Flutter 锁定小部件树时调用 setState() 或 markNeedsBuild()

Question

我无法找到此异常的来源，而且该应用程序非常复杂，因此很难显示代码的任何相关部分。 这是发生错误时我的存储库： Github repo

我怀疑以下代码可能是肇事者：

Widget buildResultCard(Map result, BuildContext context) {
    String name = result["value"];
    String description =
        result["label"].replaceAll(new RegExp(r"<(?:.|\n)*?>"), "");
    TextEditingController controller = new TextEditingController(text: name);

    Function onPressed = () {
      showDialog(
          context: context,
          child: new AlertDialog(
            title: new Text("Name your schedule"),
            content: new TextField(
              autofocus: true,
              controller: controller,
            ),
            actions: <Widget>[
              new FlatButton(
                  onPressed: () {
                    String givenName = controller.text;
                    ScheduleMeta schedule = new ScheduleMeta(
                        givenName: givenName,
                        name: name,
                        type: _selectedChoice.value,
                        description: description);

                    scheduleStore
                        .dispatch(new AddScheduleAction(schedule: schedule));

                    scheduleStore.dispatch(
                        new SetCurrentScheduleAction(schedule: schedule));

                    fetchAllSchedules(scheduleStore.state.schedules)
                        .then((weeks) {
                      scheduleStore.dispatch(
                          new SetWeeksForCurrentScheduleAction(weeks: weeks));
                    });

                    Scaffold.of(context).showSnackBar(new SnackBar(
                          content: new Text("Added " + givenName),
                          action: new SnackBarAction(
                              label: "Undo",
                              onPressed: () {
                                scheduleStore.dispatch(
                                    new RemoveScheduleAction(schedule: name));

                                Scaffold.of(context).showSnackBar(new SnackBar(
                                      content: new Text(
                                          "Deleted " + givenName),
                                    ));
                              }),
                        ));

                    Navigator.of(context).pop();
                  },
                  child: new Text("Add")),
            ],
          ));
    };

    return new Card(
      child: new Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          new ListTile(
            leading: const Icon(Icons.schedule),
            title: new Text(name),
            subtitle: new Text(description),
            isThreeLine: true,
            dense: true,
          ),
          new ButtonTheme.bar(
            child: new ButtonBar(
              children: <Widget>[
                new FlatButton(
                    child: const Text('Add Schedule'),
                    onPressed: scheduleStore.state.schedules
                            .any((schedule) => schedule.name == name)
                        ? null
                        : onPressed)
              ],
            ),
          ),
        ],
      ),
    );
  }

显示对话框时出现错误，当用户按下对话框上的按钮时，两个 Redux 商店调度直接依次发送。 对话框后面的 UI 订阅了 Redux 商店中的更改。

我认为 Dart/Flutter 是单线程的，所以不会发生这样的冲突，似乎一个线程正在调用一个小部件上的 setState() 而另一个线程已经锁定了小部件树。

有没有一种方法可以检查小部件树是否已锁定，从而可以避免这种情况？

堆栈提供以下信息：

I/flutter (13466): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (13466): The following assertion was thrown while finalizing the widget tree:
I/flutter (13466): setState() or markNeedsBuild() called when widget tree was locked.
I/flutter (13466): This _ModalScope widget cannot be marked as needing to build because the framework is locked.
I/flutter (13466): The widget on which setState() or markNeedsBuild() was called was:
I/flutter (13466):   _ModalScope([LabeledGlobalKey<_ModalScopeState>#cb4cc]; state: _ModalScopeState#d4c02())
I/flutter (13466): 
I/flutter (13466): When the exception was thrown, this was the stack:
I/flutter (13466): #0      Element.markNeedsBuild.<anonymous closure> (package:flutter/src/widgets/framework.dart:3250)
I/flutter (13466): #2      Element.markNeedsBuild (package:flutter/src/widgets/framework.dart:3226)
I/flutter (13466): #3      State.setState (package:flutter/src/widgets/framework.dart:1072)
I/flutter (13466): #4      _ModalScopeState._routeSetState (package:flutter/src/widgets/routes.dart:473)
I/flutter (13466): #5      ModalRoute.setState (package:flutter/src/widgets/routes.dart:552)
I/flutter (13466): #6      ModalRoute.changedInternalState (package:flutter/src/widgets/routes.dart:889)
I/flutter (13466): #7      TransitionRoute&&LocalHistoryRoute.removeLocalHistoryEntry (package:flutter/src/widgets/routes.dart:317)
I/flutter (13466): #8      LocalHistoryEntry.remove (package:flutter/src/widgets/routes.dart:267)
I/flutter (13466): #9      DrawerControllerState.dispose (package:flutter/src/material/drawer.dart:147)
I/flutter (13466): #10     StatefulElement.unmount (package:flutter/src/widgets/framework.dart:3550)
I/flutter (13466): #11     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1626)
I/flutter (13466): #12     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #13     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #14     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #15     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #16     MultiChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:4421)
I/flutter (13466): #17     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #18     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #19     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #20     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #21     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #22     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #23     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #24     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #25     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:4321)
I/flutter (13466): #26     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #27     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #28     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #29     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #30     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #31     SingleChildRenderObjectElement.visitChildren (package:flutter/src/widgets/framework.dart:4321)
I/flutter (13466): #32     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #33     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #34     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #35     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #36     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #37     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #38     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #39     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #40     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #41     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #42     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #43     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #44     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #45     _InactiveElements._unmount.<anonymous closure> (package:flutter/src/widgets/framework.dart:1624)
I/flutter (13466): #46     ComponentElement.visitChildren (package:flutter/src/widgets/framework.dart:3427)
I/flutter (13466): #47     _InactiveElements._unmount (package:flutter/src/widgets/framework.dart:1622)
I/flutter (13466): #48     _InactiveElements._unmountAll (package:flutter/src/widgets/framework.dart:1636)
I/flutter (13466): #49     BuildOwner.finalizeTree.<anonymous closure> (package:flutter/src/widgets/framework.dart:2228)
I/flutter (13466): #50     BuildOwner.lockState (package:flutter/src/widgets/framework.dart:2060)
I/flutter (13466): #51     BuildOwner.finalizeTree (package:flutter/src/widgets/framework.dart:2227)
I/flutter (13466): #52     BindingBase&SchedulerBinding&GestureBinding&ServicesBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:505)
I/flutter (13466): #53     BindingBase&SchedulerBinding&GestureBinding&ServicesBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:189)
I/flutter (13466): #54     BindingBase&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:688)
I/flutter (13466): #55     BindingBase&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:636)
I/flutter (13466): #56     _drawFrame (file:///b/build/slave/Linux_Engine/build/src/flutter/lib/ui/hooks.dart:70)
I/flutter (13466): (elided one frame from class _AssertionError)
I/flutter (13466): ════════════════════════════════════════════════════════════════════════════════════════════════════

