How to align the child.
The x and y values of the alignment control the horizontal and vertical alignment, respectively. An x value of -1.0 means that the left edge of the child is aligned with the left edge of the parent whereas an x value of 1.0 means that the right edge of the child is aligned with the right edge of the parent. Other values interpolate (and extrapolate) linearly. For example, a value of 0.0 means that the center of the child is aligned with the center of the parent.
Defaults to Alignment.center.
final AlignmentGeometry alignment