builder method
Implementation
Widget builder(BuildContext context, BoxConstraints constraints) {
// Check if widget is visible before wasting resources on building it
if (!widget.model.visible) return const Offstage();
// set system sizing
// this may no necessary in the future
onLayout(constraints);
// rebuild content?
children = widget.builder(context, constraints);
// build the box
var view = _buildView(context, constraints, widget.model, children!);
// add margins
view = addMargins(view);
// apply visual transforms
view = applyTransforms(view);
// apply constraints
view = applyConstraints(view, widget.model.constraints);
return view;
}