Answer 1

作为解决方法，将调用setState的代码包装到WidgetsBinding.addPostFrameCallback:

WidgetsBinding.instance
        .addPostFrameCallback((_) => setState(() {}));

这样您就可以确保在构建当前小部件后它会被执行。

Answer 2

使用Scaffold和Drawer的特例：

如果您尝试用打开的Drawer重建树，也会发生这种情况。 例如，如果您向一个强制重建整个页面/屏幕的Bloc发送消息。

考虑在您的点击处理程序中首先调用Navigator.pop(context) 。

Answer 3

这不是线程问题。 此错误意味着您在构建阶段调用了setState 。

一个典型的例子如下：

Widget build(BuildContext context) {
    myParentWidgetState.setState(() { print("foo"); });
    return Container();
}

但是 setState 调用可能不太明显。 例如Navigator.pop(context)在内部执行setState 。 所以如下：

Widget build(BuildContext context) {
    Navigator.pop(context);
    return Container();
}

也是不行的。

---

查看堆栈跟踪，似乎与Navigator.pop(context)同时，您的模式尝试使用新数据进行更新。

Answer 4

由于提供的答案似乎是正确的，所以我有一个变通办法，因为在我的情况下，不可能简单地将它从构建中删除。

我使用Future.delayed(Duration.zero, () => setState(() {... })); 而不是 setState 并且对于可能使用 setState 的方法也是如此。

编辑：来源

Answer 5

在initState的build方法和context中使用了setState的几个选项

将您的 setState 包装在其中一个中

WidgetsBinding.instance.addPostFrameCallback或Future.microTask()或Timer.run或Future.delayed(Duration.zero,

例子：

  WidgetsBinding.instance
              .addPostFrameCallback((_) {
            //valueNotifier.value = _pcm; //provider
            //setState
          });

Answer 6

在制作一个需要在返回 Widget 之前执行Navigator.pop的小部件时，我遇到了同样的问题build 。

让我调用Rémi Rousselet的代码：

Widget build(BuildContext context) {
    Navigator.pop(context);
    return Container();
}

上面这段代码会导致错误，但你只能弹出，因为context在build中，那怎么办？

Widget build(BuildContext context) {
    if(datas != null) // check datas
    {
      SchedulerBinding.instance.addPostFrameCallback((_){ // make pop action to next cycle
        Navigator.of(context).pop(datas); /*...your datas*/
      });
    }
    return Container();
}

这个方法是其他方法的结合，但我不确定这是否是最佳实践，如果有错误，请发表评论指出我！

Answer 7

使用Timer类，

import 'dart:async';

@override
void initState() {
  super.initState();

  Timer.run(() {
    // You can call setState from here
  });
}

Answer 8

我在每次打开或重新加载时在 LoginScreen 控制用户。 如果用户通过身份验证，我导航到家。 在这一点上我得到同样的错误。 据我了解，这是关于导航器自己的 setState 的。 addPostFrameCallback 或计时器修复它。

  void initState() {
    super.initState();
    if (auth.getUser != null) {
      // ERROR
      // Navigator.pushReplacementNamed(context, '/home');

      // SUCCESS
      // WidgetsBinding.instance.addPostFrameCallback((_) => setState(() {
      //       Navigator.pushReplacementNamed(context, '/home');
      //     }));

      // SUCCESS
      Timer.run(() {
        Navigator.pushReplacementNamed(context, '/home');
      });
    }
  }

Answer 9

 void initState() {
super.initState();
if (auth.getUser != null) {
  // ERROR
  // Navigator.pushReplacementNamed(context, '/home');

  // SUCCESS
  // WidgetsBinding.instance.addPostFrameCallback((_) => setState(() {
  //       Navigator.pushReplacementNamed(context, '/home');
  //     }));

  // SUCCESS
  Timer.run(() {
    Navigator.pushReplacementNamed(context, '/home');
  });
}

}