getChildUsableCrossAxisExtent method
Return usable cross-axis extent of each child.
It doesn't contain crossAxisSpacing.
Implementation
double getChildUsableCrossAxisExtent(SliverConstraints constraints) {
final int crossAxisCount = getCrossAxisCount(constraints);
final double usableCrossAxisExtent = max(0.0,
constraints.crossAxisExtent - crossAxisSpacing * (crossAxisCount - 1));
return usableCrossAxisExtent / crossAxisCount;
}