customRouteBuilder property
This builder function is passed to customRouteBuilder property in CustomPage
I couldn't type this function from here but it should match typedef CustomRouteBuilder = Route Function(BuildContext context, CustomPage page); you should only reference the function when passing it so the generator can import it into the generated file
this builder function accepts a BuildContext and a CustomPage that has all the other properties assigned to it so using them then is totally up to you.
Implementation
final Function? customRouteBuilder;