calculus-use-case-calculator-input.dart

class CalculatorInput extends StatefulWidget {
  final CalculatorUseCase useCase;

  const CalculatorInput({Key key, this.useCase}) : super(key: key);

  @override
  _CalculatorInputState createState() => _CalculatorInputState();
}

class _CalculatorInputState extends State<CalculatorInput> {
  ...
  void onOperationChanged(Operation newOperation) {
    widget.useCase.setOperation(newOperation);
    setState(() {});
  }

  void onOperandChanged() {
    // rebuilding calculation result widget
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        children: <Widget>[
          Row(
            children: <Widget>[
              DropdownButton(
                ...
                value: widget.useCase.operation,
                onChanged: onOperationChanged,
              ),
            ],
          ),
          OperandInput(
              label: 'Operand A',
              operand: Operand.A,
              useCase: widget.useCase,
              onChanged: onOperandChanged),
          widget.useCase.operation == Operation.sqrt
              ? SizedBox(height: 68)
              : OperandInput(label: 'Operand B', ...),
          Container(
              child: Text('${widget.useCase.format} = ${widget.useCase.value.toStringAsFixed(3)}')),
        ],
      ),
    );
  }
}