在颤振中显示图像一次。

Question

当我在文件中设置图像并将其设置在资产中时，我遇到了颤振Image.assets的问题：

“颤振:用途-材料-设计:真正的资产：

• images/facebook.png`

当我把它写进我的代码时：

Center(
    child: Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Image.asset(
          "images/facebook.png"

        ),
        Text(
          '$_counter',
          style: Theme.of(context).textTheme.display1,
        ),
      ],
    ),
  ),

并运行它，它运行良好，但当我第二次运行代码时，它没有显示，这是我得到的错误：

图像资源服务╞════════════════════════════════════════════════════I/

(4784)捕获的══╡异常:解析图像时抛出以下断言: I/flutter ( 4784)：无法加载资产: AssetManifest.json。。。I/颤振( 4784)：#381 (package:flutter/src/widgets/binding.dart:969:5) I/ RenderObjectToWidgetElement.mount ( 4784)：#382 RenderObjectToWidgetAdapter.attachToRenderTree.(package:flutter/src/widgets/binding.dart:915:17) I/颤振( 4784)：#383 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2328:19) I/颤振( 4784)：#384 RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:914:13) I/颤振( 4784)：#385 WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:795:7) I/颤振( 4784)：#386(package:flutter/src/widgets/binding.dart:845:7) I/ runApp ( 4784)：#387 main (软件包:runApp_app/main.dart:4:16)i/颤振( 4784)：#388 _runMainZoned.(飞镖:ui/Hooks.DAT:229:25)I/颤振( 4784)：#393 _runMainZoned。(飞镖:ui/Hooks.DAT:221:5)I/颤振( 4784)：#394

_startIsolate。(dart:isolate-patch/isolate_patch.dart:305:19) I/颤振( 4784)：#395 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:172:12) I/_RawReceivePortImpl._handleMessage( 4784)：(从包飞镖中删除7帧)I/颤振( 4784)：图像提供者: AssetImage(bundle: null，名称：“_RawReceivePortImpl._handleMessage/facebook.png”)I/_RawReceivePortImpl._handleMessage( 4784)：图像配置: ImageConfiguration(bundle: PlatformAssetBundle#b5447()，devicePixelRatio: 1.5 )I/颤振( 4784)：地区: en_US，textDirection: TextDirection.ltr，平台: android) I/颤振( 4784)：════════════════════════════════════════════════════════════════════════════════════════════════════

Answer 1

看看这个

assets:
   - images/facebook.png

确保你有相同的空间，就像这样

flutter:

[2 whitespaces or 1 tab]assets:
[4 whitespaces or 2 tabs]- images/facebook.png

每一个空间都很重要

Answer 2

我的代码：

import 'package:flutter/material.dart';
import 'dart:io';
import 'package:image_picker/image_picker.dart';
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'myApp'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  File imageFile;
  int _counter = 0;
  Color red = Colors.red;
   _incrementCounter() async{
     var picture = await ImagePicker.pickImage(source: ImageSource.camera);

    this.setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      imageFile = picture;
      _counter++;
      print("work");
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Image.asset(
              "images/facebook.png"

            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

我的pubspec.yaml：

name: flutter_app
description: A new Flutter application.

# The following defines the version and build number for your application.
# A version number is three numbers separated by dots, like 1.2.43
# followed by an optional build number separated by a +.
# Both the version and the builder number may be overridden in flutter
# build by specifying --build-name and --build-number, respectively.
# In Android, build-name is used as versionName while build-number used as versionCode.
# Read more about Android versioning at https://developer.android.com/studio/publish/versioning
# In iOS, build-name is used as CFBundleShortVersionString while build-number used as CFBundleVersion.
# Read more about iOS versioning at
# https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
version: 1.0.0+1

environment:
  sdk: ">=2.1.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  image_picker:


  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2

dev_dependencies:
  flutter_test:
    sdk: flutter


# For information on the generic Dart part of this file, see the
# following page: https://dart.dev/tools/pub/pubspec

# The following section is specific to Flutter.
flutter:
  uses-material-design: true
  assets:
   - images/

  # An image asset can refer to one or more resolution-specific "variants", see
  # https://flutter.dev/assets-and-images/#resolution-aware.

  # For details regarding adding assets from package dependencies, see
  # https://flutter.dev/assets-and-images/#from-packages

  # To add custom fonts to your application, add a fonts section here,
  # in this "flutter" section. Each entry in this list should have a
  # "family" key with the font family name, and a "fonts" key with a
  # list giving the asset and other descriptors for the font. For
  # example:
  # fonts:
  #   - family: Schyler
  #     fonts:
  #       - asset: fonts/Schyler-Regular.ttf
  #       - asset: fonts/Schyler-Italic.ttf
  #         style: italic
  #   - family: Trajan Pro
  #     fonts:
  #       - asset: fonts/TrajanPro.ttf
  #       - asset: fonts/TrajanPro_Bold.ttf
  #         weight: 700
  #
  # For details regarding fonts from package dependencies,
  # see https://flutter.dev/custom-fonts/#from-packages

Answer 3

确保在pubspec.yaml中使用2或4个空格(避免制表符)作为缩进。

你的pubspec.yaml肯定是这样的

flutter:
  uses-material-design: true
  assets:
    - images/facebook.png

不是这样的

flutter:
  uses-material-design: true
  assets:
      - images/facebook.png # error

或者像这样

flutter:
  uses-material-design: true
  assets:
  - images/facebook.png # also error

跑

flutter clean

flutter pub get

在你的航站楼