calculator.dart

@override
Widget build(BuildContext context) {
  return new Scaffold(
    backgroundColor: Theme.of(context).primaryColor,
    body: SafeArea(
      child: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Column(
          children: <Widget>[
            Expanded(
              child: Container(
                child: Align(
                  alignment: Alignment.bottomRight,
                  child: Column(
                    crossAxisAlignment: CrossAxisAlignment.end,
                    mainAxisAlignment: MainAxisAlignment.end,
                    children: <Widget>[
                      operand1 != null
                          ? SingleChildScrollView(
                              scrollDirection: Axis.horizontal,
                              child: Text(
                                operand1 is double
                                    ? operand1.toStringAsFixed(2)
                                    : operand1.toString(),
                                style: _whiteTextStyle,
                                textAlign: TextAlign.right,
                              ),
                            )
                          : Container(),
                      operator != null
                          ? Text(
                              operator.toString(),
                              style: _whiteTextStyle,
                              textAlign: TextAlign.right,
                            )
                          : Container(),
                      operand2 != null
                          ? Text(
                              operand2.toString(),
                              style: _whiteTextStyle,
                              textAlign: TextAlign.right,
                            )
                          : Container(),
                      result != null
                          ? Divider(
                              height: 5.0,
                              color: Colors.white,
                            )
                          : Container(),
                      result != null
                          ? SingleChildScrollView(
                              scrollDirection: Axis.horizontal,
                              child: Text(
                                result is double
                                    ? result.toStringAsFixed(2)
                                    : result.toString(),
                                style: _whiteTextStyle,
                                textAlign: TextAlign.right,
                              ),
                            )
                          : Container(),
                    ],
                  ),
                ),
              ),
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                UnaryOperatorButton(
                  text: "AC",
                  onPressed: () {
                    _otherOperationAction(OtherOperation.clear);
                  },
                ),
                UnaryOperatorButton(
                  text: plus_or_minus_sign,
                  onPressed: (){_unaryOperationAction(UnaryOperation.changeSign);},
                ),
                UnaryOperatorButton(
                  text: percent_sign,
                  onPressed: (){_unaryOperationAction(UnaryOperation.percent);},
                ),
                BinaryOperatorButton(
                  text: divide_sign,
                  onPressed: () {
                    _binaryOperationAction(BinaryOperation.divide);
                  },
                )
              ],
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                NumberButton(
                    text: "7",
                    onPressed: () {
                      _numberButtonAction("7");
                    }),
                NumberButton(
                    text: "8",
                    onPressed: () {
                      _numberButtonAction("8");
                    }),
                NumberButton(
                    text: "9",
                    onPressed: () {
                      _numberButtonAction("9");
                    }),
                BinaryOperatorButton(
                  text: multiply_sign,
                  onPressed: () {
                    _binaryOperationAction(BinaryOperation.multiply);
                  },
                )
              ],
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                NumberButton(
                    text: "4",
                    onPressed: () {
                      _numberButtonAction("4");
                    }),
                NumberButton(
                    text: "5",
                    onPressed: () {
                      _numberButtonAction("5");
                    }),
                NumberButton(
                    text: "6",
                    onPressed: () {
                      _numberButtonAction("6");
                    }),
                BinaryOperatorButton(
                  text: minus_sign,
                  onPressed: () {
                    _binaryOperationAction(BinaryOperation.subtract);
                  },
                )
              ],
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                NumberButton(
                    text: "1",
                    onPressed: () {
                      _numberButtonAction("1");
                    }),
                NumberButton(
                    text: "2",
                    onPressed: () {
                      _numberButtonAction("3");
                    }),
                NumberButton(
                    text: "3",
                    onPressed: () {
                      _numberButtonAction("3");
                    }),
                BinaryOperatorButton(
                  text: add_sign,
                  onPressed: () {
                    _binaryOperationAction(BinaryOperation.add);
                  },
                )
              ],
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                ZeroButton(onPressed: (){_zeroButtonAction();},),
                BinaryOperatorButton(
                  text: ".",
                  onPressed: () {
                    _otherOperationAction(OtherOperation.addDecimal);
                  },
                ),
                BinaryOperatorButton(
                  text: equal_sign,
                  onPressed: () {
                    _otherOperationAction(OtherOperation.equals);
                  },
                )
              ],
            ),
          ],
        ),
      ),
    ),
  );