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;