anchor property Null safety
The relative position of the zero scroll offset.
For example, if anchor is 0.5 and the AxisDirection determined by scrollDirection and reverse is AxisDirection.down or AxisDirection.up, then the zero scroll offset is vertically centered within the viewport. If the anchor is 1.0, and the axis direction is AxisDirection.right, then the zero scroll offset is on the left edge of the viewport.
final double anchor;