这是变量初始化
var displayResult = '';
这里是印刷代码
Padding(
padding: EdgeInsets.all(5.0),
child: Row(
children: <Widget>[
Expanded(
child: RaisedButton(
color: Theme
.of(context)
.primaryColor,
textColor: Theme
.of(context)
.primaryColorLight,
child: Text("calculate"),
onPressed: () {
setState(() {
this.displayResult = _calculateTotalReturn();
});
},
)),
我正在尝试调用的功能
String _calculateTotalReturn() {
setState(() {
double principle = double.parse(principleController.text);
double roi = double.parse(roiController.text);
double term = double.parse(termController.text);
double tap = principle + (principle * roi * term) / 100;
String result = "After $term years, your investment will worth $tap $_currentItemSelected";
return result;
});
}
这是我得到的错误
Performing hot reload...
Syncing files to device iPhone XR...
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building SIForm(dirty, dependencies:
flutter: [_LocalizationsScope-[GlobalKey#4c1f9], _InheritedTheme], state: _SIFormState#b2966):
flutter: 'package:flutter/src/widgets/text.dart': Failed assertion: line 237 pos 15: 'data != null': is not
flutter: true.
flutter:
flutter: Either the assertion indicates an error in the framework itself, or we should provide substantially
flutter: more information in this error message to help you determine and fix the underlying cause.
flutter: In either case, please report this assertion by filing a bug on GitHub:
flutter: https://github.com/flutter/flutter/issues/new?template=BUG.md
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #2 new Text (package:flutter/src/widgets/text.dart:237:15)
flutter: #3 _SIFormState.build (package:simple_cal/main.dart:153:24)
flutter: #4 StatefulElement.build (package:flutter/src/widgets/framework.dart:3825:27)
flutter: #5 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3736:15)
flutter: #6 Element.rebuild (package:flutter/src/widgets/framework.dart:3559:5)
flutter: #7 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2273:33)
flutter: #8 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:700:20)
flutter: #9 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:268:5)
flutter: #10 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:988:15)
flutter: #11 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:928:9)
flutter: #12 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:749:7)
flutter: #14 _Timer._runTimers (dart:isolate/runtime/libtimer_impl.dart:382:19)
flutter: #15 _Timer._handleMessage (dart:isolate/runtime/libtimer_impl.dart:416:5)
flutter: #16 _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)
flutter: (elided 3 frames from class _AssertionError and package dart:async)
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════
Reloaded 1 of 432 libraries in 641ms.
这是完整的main.dart文件
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
Widget MyApp() {
return MaterialApp(
title: 'Simple Intrest app',
home: SIForm (),
theme: ThemeData(
primaryColor: Colors.deepPurple,
accentColor: Colors.deepPurpleAccent
),
);
}
class SIForm extends StatefulWidget {
@override
State<StatefulWidget> createState() {
return _SIFormState ();
}
}
class _SIFormState extends State<SIForm> {
var _currencies = ["INR", "USD", "CAD"];
var _currentItemSelected = "INR";
var displayResult = '';
TextEditingController principleController = TextEditingController();
TextEditingController roiController = TextEditingController();
TextEditingController termController = TextEditingController();
@override
Widget build(BuildContext context) {
TextStyle textStyle = Theme
.of(context)
.textTheme
.title;
return Scaffold(
appBar: AppBar(
title: Text("Simple Interest Calc"),
),
body: Container(padding: EdgeInsets.only(top: 50.0),
child: ListView(
children: <Widget>[
Padding(
padding: EdgeInsets.all(5.0),
child: TextField(
keyboardType: TextInputType.number,
controller: principleController,
decoration: InputDecoration(
labelText: 'principle',
hintText: '0.00',
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(5.0)
)
),
)),
Padding(
padding: EdgeInsets.all(5.0),
child: TextField(
controller: roiController,
keyboardType: TextInputType.number,
decoration: InputDecoration(
labelText: 'Rate of Interest',
hintText: '0.00',
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(5.0)
)
),
)),
Padding(
padding: EdgeInsets.all(5.0),
child: Row(
children: <Widget>[
Expanded(child: TextField(
controller: termController,
keyboardType: TextInputType.number,
decoration: InputDecoration(
labelText: 'Terms',
hintText: '%',
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(5.0)
)
),
)),
Container(width: 10.0),
Expanded(
child: DropdownButton<String>(
items: _currencies.map((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
value: _currentItemSelected,
onChanged: (val) {
_ondropDownChange(val);
},),
),
],
)),
Padding(
padding: EdgeInsets.all(5.0),
child: Row(
children: <Widget>[
Expanded(
child: RaisedButton(
color: Theme
.of(context)
.primaryColor,
textColor: Theme
.of(context)
.primaryColorLight,
child: Text("calculate"),
onPressed: () {
setState(() {
this.displayResult = _calculateTotalReturn();
});
},
)),
Container(width: 5.0),
Expanded(
child: RaisedButton(
color: Theme
.of(context)
.accentColor,
textColor: Theme
.of(context)
.primaryColorLight,
child: Text("Reset"),
onPressed: () {
setState(() {
// _reset();
});
}
)),
],
)),
Padding(
padding: EdgeInsets.all(5.0),
child: Text(this.displayResult),
)
],
),
),
);
}
void _ondropDownChange(String value) {
setState(() {
this._currentItemSelected = value;
});
}
String _calculateTotalReturn() {
setState(() {
double principle = double.parse(principleController.text);
double roi = double.parse(roiController.text);
double term = double.parse(termController.text);
double tap = principle + (principle * roi * term) / 100;
String result = "After $term years, your investment will worth $tap $_currentItemSelected";
return result;
});
}
void _reset() {
setState(() {
this.principleController.text = " ";
this.roiController.text = " ";
this.termController.text = " ";
this.displayResult = " ";
});
}
}
答案 0 :(得分：1)
您应该摆脱setState中的_calculateTotalReturn。您提供给return result;的内联函数返回了setState，而_calculateTotalReturn实际上返回了null。