Bottom Sheet gesture

sheet_gesture.dart

 void _handleDragUpdate(DragUpdateDetails details) {
    if (_dismissUnderway) return;

    var change = details.primaryDelta / (_childHeight ?? details.primaryDelta);
    if (_isDirectionTop())
      _animationController.value += change;
    else
      _animationController.value -= change;
  }

  void _handleDragEnd(DragEndDetails details) {
    if (_dismissUnderway) return;

    if (details.velocity.pixelsPerSecond.dy > 0 && _isDirectionTop()) return;
    if (details.velocity.pixelsPerSecond.dy < 0 && !_isDirectionTop()) return;

    if (details.velocity.pixelsPerSecond.dy > 700) {
      final double flingVelocity =
          -details.velocity.pixelsPerSecond.dy / _childHeight;
      if (_animationController.value > 0.0)
        _animationController.fling(velocity: flingVelocity);
    } else if (_animationController.value < 0.5) {
      if (_animationController.value > 0.0)
        _animationController.fling(velocity: -1.0);
    } else
      _animationController.reverse();
  }