override
  Widget build(BuildContext context) {
    double screenWidth = MediaQuery.of(context).size.width;
    double screenHeight = MediaQuery.of(context).size.height;

    return Positioned(
      top: yPosition,
      left: xPosition,
      child: GestureDetector(
        onPanUpdate: (tapInfo) {
          setState(() {
            xPosition = _isXCoordinateMoreThanScreenWidth(tapInfo, screenWidth)
                ? screenWidth - containerWidth // stick to right edge of the screen
                : _isXCoordinateLessThanZero(tapInfo)
                    ? 0 // stick to left edge of the screen
                    : xPosition + tapInfo.delta.dx;
            yPosition = _isYCoordinateMoreThanScreenHeight(tapInfo, screenHeight)
                ? screenHeight - containerHeight // stick to bottom edge of the screen
                : _isYCoordinateLessThanZero(tapInfo)
                    ? 0 // stick to top edge of the screen
                    : yPosition + tapInfo.delta.dy;
          });
        },
        child: Container(width: containerWidth, height: containerHeight, color: color),
      ),
    );
  }

  bool _isXCoordinateMoreThanScreenWidth(DragUpdateDetails tapInfo, double screenWidth) {
    return xPosition + containerWidth + tapInfo.delta.dx > screenWidth;
  }

  bool _isXCoordinateLessThanZero(DragUpdateDetails tapInfo) => xPosition + tapInfo.delta.dx <= 0;

  bool _isYCoordinateMoreThanScreenHeight(DragUpdateDetails tapInfo, double screenHeight) {
    return yPosition + containerHeight + tapInfo.delta.dy > screenHeight;
  }

  bool _isYCoordinateLessThanZero(DragUpdateDetails tapInfo) => yPosition + tapInfo.delta.dy <= 0;
}

DartPad:
https://dartpad.dev/1461e9b4b297ed3cb125f54aa638c633?

Demo GIF: