flutter - `Drawer` 中的 Flutter 本地化导致`Null check operator used on a null value` 错误
问题描述
在我首次使用Dart/Flutter时,我创建了一个非常简单的演示应用程序,在Drawer
. 该应用程序在Android Emulator中编译并启动,但不久后中止并显示错误消息Null check operator used on a null value
。在 之外Drawer
,本地化工作完美无缺。
我到底做错了什么以及如何解决?
版本:
Flutter 2.2.0 • channel stable • https://github.com/flutter/flutter.git
Framework • revision b22742018b (12 days ago) • 2021-05-14 19:12:57 -0700
Engine • revision a9d88a4d18
Tools • Dart 2.13.0
GitHub 上提供的应用程序的完整源代码:https ://github.com/DarkPurpleKnight/null_issue
主要飞镖:
import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';
import 'package:flutter_gen/gen_l10n/app_localizations.dart';
void main() {
runApp(NullIssueApp());
}
class NullIssueApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Null issue',
localizationsDelegates: [
AppLocalizations.delegate,
GlobalMaterialLocalizations.delegate,
GlobalWidgetsLocalizations.delegate,
GlobalCupertinoLocalizations.delegate,
],
supportedLocales: [
const Locale('en', ''),
const Locale('de', ''),
],
home: Scaffold(
drawer: Drawer(
child: Row(
children: [Text(AppLocalizations.of(context)!.helloWorld)],
),
),
),
);
}
}
日志:
Launching lib/main.dart on sdk gphone x86 in debug mode...
Running Gradle task 'assembleDebug'...
✓ Built build/app/outputs/flutter-apk/app-debug.apk.
Debug service listening on ws://127.0.0.1:40233/5JFMCufV37s=/ws
Syncing files to device sdk gphone x86...
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
======== Exception caught by widgets library =======================================================
The following _CastError was thrown building NullIssueApp(dirty):
Null check operator used on a null value
The relevant error-causing widget was:
NullIssueApp file:///home/dpk/source/Android/null_issue/lib/main.dart:7:10
When the exception was thrown, this was the stack:
#0 NullIssueApp.build (package:percent_clock/main.dart:28:57)
#1 StatelessElement.build (package:flutter/src/widgets/framework.dart:4648:28)
#2 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4574:15)
#3 Element.rebuild (package:flutter/src/widgets/framework.dart:4267:5)
#4 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4553:5)
...
====================================================================================================
D/skia (20273): 1 Shader compilation error
D/skia (20273): 2 ------------------------
D/skia (20273): 3 Errors:
D/skia (20273): 4
解决方案
这是因为此时您的应用程序AppLocalization
尚未完全初始化,因此在您使用空检查运算符AppLocalizations.of(context)
时返回一个null
导致崩溃的值!
。
您需要将您的小部件包装Scaffold
在一个小部件中,以便在这个新的小部件中context
准备AppLocalization
好。
这是我运行良好的代码示例:
class NullIssueApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Null issue',
localizationsDelegates: [
AppLocalizations.delegate,
GlobalMaterialLocalizations.delegate,
GlobalWidgetsLocalizations.delegate,
GlobalCupertinoLocalizations.delegate,
],
supportedLocales: [
const Locale('en', ''),
const Locale('de', ''),
],
home: HomePage(),
);
}
}
class HomePage extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(),
drawer: Drawer(
child: Row(
children: [Text(AppLocalizations.of(context)!.helloWorld)],
),
),
);
}
}
截屏
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