layout method
      
  
void
layout(
    
- Context context,
- BoxConstraints constraints, {
- bool parentUsesSize = false,
override
    First widget pass to calculate the children layout and bounding box
Implementation
@override
void layout(Context context, BoxConstraints constraints,
    {bool parentUsesSize = false}) {
  box = PdfRect.fromPoints(PdfPoint.zero, constraints.smallest);
  if (child != null) {
    child!.layout(context, _getInnerConstraints(constraints),
        parentUsesSize: true);
    assert(child!.box != null);
    final resolvedAlignment = alignment.resolve(Directionality.of(context));
    child!.box = resolvedAlignment.inscribe(child!.box!.size, box!);
  }
}