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社区首页 >问答首页 >对空值使用的颤振提供程序Null检查运算符

问对空值使用的颤振提供程序Null检查运算符
EN

Stack Overflow用户
提问于 2022-04-06 22:22:49
回答 2查看 596关注 0票数 1

我做了一个非常简单的提供者测试，但是我一直收到这个错误。我切换到颤振稳定通道，升级，并清理它。结果是一样的。当我编写函数来处理任何屏幕的方法时，它都会产生这个错误。

我使用dipsos方法的目的是：

处理ValueNotifiers:一旦所有ValueNotifiers不再使用，就可以将它们全部释放，这是一个很好的做法，可以避免应用程序内存丢失。

为了提高性能，我希望当前订阅在退出屏幕时过期。当屏幕初始化时，我会创建一个新的值化器，并且当我退出屏幕时，我想要释放。

main.dart文件

代码语言：javascript
复制
void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(
    options: DefaultFirebaseOptions.currentPlatform,
  );
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (context) => ScreenChangeNotifier(),),
        Provider(create: (context) => Genel(),)
      ],
      child: MaterialApp(
        theme: ThemeData(
            primarySwatch: Colors.blue
        ),
        title: 'Test',
        home: HomeScreen(),

      ),
    );
  }
}

ScreenChangeNotifier.dart文件

代码语言：javascript
复制
class ScreenChangeNotifier extends ChangeNotifier
{
    void hello() => print("say hello");
}

HomeScreen.dart文件

代码语言：javascript
复制
class HomeScreen extends StatefulWidget {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  @override
  void dispose() {
    context.read<ScreenChangeNotifier>().hello();
    super.dispose();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(),
    );
  }
}

这一错误

代码语言：javascript
复制
======== Exception caught by widgets library =======================================================
The following _CastError was thrown while finalizing the widget tree:
Null check operator used on a null value

When the exception was thrown, this was the stack: 
#0      Element.widget (package:flutter/src/widgets/framework.dart:3203:31)
#1      Provider._inheritedElementOf (package:provider/src/provider.dart:341:17)
#2      Provider.of (package:provider/src/provider.dart:293:30)
#3      ReadContext.read (package:provider/src/provider.dart:656:21)
#4      _HomeScreenState.dispose (package:bencebilirpro/screens/HomeScreen.dart:11:13)
#5      StatefulElement.unmount (package:flutter/src/widgets/framework.dart:4983:11)
#6      _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1926:13)
#7      _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#8      SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#9      _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#10     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#11     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#12     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#13     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#14     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#15     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#16     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#17     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#18     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#19     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#20     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#21     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#22     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#23     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#24     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#25     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#26     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#27     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#28     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#29     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#30     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#31     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#32     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#33     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#34     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#35     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#36     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#37     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#38     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#39     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#40     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#41     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#42     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#43     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#44     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#45     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#46     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#47     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#48     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#49     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#50     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#51     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#52     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#53     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#54     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#55     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#56     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#57     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#58     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#59     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#60     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#61     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#62     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#63     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#64     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#65     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#66     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#67     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#68     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#69     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#70     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#71     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#72     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#73     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#74     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#75     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#76     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#77     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#78     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#79     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#80     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#81     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#82     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#83     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#84     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#85     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#86     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:6271:14)
#87     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#88     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#89     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#90     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#91     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#92     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#93     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#94     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1924:7)
#95     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:4807:14)
#96     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1922:13)
#97     ListIterable.forEach (dart:_internal/iterable.dart:39:13)
#98     _InactiveElements._unmountAll (package:flutter/src/widgets/framework.dart:1935:25)
#99     BuildOwner.lockState (package:flutter/src/widgets/framework.dart:2519:15)
#100    BuildOwner.finalizeTree (package:flutter/src/widgets/framework.dart:2932:7)
#101    WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:884:19)
#102    RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:363:5)
#103    SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1144:15)
#104    SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1081:9)
#105    SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:995:5)
#109    _invoke (dart:ui/hooks.dart:151:10)
#110    PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:308:5)
#111    _drawFrame (dart:ui/hooks.dart:115:31)
(elided 3 frames from dart:async)
====================================================================================================
provider
flutter-provider
flutter
EN

回答 2

Stack Overflow用户

发布于 2022-04-07 04:15:09

首先，为什么要在dispose()方法中调用您的方法?当这个对象被永久从树中删除时，您不应该试图在此时找到这个小部件的祖先，因为您的小部件已经从小部件树中删除了。

如果您想要这样做，请尝试在State中创建变量，在initState()中用context.read<ScreenChangeNotifier>()初始化它，在dispose call variable.hello()中初始化它。

代码语言：javascript
复制
class HomeScreen extends StatefulWidget {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  ScreenChangeNotifier _notifier;

  @override
  void initState() {
    _notifier = context.read<ScreenChangeNotifier>();
    super.initState();
  }

  @override
  void dispose() {
    _notifier.hello();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(),
    );
  }
}
票数 1
EN

Stack Overflow用户

发布于 2022-04-07 04:07:30

为什么要在ScreenChangeNotifier函数中调用dispose？您必须在initstate或build方法中调用它。

代码语言：javascript
复制
class HomeScreen extends StatefulWidget {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  @override
  void initState() {
    super.initState();
    context.read<ScreenChangeNotifier>().hello();
  }
  @override
  Widget build(BuildContext context) {
// or you can call it here..
 // context.read<ScreenChangeNotifier>().hello();
    return Scaffold(
      body: Container(),
    );
  }
}
票数 0
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/71774249

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