52

我正在尝试创建一个简单的布局，flutter但不断收到此错误：

在构建 Text("Deliver features faster", textAlign: center): No Directionality widget found 时抛出了以下断言。

我认为问题可能出在 TextDirection 类属性中，但我一直找不到。

这是我的代码：

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';

void main(){

  runApp( new MyHome());
}

class MyHome extends StatelessWidget{

  @override

  Widget build(BuildContext context) {
    // TODO: implement build
    return new Material(
      child: new Container(
        color: Colors.red,
        margin: const EdgeInsets.all(5.0),
        child: new Row(
          children: <Widget>[
            new Expanded(
              child: new Text('Deliver features faster', textAlign: TextAlign.center),
            ),
            new Expanded(
              child: new Text('Craft beautiful UIs', textAlign: TextAlign.center),
            ),
            new Expanded(
              child: new FittedBox(
                fit: BoxFit.contain, // otherwise the logo will be tiny
                child: const FlutterLogo(),
              ),
            ),
          ],
        )
      ),
    );
  }
}

这是错误的完整堆栈跟踪：

The following assertion was thrown building Text("Deliver features faster", textAlign: center):
No Directionality widget found.
RichText widgets require a Directionality widget ancestor.
The specific widget that could not find a Directionality ancestor was:
  RichText(textAlign: center, softWrap: wrapping at box width, maxLines: unlimited, text: "Deliver
  features faster")
The ownership chain for the affected widget is:
  RichText ← Text ← Expanded ← Row ← DecoratedBox ← Padding ← Container ← DefaultTextStyle ←
  AnimatedDefaultTextStyle ← _InkFeatures-[GlobalKey#978b7 ink renderer] ← ⋯
Typically, the Directionality widget is introduced by the MaterialApp or WidgetsApp widget at the
top of your application widget tree. It determines the ambient reading direction and is used, for
example, to determine how to lay out text, how to interpret "start" and "end" values, and to resolve
EdgeInsetsDirectional, AlignmentDirectional, and other *Directional objects.
When the exception was thrown, this was the stack:
#0      debugCheckHasDirectionality.<anonymous closure> (package:flutter/src/widgets/debug.dart:223:7)
#1      debugCheckHasDirectionality (package:flutter/src/widgets/debug.dart:239:4)
#2      RichText.createRenderObject (package:flutter/src/widgets/basic.dart:4245:37)
#3      RenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4259:28)
#4      Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#5      Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#6      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#7      Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#8      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#9      ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#10     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#11     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#12     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#13     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#14     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#15     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#16     ParentDataElement.mount (package:flutter/src/widgets/framework.dart:3938:16)
#17     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#18     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4738:32)
#19     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#20     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#21     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4633:14)
#22     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#23     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#24     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4633:14)
#25     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#26     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#27     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#28     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#29     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#30     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#31     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#32     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#33     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#34     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#35     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#36     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#37     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#38     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#39     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#40     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#41     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#42     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3752:22)
#43     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#44     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#45     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#46     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4633:14)
#47     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#48     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#49     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#50     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#51     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#52     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#53     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#54     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#55     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4633:14)
#56     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#57     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#58     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#59     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#60     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#61     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3752:22)
#62     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#63     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#64     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#65     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#66     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#67     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#68     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3752:22)
#69     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#70     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#71     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#72     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3636:16)
#73     Element.rebuild (package:flutter/src/widgets/framework.dart:3478:5)
#74     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3605:5)
#75     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3600:5)
#76     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2890:14)
#77     Element.updateChild (package:flutter/src/widgets/framework.dart:2693:12)
#78     RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:852:16)
#79     RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:823:5)
#80     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:769:17)
#81     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2205:19)
#82     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:768:13)
#83     BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&RendererBinding&WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:657:7)
#84     runApp (package:flutter/src/widgets/binding.dart:699:7)
#85     main (/data/user/0/com.yourcompany.flutterproject/cache/flutter_projectENWZDI/flutter_project/lib/main.dart:6:3)
#86     _startIsolate.<anonymous closure> (dart:isolate-patch/dart:isolate/isolate_patch.dart:279)
#87     _RawReceivePortImpl._handleMessage (dart:isolate-patch/dart:isolate/isolate_patch.dart:165)
════════════════════════════════════════════════════════════════════════════════════════════════════
Another exception was thrown: No Directionality widget found.
Another exception was thrown: Horizontal RenderFlex with multiple children has a null textDirection, so the layout order is undefined.
Another exception was thrown: 'package:flutter/src/rendering/box.dart': Failed assertion: line 1446 pos 12: 'hasSize': is not true.
uid=10079(com.yourcompany.flutterproject) Thread-5 identical 2 lines
Another exception was thrown: 'package:flutter/src/rendering/box.dart': Failed assertion: line 1446 pos 12: 'hasSize': is not true.
Another exception was thrown: 'package:flutter/src/rendering/proxy_box.dart': Failed assertion: line 1564 pos 12: 'hasSize': is not true.
4

5 回答 5

71

flutter 不知道文本是 LTR 还是 RTL，所以需要明确告诉他textDirection

new Text("Hello", textDirection: TextDirection.ltr)

或者你可以只用方向性小部件包装文本

new Directionality(
          textDirection: TextDirection.ltr,
          child: new Text('Hello')

其目的是：

确定文本和文本方向敏感渲染对象的环境方向性的小部件。

并且 MaterialApp 小部件范围内的 Text 小部件不需要明确的书写方向，因为小部件和材质库中的默认本地化是 LTR

于 2018-04-06T09:47:22.660 回答
49

如果您不使用，则MaterialApp需要自己将应用程序包装到一些小部件中

import 'dart:ui' as ui;
...
runApp(
    new MediaQuery(
        data: new MediaQueryData.fromWindow(ui.window),
        child: new Directionality(
            textDirection: TextDirection.rtl,
            child: new MyHome())))
于 2018-04-06T07:25:07.563 回答
17

如果您没有使用MaterialApp或不在WidgetsApp树层次结构的根级别，则会收到此错误。

解决方案： 

void main() {
  runApp(
    MaterialApp(home: MyHome()), // use MaterialApp
  );
}

class MyHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold( // use Scaffold also in order to provide material app widgets
      body: Center(child: Text("Something")),
    );
  }
}
于 2020-02-28T16:32:41.793 回答
5

在 Dart 2 中，使用new是可选的。此外，Effective Dart 指南不鼓励使用new 。

您需要做的就是将 MyHome() 类包装到 MaterialApp()..

void main() {
  runApp(MaterialApp(home: MyHome()));
}
于 2021-02-21T11:21:50.507 回答
3

问题不在于您没有将小部件包装到 MaterialApp 中。正如文档所说，此错误是由于嵌套了相同类型的小部件，例如将行嵌套到行、列到列或列到 ListView 中，所以问题出现了，因为它成为 Widget 的无界约束，因此无法识别方向。

因此，为了避免这种问题，使用灵活或扩展来标识剩余空间。

在您的情况下，您可以简单地将值为 的 crossAxisAlignment 赋予startRow 小部件。

有关更多信息，请访问此文档页面

于 2018-05-18T19:38:25.667 回答