Flutter draggable widget

w_draggable.dart

/// Draggable widget
class _WDraggable extends StatefulWidget {
  const _WDraggable({Key key, this.child, this.valueChanged}) : super(key: key);

  final Widget child;
  final ValueChanged<Offset> valueChanged;

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

class _WDraggableState extends State<_WDraggable> {
  ValueNotifier<Offset> valueListener = ValueNotifier<Offset>(const Offset(1, 1));

  @override
  void initState() {
    valueListener.addListener(_notifyParent);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Builder(
      builder: (BuildContext context) {
        final GestureDetector handle = GestureDetector(
          onPanUpdate: (DragUpdateDetails details) {
            final double dx = (valueListener.value.dx +
                (details.delta.dx / context.size.width))
                    .clamp(0.0, 1.0)
                    .toDouble();
            final double dy = (valueListener.value.dy -
                (details.delta.dy / context.size.height))
                    .clamp(0.0, 1.0)
                    .toDouble();
            valueListener.value = Offset(dx, dy);
          },
          child: widget.child,
        );

        return AnimatedBuilder(
          animation: valueListener,
          builder: (_, Widget child) {
            return Align(
              alignment: Alignment(valueListener.value.dx * 2 - 1,
                  1 - valueListener.value.dy * 2),
              child: child,
            );
          },
          child: handle,
        );
      },
    );
  }

  // Notify change to parent
  void _notifyParent() {
    if (widget.valueChanged != null) {
      widget.valueChanged(valueListener.value);
    }
  }
}

w_draggable_h.dart

import 'package:flutter/material.dart';

/// Draggable widget Horizontal only, constraining with parent size
class WDraggableH extends StatefulWidget {
  const WDraggableH(this.child, {Key key, this.valueChanged}) : super(key: key);

  final Widget child;
  final ValueChanged<double> valueChanged;

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

class _WDraggableHState extends State<WDraggableH> {
  ValueNotifier<double> valueListener = ValueNotifier<double>(0);

  @override
  void initState() {
    valueListener.addListener(_notifyParent);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Builder(
      builder: (BuildContext context) {
        final GestureDetector handle = GestureDetector(
          onHorizontalDragUpdate: (DragUpdateDetails details) {
            valueListener.value =
                (valueListener.value + (details.delta.dx / context.size.width))
                    .clamp(0.0, 1.0)
                    .toDouble();
          },
          child: widget.child,
        );

        return AnimatedBuilder(
          animation: valueListener,
          builder: (_, Widget child) {
            return Align(
              alignment: Alignment(valueListener.value * 2 - 1, 0),
              child: child,
            );
          },
          child: handle,
        );
      },
    );
  }

  // Notify change to parent
  void _notifyParent() {
    if (widget.valueChanged != null) {
      widget.valueChanged(valueListener.value);
    }
  }
} 

w_draggable_v.dart

import 'package:flutter/material.dart';

/// Draggable widget Vertical only, constraining with parent size
class WDraggableV extends StatefulWidget {
  const WDraggableV({Key key, this.child, this.valueChanged}) : super(key: key);

  final Widget child;
  final ValueChanged<double> valueChanged;

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

class _WDraggableVState extends State<WDraggableV> {
  ValueNotifier<double> valueListener = ValueNotifier<double>(0);

  @override
  void initState() {
    valueListener.addListener(_notifyParent);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Builder(
      builder: (BuildContext context) {
        final GestureDetector handle = GestureDetector(
          onVerticalDragUpdate: (DragUpdateDetails details) {
            valueListener.value =
                (valueListener.value - (details.delta.dy / context.size.height))
                    .clamp(0.0, 1.0)
                    .toDouble();
          },
          child: widget.child,
        );

        return AnimatedBuilder(
          animation: valueListener,
          builder: (_, Widget child) {
            return Align(
              alignment: Alignment(0, 1 - valueListener.value * 2),
              child: child,
            );
          },
          child: handle,
        );
      },
    );
  }

  // Notify change to parent
  void _notifyParent() {
    if (widget.valueChanged != null) {
      widget.valueChanged(valueListener.value);
    }
  }
}