获取userQuestion和userAnswer错误

Question

Dart不仅用于移动应用程序开发，而且是一种编程语言。由Ecma(ECMA-408)批准为标准，它用于构建web上的任何东西，服务器，桌面，当然还有移动应用程序（是的，正是这些人标准化了我们最喜欢的ES5和ES6。）

import 'package:flutter/material.dart';
import 'buttons.dart';
import 'package:math_expressions/math_expressions.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

bool isOperator(String x) {
  if (x == '%' || x == '/' || x == 'x' || x == '+' || x == '-' || x == '=') {
    return true;
  }
  return false;
}

void equalPressed() {
  String finalQuestion = userQuestion;
  finalQuestion = finalQuestion.replaceAll('x', '*');

  Parser p = Parser();
  Expression exp = p.parse(finalQuestion);
  ContextModel cm = ContextModel();
  double eval = exp.evaluate(EvaluationType.REAL, cm);

 userAnswer = eval.toString();
}

class _HomePageState extends State<HomePage> {
  var userQuestion = '';
  var userAnswer = '';

  final List<String> buttons = [
    'C',
    'DEL',
    '%',
    '/',
    '7',
    '8',
    '9',
    'x',
    '4',
    '5',
    '6',
    '-',
    '1',
    '2',
    '3',
    '+',
    '0',
    '00',
    '.',
    '=',
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color(0xFFE5CCFF),
      body: Column(
        children: <Widget>[
          Expanded(
            child: Container(
              child: Column(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    SizedBox(
                      height: 50,
                    ),
                    Container(
                      padding: EdgeInsets.all(20),
                      alignment: Alignment.centerLeft,
                      child: Text(
                        userQuestion,
                        style: TextStyle(fontSize: 20),
                      ),
                    ),
                    Container(
                      padding: EdgeInsets.all(20),
                      alignment: Alignment.centerRight,
                      child: Text(
                        userAnswer,
                        style: TextStyle(fontSize: 20),
                      ),
                    ),
                  ]),
            ),
          ),
          Expanded(
            flex: 2,
            child: Container(
              child: GridView.builder(
                  itemCount: buttons.length,
                  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
                      crossAxisCount: 4),
                  itemBuilder: (BuildContext context, int index) {
                    // clear button
                    if (index == 0) {
                      return MyButton(
                        buttonTapped: () {
                          setState(() {
                            userQuestion = '';
                          });
                        },
                        buttonText: buttons[index],
                        color: Colors.green,
                        textColor: Colors.white,
                      );
                    } else if (index == 1) {
                      // Delete Button
                      return MyButton(
                        buttonTapped: () {
                          setState(() {
                            userQuestion = userQuestion.substring(
                                0, userQuestion.length-1);
                          });
                        },
                        buttonText: buttons[index],
                        color: Colors.red,
                        textColor: Colors.white,
                      );
                    }
                    // equal button
                    else if (index == buttons.length-1) {
                      return MyButton(
                        buttonTapped: () {
                          setState(() {
                            equalPressed();
                          });
                        },
                        buttonText: buttons[index],
                        color: Colors.deepPurple,
                        textColor: Colors.white,
                      );
                    } else {
                      // Rest of the Button
                      return MyButton(
                        buttonTapped: () {
                          setState(() {
                            userQuestion += buttons[index];
                          });
                        },
                        buttonText: buttons[index],
                        color: isOperator(buttons[index])
                            ? Colors.deepPurple
                            : Colors.deepPurple[50],
                        textColor: isOperator(buttons[index])
                            ? Colors.white
                            : Colors.deepPurple,
                      );
                    }
                  }),
            ),
          ),
        ],
      ),
    );
  }
}

Answer 1

在_homePagestate中设置isoperator和equalpressed方法