calculus-use-case-calculator-input-with-operation.dart

class _CalculatorInputState extends State<CalculatorInput> {
  // Map replaced with the list
  final operations = <Operation>[Add, Sub, Mul, Div, Sqrt];
  ...
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        children: <Widget>[
          Row(
            children: <Widget>[
              DropdownButton(
                // items builder updated
                items: operations
                    .map((entry) =>
                        DropdownMenuItem(child: Text(entry.name), value: entry))
                    .toList(),
              ),
            ],
          ),
          OperandInput(label: 'Operand A', ...)
          // an instance of Operation used to switch UI view 
          widget.useCase.operation == Sqrt
              ? SizedBox(height: 68)
              : OperandInput(label: 'Operand B', ...),
        ],
      ),
    );
  }
}