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}) {
  if (child != null) {
    child!.layout(context, this.constraints.enforce(constraints),
        parentUsesSize: true);
    assert(child!.box != null);
    box = child!.box;
  } else {
    box = PdfRect.fromPoints(
        PdfPoint.zero, this.constraints.enforce(constraints).smallest);
  }
}