// 三元运算符
int a = 10;
int b = 20;
int max = (a > b) ? a : b; // max = 20

// 空值合并运算符
String? name; // 可空类型
String displayName = name ?? 'Guest'; // 如果 name 为 null，则 displayName 为 'Guest'

// 不使用级联运算符
var sb = StringBuffer();
sb.write('Hello');
sb.write(' ');
sb.write('World');
print(sb.toString()); // 输出: Hello World

// 使用级联运算符
var sb2 = StringBuffer()
  ..write('Hello')
  ..write(' ')
  ..write('World');
print(sb2.toString()); // 输出: Hello World

import 'package:flutter/material.dart';

class CalculatorApp extends StatefulWidget {
  const CalculatorApp({super.key});

  @override
  State<CalculatorApp> createState() => _CalculatorAppState();
}

class _CalculatorAppState extends State<CalculatorApp> {
  String _output = '0';
  String _currentNumber = '';
  double _num1 = 0.0;
  double _num2 = 0.0;
  String _operator = '';
  bool _isOperatorPressed = false;

  void _buttonPressed(String buttonText) {
    if (buttonText == 'CLEAR') {
      _output = '0';
      _currentNumber = '';
      _num1 = 0.0;
      _num2 = 0.0;
      _operator = '';
      _isOperatorPressed = false;
    } else if (buttonText == '+' ||
        buttonText == '-' ||
        buttonText == '×' ||
        buttonText == '÷') {
      _num1 = double.parse(_output);
      _operator = buttonText;
      _isOperatorPressed = true;
      _currentNumber = ''; // 清空当前数字，准备输入下一个数字
    } else if (buttonText == '.') {
      if (_currentNumber.contains('.')) {
        return; // 已经包含小数点，不再添加
      }
      _currentNumber += buttonText;
    } else if (buttonText == '=') {
      _num2 = double.parse(_currentNumber);

      if (_operator == '+') {
        _output = (_num1 + _num2).toString();
      } else if (_operator == '-') {
        _output = (_num1 - _num2).toString();
      } else if (_operator == '×') {
        _output = (_num1 * _num2).toString();
      } else if (_operator == '÷') {
        // 使用条件运算符处理除数为零的情况
        _output = (_num2 != 0) ? (_num1 / _num2).toString() : 'Error';
      }

      _num1 = double.parse(_output); // 将结果作为第一个操作数，方便连续计算
      _operator = '';
      _currentNumber = _output; // 将结果显示在当前数字中
      _isOperatorPressed = false;
    } else {
      // 如果是数字键
      if (_output == '0' || _isOperatorPressed) {
        _output = buttonText;
        _isOperatorPressed = false;
      } else {
        _output += buttonText;
      }
      _currentNumber += buttonText;
    }

    setState(() {});
  }

  Widget _buildButton(String buttonText) {
    return Expanded(
      child: OutlinedButton(
        style: OutlinedButton.styleFrom(
          padding: const EdgeInsets.all(24.0),
        ),
        onPressed: () => _buttonPressed(buttonText),
        child: Text(
          buttonText,
          style: const TextStyle(fontSize: 20.0, fontWeight: FontWeight.bold),
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('简易计算器'),
      ),
      body: Column(
        children: <Widget>[
          Container(
            alignment: Alignment.centerRight,
            padding: const EdgeInsets.symmetric(vertical: 24.0, horizontal: 12.0),
            child: Text(
              _output,
              style: const TextStyle(fontSize: 48.0, fontWeight: FontWeight.bold),
            ),
          ),
          const Expanded(child: Divider()),
          Column(
            children: <Widget>[
              Row(
                children: <Widget>[
                  _buildButton('7'),
                  _buildButton('8'),
                  _buildButton('9'),
                  _buildButton('÷'),
                ],
              ),
              Row(
                children: <Widget>[
                  _buildButton('4'),
                  _buildButton('5'),
                  _buildButton('6'),
                  _buildButton('×'),
                ],
              ),
              Row(
                children: <Widget>[
                  _buildButton('1'),
                  _buildButton('2'),
                  _buildButton('3'),
                  _buildButton('-'),
                ],
              ),
              Row(
                children: <Widget>[
                  _buildButton('.'),
                  _buildButton('0'),
                  _buildButton('CLEAR'),
                  _buildButton('+'),
                ],
              ),
              Row(
                children: <Widget>[
                  _buildButton('='),
                ],
              ),
            ],
          )
        ],
      ),
    );
  }
}

void main() {
  runApp(const MaterialApp(home: CalculatorApp()));
}