如何修复方法'>='在null上调用.接收方:null尝试调用:>=(25)飞镖摆动

Question

伙计们，我正在尝试创建一个BMI计算器应用程序。一切都很好，但我在传递条件语句中的数据时遇到了一个错误，如下所示。所以有人能帮我解决这个问题吗？这将对我有很大帮助，谢谢：(

The following NoSuchMethodError was thrown building Builder(dirty):
The method '>=' was called on null.
Receiver: null
Tried calling: >=(25)
The relevant error-causing widget was: 
MaterialApp file:///C:/Users/Prajesh/Desktop/Desktop/Flutter/bmi-calculator-flutter/lib/main.dart:8:12
When the exception was thrown, this was the stack: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
#1      CalculatorBrain.getResult (package:bmi_calculator/bmi_brain.dart:17:14)
#2      _InputPageState.build.<anonymous closure>.<anonymous closure> (package:bmi_calculator/inputPage.dart:225:25)
#3      MaterialPageRoute.buildContent (package:flutter/src/material/page.dart:54:55)
#4      MaterialRouteTransitionMixin.buildPage (package:flutter/src/material/page.dart:107:27)
...

因此，基本上我已经创建了一个类，所有的逻辑都在其中发生，但当我试图将方法初始化为一个命名参数时，它会出错。

下面是我的代码，我已经完成了所有的逻辑

import 'dart:math';
class CalculatorBrain {
CalculatorBrain({this.height, this.weight});
final int height;
final int weight;
double _bmi;
String calculateBMI() {
_bmi = weight / pow(height / 100, 2);
return _bmi.toStringAsFixed(1);
}
String getResult() {
if (_bmi >= 25) {
return 'Overweight';
} else if (_bmi > 18.5) {
return 'Normal';
} else {
return 'Underweight';
}
}
String getInterpretation() {
if (_bmi >= 25) {
return 'You have a higher than normal body weight. Try to exercise more.';
} else if (_bmi >= 18.5) {
return 'You have a normal body weight. Good job!';
} else {
return 'You have a lower than normal body weight. You can eat a bit more.';
}
}
}

现在我在下面的代码中使用这个类，这是我的应用的首页

import 'package:bmi_calculator/result_page.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:font_awesome_flutter/font_awesome_flutter.dart';
import 'custom_card_child.dart';
import 'reusable_container.dart';
import 'Constant.dart';
import 'bottom_button.dart';
import 'bmi_brain.dart';

enum Gender { male, female, }
class InputPage extends StatefulWidget {
@override
_InputPageState createState() => _InputPageState();
}
class _InputPageState extends State<InputPage> {
Gender selectedGender;
int height = 180;
int weight = 60;
int age = 20;

@override
Widget build(BuildContext context) {
return SafeArea(
child: Scaffold(
appBar: AppBar(
title: Text('BMI CALCULATOR'),
centerTitle: true,
elevation: 10.0,
),
body: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Expanded(child: Row(
children: [
Expanded(child: reusableContainer(
onPress: (){
setState(() {
selectedGender = Gender.male;
});
},
colour: selectedGender == Gender.male ? Kactiverang : Kinactiverang,
cardChild: CustomCardChild(
icon: FontAwesomeIcons.mars,
text: 'MALE',
),
),),
Expanded(child: reusableContainer(
onPress: (){
setState(() {
selectedGender = Gender.female;
});
},
colour: selectedGender == Gender.female ? Kactiverang : Kinactiverang,
cardChild: CustomCardChild(
icon: FontAwesomeIcons.venus,
text: 'FEMALE',
),
),),
],
),
),
Expanded(child: reusableContainer(
colour: Kactiverang,
cardChild: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(
'HEIGHT',
style: KtextStyle,
),
Row(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.baseline,
textBaseline: TextBaseline.alphabetic,
children: [
Text(
height.toString(),
style: KnumTextStyle,
),
Text(
'cm'
)
],
),
SliderTheme(
data: SliderTheme.of(context).copyWith(
thumbShape: RoundSliderThumbShape(enabledThumbRadius: 15.0),
overlayShape: RoundSliderOverlayShape(overlayRadius: 30.0),
thumbColor: Color(0xFFEB1555),
activeTrackColor: Colors.white,
inactiveTrackColor: Color(0xFF8D8E98),
overlayColor: Color(0x29EB1555)
),
child: Slider(
value: height.toDouble(),
min: 120.0,
max: 230.0,
//activeColor: Color(0xFFEB1555),
//inactiveColor: Color(0xFF8D8E98),
onChanged: (double newValue){
setState(() {
height = newValue.round();
});
},
),
),
],
),
),),
Expanded(child: Row(
children: [
Expanded(child: reusableContainer(
colour: Kactiverang,
cardChild: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(
'WEIGHT',
style: KtextStyle,
),
Text(
weight.toString(),
style: KnumTextStyle,
),
Row(
mainAxisAlignment: MainAxisAlignment.center,
children: [
RoundIconButton(
onPress: (){
setState(() {
weight--;
});
},
icon: FontAwesomeIcons.minus,
),
SizedBox(width: 10.0,),
RoundIconButton(
onPress: (){
setState(() {
weight++;
});
},
icon: FontAwesomeIcons.plus,
),
],
)
],
),
)),
Expanded(child: reusableContainer(
colour: Kactiverang,
cardChild: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(
'AGE',
style: KtextStyle,
),
Text(
age.toString(),
style: KnumTextStyle,
),
Row(
mainAxisAlignment: MainAxisAlignment.center,
children: [
RoundIconButton(
onPress: (){
setState(() {
age--;
});
},
icon: FontAwesomeIcons.minus,
),
SizedBox(width: 10.0,),
RoundIconButton(
onPress: (){
setState(() {
age++;
});
},
icon: FontAwesomeIcons.plus,
),
],
)
],
),
)),
],
),
),
Bottom_button(
onPress: (){
CalculatorBrain myBMI = CalculatorBrain(height: height, weight: weight);
//CalculateBMI myBMI = CalculateBMI();
Navigator.push(context,
MaterialPageRoute(builder: (context) => ResultPage(
result: myBMI.getResult(),
remark: myBMI.getInterpretation(),
bmiValue: myBMI.calculateBMI(),
)));
},
button_titile: 'CALCULATE',
),
],
),
),
);
}
}

