calculus-provider-calculator-input.dart

class CalculatorInput extends StatelessWidget { // StatefulWidget replaced with StatelessWidget
  @override
  Widget build(BuildContext context) {
    CalculatorState state = Provider.of<CalculatorState>(context);

    final operations = <Operation>[Add, Sub, Mul, Div, Sqrt];

    return Container(
      child: Column(
        children: <Widget>[
          Row(
            children: <Widget>[
              Text('Operation', style: Theme.of(context).textTheme.bodyText2),
              DropdownButton(
                value: state.operation, // values read directly from state
                items: operations
                    .map((entry) =>
                        DropdownMenuItem(child: Text(entry.name), value: entry))
                    .toList(),
                onChanged: state.setOperation,
              ),
            ],
          ),
          OperandInput(label: 'Operand A', operand: Operand.A),
          state.operation == Sqrt
              ? SizedBox(height: 68)
              : OperandInput(label: 'Operand B', operand: Operand.B),
          Container(
              child: Text('${state.format} = ${state.value.toStringAsFixed(3)}')),
        ],
      ),
    );
  }
}