我是Flutter的新手
当我运行简单的颤振APP时，我得到了这个错误。我不知道为什么会发生这个错误。
错误
用于空值的空检查运算符
我在main. dart中的代码
import 'package:flutter/material.dart';
import './ui/login.dart';
void main() {
runApp(new MaterialApp(
title: "Login Template",
home: new Login(),
));
}
我在login. dart中的代码
import 'package:flutter/material.dart';
class Login extends StatefulWidget {
@override
State<StatefulWidget> createState() {
return new LoginState();
}
}
class LoginState extends State<Login> {
@override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text("Login"),
centerTitle: true,
backgroundColor: Colors.blueAccent.shade50,
),
backgroundColor: Colors.blueGrey,
body: new Container(
),
);
}
}
代码的错误跟踪
Running Gradle task 'assembleDebug'...
✓ Built build/app/outputs/flutter-apk/app-debug.apk.
Installing build/app/outputs/flutter-apk/app.apk...
Waiting for SM J710F to report its views...
D/vndksupport(29495): Loading /vendor/lib/hw/android.hardware.graphics.mapper@2.0-impl.so from current namespace instead of sphal namespace.
Debug service listening on ws://127.0.0.1:39899/9RorUiKtUb4=/ws
Syncing files to device SM J710F...
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_RESIZED_REPORT: frame=Rect(0, 0 - 720, 1280) ci=Rect(0, 48 - 0, 582) vi=Rect(0, 48 - 0, 582) or=1
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_WINDOW_FOCUS_CHANGED 1
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@3049fea nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=96
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@aad92db nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_RESIZED: frame=Rect(0, 0 - 720, 1280) ci=Rect(0, 48 - 0, 0) vi=Rect(0, 48 - 0, 0) or=1
D/ViewRootImpl@4ac1ef4[MainActivity](29495): Relayout returned: old=[0,0][720,1280] new=[0,0][720,1280] result=0x1 surface={valid=true 3791374336} changed=false
D/libGLESv2(29495): STS_GLApi : DTS, ODTC are not allowed for Package : com.sivaram.login_template
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
Null check operator used on a null value
Login file:///home/kadavul/IdeaProjects/login_template/lib/main.dart:8:15
════════════════════════════════════════════════════════════════════════════════════════════════════
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@a0ff0af nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=87
D/InputTransport(29495): Input channel destroyed: fd=96
D/SurfaceView(29495): windowStopped(true) false 77b9092 of ViewRootImpl@4ac1ef4[MainActivity]
D/SurfaceView(29495): BG show() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@0) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/SurfaceView(29495): surfaceDestroyed 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@a78fcbc nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=91
D/InputTransport(29495): Input channel destroyed: fd=87
D/SurfaceView(29495): windowStopped(false) true 77b9092 of ViewRootImpl@4ac1ef4[MainActivity]
D/SurfaceView(29495): BG show() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@1) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
V/Surface (29495): sf_framedrop debug : 0x4f4c, game : false, logging : 0
D/SurfaceView(29495): surfaceCreated 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/mali_winsys(29495): EGLint new_window_surface(egl_winsys_display *, void *, EGLSurface, EGLConfig, egl_winsys_surface **, egl_color_buffer_format *, EGLBoolean) returns 0x3000, [720x1280]-format:1
D/SurfaceView(29495): surfaceChanged (720,1280) 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/SurfaceView(29495): BG destroy() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@0) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/ViewRootImpl@4ac1ef4[MainActivity](29495): Relayout returned: old=[0,0][720,1280] new=[0,0][720,1280] result=0x3 surface={valid=true 3791374336} changed=false
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_RESIZED_REPORT: frame=Rect(0, 0 - 720, 1280) ci=Rect(0, 48 - 0, 0) vi=Rect(0, 48 - 0, 0) or=1
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@7ed1445 nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=92
D/InputTransport(29495): Input channel destroyed: fd=91
D/SurfaceView(29495): windowStopped(true) false 77b9092 of ViewRootImpl@4ac1ef4[MainActivity]
D/SurfaceView(29495): BG show() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@1) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/SurfaceView(29495): surfaceDestroyed 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
我的颤振医生输出
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Doctor summary (to see all details, run flutter doctor -v):
Failed to find the latest git commit date: VersionCheckError: Command exited with code 128: git -c log.showSignature=false log -n 1 --pretty=format:%ad --date=iso
Standard out:
Standard error: fatal: your current branch 'master' does not have any commits yet
Returning 1970-01-01 05:30:00.000 instead.