///////////////////////////////////////////////////////////////////////////////////////////////////////

class RoundIconButton extends StatelessWidget {
RoundIconButton({@required this.onPress, @required this.icon});
final Function onPress;
final IconData icon;
@override
Widget build(BuildContext context) {
return RawMaterialButton(
onPressed: onPress,
child: Icon(icon),
elevation: 6.0,
fillColor: Color(0xFF4C4F5E),
shape: CircleBorder(),
constraints: BoxConstraints.tightFor(width: 56.0, height: 56.0),

);
}
}

我想把这些数据传递到下面的下一页

import 'package:bmi_calculator/Constant.dart';
import 'package:bmi_calculator/reusable_container.dart';
import 'package:flutter/material.dart';
import 'bottom_button.dart';
class ResultPage extends StatelessWidget {
final String result;
final String bmiValue;
final String remark;
// ResultPage({@required this.bmiValue, @required this.remark, @required this.result});
ResultPage({this.bmiValue, this.remark, this.result});
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('BMI CALCULATOR'),
centerTitle: true,
),
body: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Expanded(
child: Container(
padding: EdgeInsets.all(15.0),
alignment: Alignment.bottomLeft,
child: Text('Your Result', style: KresultTextStyle,),
),
),
Expanded(
flex: 5,
child: reusableContainer(
colour: Kactiverang,
cardChild: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
Text(result.toUpperCase(), style: KnormalTextStyle,),
Text(bmiValue, style: KBMItextStyle,),
Text('all well',
style: KremarkTextStyle,
textAlign: TextAlign.center,
),
],
),
),
),
Bottom_button(onPress: (){Navigator.pop(context);}, button_titile: 'RE-CALCULATE'),
],
),
);
}
}

所以，有人能帮我解决这个问题吗？或者至少告诉我为什么会出现这个错误？谢谢：(

Answer 1

您在calculateBMI((之前调用getResult((，其中您的_bmi为null，因此会出现异常。

您可以通过初始化值来解决它，但它不会在结果页中修复值。

你可能想先计算Bmi，然后保存它，然后传递它。就像这样。

onPress: (){
CalculatorBrain myBMI = CalculatorBrain(height: height, weight: weight);
final val = mBmi.calculateBMI();
Navigator.push(context,
MaterialPageRoute(builder: (context) => ResultPage(
result: myBMI.getResult(),
remark: myBMI.getInterpretation(),
bmiValue: val,
)));
},

Answer 2

有点晚了。然而，加起来就是@Sahdeep Singh的答案。实际上，您在calculateBMI((之前调用getResult((，但不需要将BMI值初始化为Val。

您所要做的就是调用calculateBMI((作为构造函数中的第一个参数；这样你就可以初始化它，就像这样：

onPress: (){
CalculatorBrain myBMI = CalculatorBrain(height: height, weight: weight);
Navigator.push(context,
MaterialPageRoute(builder: (context) => ResultPage(
bmiValue: myBMI.calculateBMI(),
result: myBMI.getResult(),
remark: myBMI.getInterpretation(),
)));
},