calculus-logic-calculator-input.dart

enum Operand { A, B } // Added to identify operands

class CalculatorInput extends StatefulWidget {  
  final num memory;                  // Memory state passed by parent widget
  final ValueChanged<num> onChanged; // Calculator state listener

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

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

class _CalculatorInputState extends State<CalculatorInput> {
  ...

  // Added to handle operand values
  num operandA = 0;
  num operandB = 0;

  void onOperationChanged(Operation newOperation) {
    ...
    widget.onChanged(calculatorValue);
  }

  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);
      };

  num get calculatorValue {
    switch (operation) {
      case Operation.addition:
        return operandA + operandB;
      case Operation.subtraction:
        return operandA - operandB;
      case Operation.multiplication:
        return operandA * operandB;
      case Operation.division:
        return operandA / operandB;
      case Operation.sqrt:
        return sqrt(operandA);
      default:
        return 0;
    }
  }

  String get calculatorFormat {
    switch (operation) {
      case Operation.addition:
        return '$operandA + $operandB';
      case Operation.subtraction:
        return '$operandA - $operandB';
      case Operation.multiplication:
        return '$operandA * $operandB';
      case Operation.division:
        return '$operandA / $operandB';
      case Operation.sqrt:
        return 'sqrt($operandA)';
      default:
        return '';
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: const EdgeInsets.only(top: 10),
      child: Column(
        children: <Widget>[
          ...
          OperandInput(
              label: 'Operand A',
              initValue: operandA,
              memory: widget.memory,
              onChanged: onOperandChanged(Operand.A)),
          operation == Operation.sqrt
              ? SizedBox(height: 68)
              : OperandInput(
                  label: 'Operand B',
                  initValue: operandB,
                  memory: widget.memory,
                  onChanged: onOperandChanged(Operand.B)),
          Container(
              margin: const EdgeInsets.only(top: 10, bottom: 20),
              child: Text(
                  '$calculatorFormat = ${calculatorValue.toStringAsFixed(3)}')), // Output operation result
        ],
      ),
    );
  }
}