FractionalOffset alignment

How to align the child.

The x and y values of the alignment control the horizontal and vertical alignment, respectively. An x value of 0.0 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 0.5 means that the center of the child is aligned with the center of the parent.

Source 

FractionalOffset get alignment => _alignment;
void alignment=(FractionalOffset newAlignment)

Sets the alignment to a new value, and triggers a layout update.

The new alignment must not be null or have any null properties.

Source 

set alignment (FractionalOffset newAlignment) {
  assert(newAlignment != null && newAlignment.dx != null && newAlignment.dy != null);
  if (_alignment == newAlignment)
    return;
  _alignment = newAlignment;
  markNeedsLayout();
}