dart - 未找到方向性小部件

Question

我正在尝试在 flutter 中创建一个简单的布局，但不断收到此错误:

The following assertion was thrown building Text("Deliver features faster", textAlign: center): No Directionality widget found.

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.

Answer 1

flutter 不知道文本是 LTR 还是 RTL，所以你需要告诉他 textDirection明确的

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

或者你可以用 Directionality 包裹文本小部件

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

其目的是:

A widget that determines the ambient directionality of text and text-direction-sensitive render objects.

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 Material 库中的默认本地化是 LTR