[✓] Flutter (Channel unknown, 0.0.0-unknown, on Linux, locale en_US.UTF-8)
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
[✓] Android Studio (version 4.0)
[!] VS Code (version 1.50.0)
✗ Flutter extension not installed; install from
https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter
[✓] Connected device (1 available)
! Doctor found issues in 1 category.
有人能为此提供解决方案吗？
当您在未初始化的可空实例上使用刘海运算符（！）时，会发生此错误。
例如：
String? string; // Nullable String
void main() {
var len = string!.length; // Runtime error: Null check operator used on a null value
}
打开日志，必须有一行指向项目中发生错误的文件：
对空值使用的空检查运算符
#0 main（包：example/main. dart：22:16）
一旦你到了那里，你可以使用以下任何一种方法来修复它：
>
使用局部变量
var s = string;
if (s != null) {
var len = s.length; // Safe
}
使用？.和？？
var len = string?.length ?? 0; // Provide a default value if string was null.
堆栈跟踪也可以指向不属于您项目的文件。例如：
当您尝试异步访问BuildContext时也会发生此错误。
因此，在访问BuildContext之前，您应该首先检查小部件是否安装了。
Future<void> foo() async {
// Some async operation
await compute();
// Check `mounted` before accessing 'context'.
if (mounted) {
MediaQuery.of(context).size;
Navigator.of(context).pop();
}
}
你在用
Colors.blueAccent.shade50
它没有50th阴影。如果您查看源代码，您会发现：
Color get shade50 => this[50]!; // <-- This bang operator is causing the error.
要解决此错误，您应该使用其他不是null的颜色，可能是100的阴影。
Colors.blueAccent[100]
// or
Colors.blue.shade100
您可以通过两种方式解决错误：
>
为您的FutureBuilder/StreamBuilder指定类型
FutureBuilder<List<int>>( // <-- type 'List<int>' is specified.
future: _listOfInt(),
builder: (_, snapshot) {
if (snapshot.hasData) {
List<int> myList = snapshot.data!; // <-- Your data
}
return Container();
},
)
使用作为将Object向下转换为您的类型，例如List或Map。
FutureBuilder(
future: _listOfInt(),
builder: (_, snapshot) {
if (snapshot.hasData) {
var myList = snapshot.data! as List<int>; // <-- Your data using 'as'
}
return Container();
},
)
任何使用get_it软件包并有类似问题的人，这里是最简单的解决方案。只需在main函数的顶部添加WidgetsFlutterBind. ensureCentalize（）；。
像这样更改您的主要功能：
Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
await di.init()
runApp(MyApp());}
我们需要解决上述问题的步骤如下
- flutter channel stable
- flutter upgrade
- flutter pub cache repair //To perform a clean reinstall of the packages in your system cache, use pub cache repair
- flutter clean //flutter clean will delete the /build folder
当我尝试flutter通道稳定时。我收到了另一个错误，因为我有两个flutter，一个来自snapd，另一个来自git clone方法。
然后，我用git clone配置了SDK。最后，我使用Git cloneSDKflutter执行所有命令，如下所示
flutter channel stable
- ~/flutter/bin/flutter upgrade
- ~/flutter/bin/flutter pub cache repair //To perform a clean reinstall of the packages in your system cache, use pub cache repair
- ~/flutter/bin/flutter clean //flutter clean will delete the /build folder
