calculus-logic-memory.dart

class CalculusScreen extends StatefulWidget {
  @override
  _CalculusScreenState createState() => _CalculusScreenState();
}

class _CalculusScreenState extends State<CalculusScreen> {
  num memory;
  num calculatorResult; // current operation result

  void onMemorise() {
    setState(() {
      memory = calculatorResult;
    });
  }

  void onResetMemory() {
    setState(() {
      memory = null;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      ...
      // memory and callbacks passed to nested elements
      MemoryInfo(memory: memory),
      MemoryManagement(onMemorise: onMemorise, onResetMemory: onResetMemory),
    );
  }
}