未找到材质小部件

Question

我是 Flutter 的新手，我正在尝试执行示例 here。我只想使用 TextField 小部件来获取一些用户输入。问题是我得到一个“没有找到材料小部件”。错误。我究竟做错了什么 ？谢谢。

代码：

import 'package:flutter/material.dart';    

void main() {
  runApp(new MyApp());
}


class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      home: new ExampleWidget(),
    );
  }
}


/// Opens an [AlertDialog] showing what the user typed.
class ExampleWidget extends StatefulWidget {
  ExampleWidget({Key key}) : super(key: key);

  @override
  _ExampleWidgetState createState() => new _ExampleWidgetState();
}

/// State for [ExampleWidget] widgets.
class _ExampleWidgetState extends State<ExampleWidget> {
  final TextEditingController _controller = new TextEditingController();

  @override
  Widget build(BuildContext context) {
    return new Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        new TextField(
          controller: _controller,
          decoration: new InputDecoration(
            hintText: 'Type something',
          ),
        ),
        new RaisedButton(
          onPressed: () {
            showDialog(
              context: context,
              child: new AlertDialog(
                title: new Text('What you typed'),
                content: new Text(_controller.text),
              ),
            );
          },
          child: new Text('DONE'),
        ),
      ],
    );
  }
}

这是错误堆栈：

Launching lib/main.dart on Android SDK built for x86 in debug mode...
Built build/app/outputs/apk/app-debug.apk (21.5MB).
I/flutter ( 5187): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 5187): The following assertion was thrown building InputDecorator(decoration: InputDecoration(hintText:
I/flutter ( 5187): "Type something"); baseStyle: null; isFocused: false; isEmpty: true; dirty):
I/flutter ( 5187): No Material widget found.
I/flutter ( 5187): InputDecorator widgets require a Material widget ancestor.
I/flutter ( 5187): In material design, most widgets are conceptually "printed" on a sheet of material. In Flutter's
I/flutter ( 5187): material library, that material is represented by the Material widget. It is the Material widget
I/flutter ( 5187): that renders ink splashes, for instance. Because of this, many material library widgets require that
I/flutter ( 5187): there be a Material widget in the tree above them.
I/flutter ( 5187): To introduce a Material widget, you can either directly include one, or use a widget that contains
I/flutter ( 5187): Material itself, such as a Card, Dialog, Drawer, or Scaffold.
I/flutter ( 5187): The specific widget that could not find a Material ancestor was:
I/flutter ( 5187):   InputDecorator(decoration: InputDecoration(hintText: "Type something"); baseStyle: null;
I/flutter ( 5187):   isFocused: false; isEmpty: true)
I/flutter ( 5187): The ownership chain for the affected widget is:
I/flutter ( 5187):   InputDecorator ← AnimatedBuilder ← Listener ← _GestureSemantics ← RawGestureDetector ←
I/flutter ( 5187):   GestureDetector ← TextField ← Column ← ExampleWidget ← _ModalScopeStatus ← ⋯
I/flutter ( 5187): 
I/flutter ( 5187): When the exception was thrown, this was the stack:
I/flutter ( 5187): #0      debugCheckHasMaterial.<anonymous closure> (package:flutter/src/material/debug.dart:26)
I/flutter ( 5187): #2      debugCheckHasMaterial (package:flutter/src/material/debug.dart:23)
I/flutter ( 5187): #3      InputDecorator.build (package:flutter/src/material/input_decorator.dart:334)
... <output omitted>
I/flutter ( 5187): (elided one frame from class _AssertionError)
I/flutter ( 5187): ════════════════════════════════════════════════════════════════════════════════════════════════════
I/flutter ( 5187): Another exception was thrown: No Material widget found.

Answer 1

错误信息说：

要引入Material 小部件，您可以直接包含一个， 或使用包含Material 本身的小部件，例如Card， Dialog、Drawer 或 Scaffold。

在您的情况下，我可能会将您的 Column 包装在 Scaffold 中。这将便于以后将其他材质小部件添加到您的应用中，例如 AppBar、Drawer 或 FloatingActionButton。

@override
Widget build(BuildContext context) {
  return new Scaffold(
    body: new Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        new TextField(
            controller: _controller,
            decoration: new InputDecoration(
                hintText: 'Type something',
            ),
        ),
        new RaisedButton(
            onPressed: () {
              showDialog(
                  context: context,
                  child: new AlertDialog(
                      title: new Text('What you typed'),
                      content: new Text(_controller.text),
                  ),
              );
            },
            child: new Text('DONE'),
        ),
      ],
    ),
  );
}

Answer 2

就我而言，我在类似这样的脚手架内使用了一个英雄小部件

Scaffold(
  body:Hero(
    child:ListView(
      children:<Widget>[
        TextField(),
           ...
           ...
      ]
    )
  )
);

我只是将 Hero Widget 移到了 Scaffold 外，它就解决了问题

Hero(
  child:Scaffold(
    body:ListView(
      children:<Widget>[
        TextField(),
        ...
        ...
      ]
    )
  )
);

Answer 3

将您的小部件放在脚手架内，如下所示：

    import 'package:flutter/material.dart';

    void main() => runApp(MyApp());

    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          title: 'Demo',
          //home: MyWidget(), --> do not do this !!!
          home: Home() --> this will wrap it in Scaffold
        );
      }
    }

    class Home extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return Scaffold(
            appBar: AppBar(
              title: Text('Demo'),
            ),
            body: MyWidget()); --> put your widget here
      }
    }

class MyWidget extends StatefulWidget {
...

Answer 4

该错误意味着您必须将列包装在 Material 小部件中，您可以将其作为主体放置在脚手架上，或者将其作为 Materialapp 的主页。例如：

 return MaterialApp(
   home: Column(
     mainAxisAlignment: MainAxisAlignment.center,
     children: [
       TextField(
         controller: _controller,
         decoration: new InputDecoration(hintText: 'Type something'),
       ),
     ]
   ),
 );

或

return MaterialApp(
   body: Column(
     mainAxisAlignment: MainAxisAlignment.center,
     children: [
       TextField(
         controller: _controller,
         decoration: new InputDecoration(hintText: 'Type something'),
       ),
     ]
   ),
 );

Answer 5

简单的解决方法是：

而不是使用：

void main() {
  runApp(new MyApp());
}

你应该使用：

void main() => runApp(
  MaterialApp(
    home: MyApp()
  )
);

希望对你有帮助！