calculus-state-use-case.dart

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

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

  ...
  
  void setOperation(Operation value) {
    operation = value;
    onOperationChanged?.call(); // notify state changes
  }

  void Function(num) setOperand(Operand operand) {
    onOperationChanged?.call();
    return (operand == Operand.A) ? _setOperandA : _setOperandB;
  }

  void memorise() {
    memory = value;
    onMemoryChanged?.call(); // notify state changes
  }

  void resetMemory() {
    memory = null;
    onMemoryChanged?.call(); // notify state changes
  }
}