calculus-logic-calculator-input.dart

class _CalculatorInputState extends State<CalculatorInput> {
  final operations = <Operation, String>{
    Operation.addition: 'Addition',
    ...
  };

  Operation operation = Operation.addition;
  num operandA = 0;
  num operandB = 0;

  void onOperationChanged(Operation newOperation) {
    setState(() {
      operation = newOperation;
    });
    widget.onChanged(calculatorValue); // notify parent about state change
  }

  ValueChanged<String> onOperandChanged(Operand operand) => (String v) {
        double value = double.tryParse(v) ?? 0;
        setState(() {
          if (operand == Operand.A) operandA = value;
          if (operand == Operand.B) operandB = value;
        });
        widget.onChanged(calculatorValue); // notify parent about state change
      };

  ... // calculator value and format getters

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        children: <Widget>[
          Row(
            children: <Widget>[
              Text('Operation', style: Theme.of(context).textTheme.bodyText2),
              DropdownButton(
                value: operation,
                items: operations.entries
                    .map((entry) => DropdownMenuItem(
                        child: Text(entry.value), value: entry.key))
                    .toList(),
                onChanged: onOperationChanged,
              ),
            ],
          ),
          OperandInput(
              label: 'Operand A',
              initValue: operandA,
              memory: widget.memory,
              onChanged: onOperandChanged(Operand.A)),
          operation == Operation.sqrt
              ? SizedBox(height: 68)
              : OperandInput(label: 'Operand B', ...),
          Container(
              child: Text('$calculatorFormat = ${calculatorValue.toStringAsFixed(3)}')),
        ],
      ),
    );
  }
}