verticalDirection property
Determines the order to lay children out vertically and how to interpret
start and end in the vertical direction.
If the direction is Axis.vertical, this controls which order children are painted in (down or up), the meaning of the alignment property's WrapAlignment.start and WrapAlignment.end values.
If the direction is Axis.vertical, and either the alignment is either WrapAlignment.start or WrapAlignment.end, or there's more than one child, then the verticalDirection must not be null.
If the direction is Axis.horizontal, this controls the order in which runs are positioned, the meaning of the runAlignment property's WrapAlignment.start and WrapAlignment.end values, as well as the crossAxisAlignment property's WrapCrossAlignment.start and WrapCrossAlignment.end values.
If the direction is Axis.horizontal, and either the runAlignment is either WrapAlignment.start or WrapAlignment.end, the crossAxisAlignment is either WrapCrossAlignment.start or WrapCrossAlignment.end, or there's more than one child, then the verticalDirection must not be null.
Implementation
final VerticalDirection verticalDirection;