Flutter BMI计算器报'>='方法空值调用错误排查

问题表现 #

• 跟随App Brewery平台Angela教程开发BMI计算器Flutter应用时，实例化CalculatorBrain对象并传入height、weight构造参数后，calculator_brain.dart中的私有属性_bmi始终为null
• 仅手动给_bmi初始化固定数值时程序可正常运行，触发报错为NoSuchMethod '>=' was called on null

根因定位 #

• 变量作用域错误：CalculateBMI()方法内重复声明了局部变量double _bmi，计算逻辑仅给方法内的局部变量赋值，类级别的私有属性_bmi从未被赋值，默认值为null
• 方法调用顺序错误：路由传参时优先调用了依赖_bmi值的getInterpritation()、getResult()方法，此时尚未执行BMI计算逻辑给_bmi赋值，即使修正作用域问题，顺序错误仍会导致读取到null值

修复步骤 #

1. 修正CalculatorBrain类的变量作用域问题，删除CalculateBMI()方法内重复的_bmi类型声明，直接给类属性赋值：

class CalculatorBrain {
  CalculatorBrain({
    @required this.Getheight,
    @required this.Getweight,
  });

  final int Getheight;
  final int Getweight;
  double _bmi;

  String CalculateBMI() {
    // 移除前面的double关键字，不再声明局部变量，直接赋值给类属性_bmi
    _bmi = Getweight / pow(Getheight / 100, 2);
    return _bmi.toStringAsFixed(1);
  }

  String getResult() {
    if (_bmi >= 25) {
      return 'Overweight';
    } else if (_bmi > 18.5) {
      return 'Normal';
    } else {
      return 'Underweight';
    }
  }

  String getInterpritation() {
    if (_bmi >= 25) {
      return 'You have a higher than normal body weight, Try excerise more';
    } else if (_bmi > 18.5) {
      return 'You have a Normal body weight, Good Job ! ';
    } else {
      return 'You have a Low than normal body weight, Try eating More !';
    }
  }
}

2. 调整计算按钮点击逻辑的方法调用顺序，先执行BMI计算完成_bmi赋值，再获取结果文本和解读文本传参：

bottonButton(
  buttonTitle: 'Calculate',
  onTap: () {
    CalculatorBrain calc =
        CalculatorBrain(Getheight: height, Getweight: weight);
    // 优先执行计算逻辑，完成_bmi赋值
    final bmiVal = calc.CalculateBMI();
    Navigator.push(
      context,
      MaterialPageRoute(
        builder: ((context) => ResultsPage(
              interpritation: calc.getInterpritation(),
              ResultText: calc.getResult(),
              bmiResults: bmiVal,
            )),
      ),
    );
  },
),

可选优化 #

可将BMI计算逻辑移入CalculatorBrain的构造函数，实例化时自动完成_bmi赋值，从根源避免调用顺序错误：

class CalculatorBrain {
  CalculatorBrain({
    @required this.Getheight,
    @required this.Getweight,
  }) {
    // 构造实例时直接计算_bmi值
    _bmi = Getweight / pow(Getheight / 100, 2);
  }

  final int Getheight;
  final int Getweight;
  double _bmi;

  String CalculateBMI() {
    return _bmi.toStringAsFixed(1);
  }
  // getResult、getInterpritation方法逻辑保持不变
}

内容的提问来源于stack exchange，提问作者Ram Gupta