antklim

typedef Operator = num Function(num) Function(num);
typedef OperatorFmt = String Function(num) Function(num);

class Operation {
  final String name;
  final Operator oper;
  final OperatorFmt format;

  Operation({this.name, this.oper, this.format});

antklim

class CalculatorUseCase {
  num memory;
  Operation operation; // an instance of Operation class
  num operandA;
  num operandB;

  CalculatorUseCase(
      {Operation operation, this.operandA = 0, this.operandB = 0}) {
    this.operation = operation ?? Add;
  }

antklim

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(

antklim

class CalculatorUseCase {
  ...
  // callbacks to notify state changes
  void Function() onMemoryChanged;
  void Function() onOperationChanged;

  CalculatorUseCase(
      {Operation operation, 
      this.operandA = 0,
      this.operandB = 0,

antklim

class CalculatorState extends ChangeNotifier {
  CalculatorUseCase _useCase;

  CalculatorState() {
    _useCase = CalculatorUseCase(
        onMemoryChanged: onMemoryChanged,
        onOperationChanged: onOperationChanged);
  }

  void onMemoryChanged() => notifyListeners();

antklim

class CalculusScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<CalculatorState>(
        create: (_) => CalculatorState(),
        builder: (context, _) => CalculusScreenContainer());
  }
}

class CalculusScreenContainer extends StatelessWidget {

antklim

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>[

antklim

void main() {
  group('Calculus screen', () {
    ...
    testWidgets('has one operand when operation is SQRT', (WidgetTester tester) async {
      // initiate required state
      CalculatorState state = CalculatorState();
      state.setOperation(Sqrt);

      await tester.pumpWidget(MaterialApp(
        home: ChangeNotifierProvider<CalculatorState>(

antklim

type HTTPClient interface {
	Do(req *http.Request) (*http.Response, error)
}

func Routes(client HTTPClient) map[string]http.HandlerFunc {
	return map[string]http.HandlerFunc{
		"/do":     doHandler(),
		"/remote": remoteHandler(client),
		"/":       notFoundHandler,
	}

antklim

func Start(address string) error {
	mux := http.NewServeMux()
	client := &http.Client{}

	for route, handler := range Routes(client) {
		mux.Handle(route, handler)
	}

	s := &http.Server{
		Addr:           address,